Star Tracktracking of astrophysical sources with neutrinos

Prof. dr. Maarten de Jong

Nikhef – National institute for subatomic physics

Amsterdam, the Netherlands

www.nikhef.nl

Project duration: 60 months

abstractAlthough a visit may seem the ideal way to study astrophysical sources in

detail, the required travel time makes this idea impractical. The alterna-

tive is to boldly view the sources with Earth-based telescopes. The prime

objective of this proposal is the scientific capitalization of a new generation

of telescopes. Unlike conventional telescopes, these telescopes will detect

neutrinos and not light. The detection of neutrinos from the cosmos will

break new grounds in the study of various frontier questions in science

such as those related to the origin of cosmic rays, the mechanism of astro-

physical particle acceleration and the birth of relativistic jets.

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Section 1A: The Principal Investigator

Maarten de JongMinervalaan 8"1077 NX Amsterdam, the Netherlands Single, 43 years old, Dutch nationalitye-mail: Maarten.de.Jong@nikhef.nltel: +31 20 592.2121

Work Experience

Nikhef 1997–present

EU Work PackagE coordinator: Responsible for the coordination of the information technology aspects of the KM3NeT design study. Contact person of the proposal “KM3NeT: The next generation neutrino telescope” submitted to the Dutch fund-ing agency NWO with a total investment budget of 8.8 M€. This proposal in under evaluation by NWO.

ProfEssor: Appointed as the first professor in the Netherlands (Leiden University) in the field of experimental astro-particle physics. Lecturing one semester per year.

antarEs dEPUty sPokEsPErson: Responsible for the physics coordination of the Antares project and the dissemination of scientific results in refereed journals and at conferences. Organising 2 workshops per year (about 50 participants) and 3 collaboration meetings per year (about 100 participants).

tEam LEadEr: Responsible for the coordination of the Dutch participation in the Antares project (5 PhD students, 1 post-doc, 4 senior physicists and 6 techni-cians), the project planning and management and the spending of the investment budget. Supervising PhD students (subjects: Monopole, neutralino and gamma-ray burst detection). Recruiting new people and building a team. Developed a new data filter pro-gram for the Antares neutrino telescope (2.5 × faster, 100 × better signal-to-noise ratio).

sEnior staff: Contact person of the proposal “Antares: a cosmic neutrino observatory”, sub-mitted to the Dutch funding agency NWO with a total investment budget of 3.6 M€. Invented a new readout system. Nominated readout project leader for the Antares experiment. Published a study of neutrino physics at a future muon collider. Organised the Antares collaboration meeting at Nikhef (about 100 participants). Contributed to the Topical Lectures 1998 and 2001 at Nikhef, the European Graduate School in 2002 and the Fantom study week in 2003. Organised the Chorus collaboration meeting at Nikhef (about 100 participants). Supervising PhD students (co-promotor). Developed new track fit algorithms for the Chorus experiment which yielded 20% more physics events found in target.

CERN, Chorus experiment 1993–1997

fELLoW (CERN): Conducting the design, construc-tion and operation of a new detector. Proposed an upgrade of the detector to improve the charge and momentum determination of hadrons. Approval of the project: nominated project leader (2 PhD students and 5 technicians). Coordinated the design and the manufacture of three so-called honeycomb wire-chambers at Nikhef. Designed and implemented the necessary infrastructure (gas, cooling, low-voltage, high-voltage, slow control, DAQ, alignment). In charge of the testing and final assembly and installa-tion. The new detectors have been successfully used.

rEsEarch associatE (Nikhef): Participating in the construction, calibration (~1 ns precision) and main-tenance of the trigger system. Improved the neutrino oscillation trigger such that 10% more physics events in target were detected.

CERN, NMC & SMC experiments 1991–1993

rEsEarch associatE (University of Mainz): Supervising an international group of 5 PhD stu-dents. Established the key point in the data analysis and developed a Monte Carlo simulation program with enhanced statistical significance (up to a factor of 10). Determined the radius of the J/ψ meson by a novel comparison of the cross section of J/ψ produc-tion to that of elastic π scattering.

rEsEarch associatE (Nikhef): Preparing the detector and coordinating the startup of data taking. Adapted the inclusive muon trigger to the increased beam intensity.

Scientific Committees

Peer Review Committee of the ApPEC 2002–2005.CERN SPS-Committee 2001–2004.Scient. Adv. Committee of Nikhef 2003–present.

Outreach

Contributing to public relations.Citations in seven national newspaper articles.One appearance on local TV.One appearance in a theatre play (AdHoc).Giving seminars to the general public (1–2 per year).

Education

NIMO, Project Management, 2000, 6 days training.Vrije Universiteit, Amsterdam, 1986–1991, PhD.CERN summer student, 1986.Vrije Universiteit, Amsterdam, 1982–1986, Cum Laude.Sweelinck Gymnasium, Amsterdam, (1976–1982).

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Scientific leadership profile

Having performed various particle physics experi-ments at CERN*, I have now focussed my career on the study of cosmic neutrinos. The study of cosmic neutrinos is one of the key components in the field of astro-particle physics. This is a relative new field emerging at the interface between astrophys-ics and accelerator-based particle physics. In 2006, I have been appointed as the first professor in the Netherlands in the field of experimental astro-par-ticle physics. The election as deputy spokesperson of the Antares¶ project and the nomination of work-package coordinator of the KM3NeT† design study speak for my leadership in this field.

My ideas can be considered as creative and effective. Most recently, I have invented the ‘All-data-to-shore’ concept for the readout of the Antares prototype neutrino telescope. In this, the rare neutrino signal is filtered on shore from the background using a farm of commodity PCs and state-of-the-art software. The software has been realised by me and is now fully operational in the Antares experiment. This achievement is remarkable (few considered this possible) and changed the picture dramatically. Now, my idea has become a seminal part of the astrophys-ics program with the next generation neutrino telescope. Some of my ideas have led to new results (e.g. measurement of the size of a very short lived particle), others have disproved an initial claim for a new theory (e.g. colour transparency).

Both in particle physics and astro-particle physics, a key issue is the reconstruction of particle trajectories in a detector. For the Chorus experiment at CERN, I have developed a new track-fit algorithm. This has improved the final measurement of νµ→ντ oscilla-tions by such an amount that the most precise result ever could be achieved. For the Antares experiment, I have found a linear solution of the track-fit problem by considering a subset of the phase space. Although its impact has not been quantified yet, preliminary studies have shown that it can improve the angular resolution of a neutrino telescope significantly.

I have coordinated the construction and opera-tion of a new kind of particle detector (so-called ‘Honeycomb chamber’) for the Chorus experiment. This marked the first successful application of this kind of detector in a large scale experiment. For the Antares experiment I have coordinated the design efforts of the submarine power system. These efforts have led to a system consisting of DC-DC convert-ers with an unprecedented efficiency. By now, the system is operational for more than two years indicating that the high level of reliability required for submarine operations has been achieved.

For the funding of scientific projects I have (co-)written various proposals which have provided so far a total amount of about 3.6 M€ for (hardware) investments and about 4.8 M€ for personnel. In addition, I am contact person of a proposal submit-ted to the Dutch funding agency NWO with a total investment budget of 8.8 M€. At the moment of this writing, this proposal is still under evaluation.

I initiated the measurements and supervised the analysis that has led to the publication “Measurement of nucleon structure functions in neutrino scattering”. At that time, there were only two measurements made worldwide which yielded contradicting results. This paper has been seminal in two ways. It has resolved the long standing discrepancy and it is today the one and only measurement of all three structure functions of the nucleon known to exist.

I have been supervising master and PhD students throughout my scientific career. About half of these students continued a career in science, the others have found a job in industry relatively fast. One of my students originally applied to a permanent post in the computing department of Nikhef. I was member of the search committee and I persuaded this person to become a PhD student in my group. After a successful PhD, this person has won two prestigious awards in the Netherlands.

In the following, I list a few quotes from a selection of PhD theses of students whom I have supervised:

“Maarten de Jong took a key role in all stages of this work. He convinced the collaboration that a DIS program in 1998 was possible and thus obligatory. In the analysis, he brought me back on track when I was once again in panic and desperation. This text would have been unreadable without Maarten’s ex-tensive commenting.” R.G.C. Oldeman, PhD thesis, University of Amsterdam (2000).

“Maarten, to you I owe most thanks. I could not have wished a better supervisor! Your enthusiasm and inexhaustible drive have continuously motivated me. Without your knowledge and bright ideas, I could never have written this thesis.” B.A.P van Rens, PhD thesis, University of Amsterdam (2006).

“Maarten was so kind to become my thesis advisor and read the whole manuscript very carefully.” J. Uiterwijk, PhD thesis, University of Leiden (2007).

* www.cern.ch¶ antares.in2p3.fr† www.km3net.org

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Short list of selected publications

Final results from a search for νµ→ντ oscillations with the CHORUS experimentNucl. Phys. B 793, 326-343, 2008.citations: not yet applicable.

The data acquisition system for the ANTARES Neutrino TelescopeNucl. Instrum. Meth. A 570, 107-116, 2007. citations: 5

Measurement of nucleon structure functions in neutrino scatteringPhys. Lett. B 632, 65-75, 2006.citations: 18

Measurement of the Z/A dependence of neutrino charged-current total cross-sectionsEur. Phys. J. C 30, 159-167, 2003.citations: 6

Measurement of Λc+ production in neutrino charged-

current interactionsPhys. Lett. B 555, 156-166, 2003.citations: 20

Cross-section measurement for quasi-elastic pro-duction of charmed baryons in neutrino-Nucleon interactionsPhys. Lett. B 575, 198-207, 2003.citations: 14

Measurement of D0 production in neutrino charged-current interactionsPhys. Lett. B 527, 173-181, 2002.citations: 26

The neutrino factory: Beam and experimentsNucl. Instrum. Meth. A 451, 102-122, 2000.citations: 82

Search for νµ→ντ oscillation using the τ decay modes into a single charged particlePhys. Lett. B 434, 205-213, 1998.citations: 53

Observation of neutrino induced diffractive Ds*+

production and subsequent decay Ds*+→Ds

+→τ+→µ+

Phys. Lett. B 435, 458-464, 1998.citations: 21

Contributions to advanced international schools

The 19th general FANTOM study week“Interplay between Theory and Experiment”2003, Amsterdam, the Netherlands

European Granduate School“Neutrino: masses, mixing and oscillations”2002, Giessen, Germany

Topical lectures“The Antares neutrino telescope”2001, Nikhef, Amsterdam, the Netherlands.

Topical lectures“Neutrino physics”1998, Nikhef, Amsterdam, the Netherlands.

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Section 1B: Synopsis of the Project Proposal

The prime objective of this proposal is the scientific capitalization of a new generation of telescopes. Unlike conventional telescopes, these telescopes will detect neutrinos and not light. The detection of neutrinos from the cosmos will break new grounds in the study of various frontier questions in science such as those related to the origin of cosmic rays, the mechanism of astrophysical particle acceleration and the birth of relativistic jets. The study of cosmic neutrinos is a key component in the field of astro-particle physics, a new interdisciplinary research field with strong links to its two progenitors: astro-physics and accelerator-based particle physics.

The common way to study the cosmos is by the detection of light. Light, however, is not in every respect the most suitable probe for observational astronomy. For example, there are regions in the Universe from which light cannot escape. Light can also be absorbed on its way to Earth. An alternative is to study the cosmos by the detection of cosmic rays. Cosmic rays are energetic particles originating from space that bombard the Earth continuously. Their interactions with the atoms in the atmosphere give rise to extended showers of secondary parti-cles which are observed on Earth. Although this phenomenon has been known for almost a century, the origin of these cosmic rays remains unclear. It is suspected that the most energetic cosmic particles are accelerated in astrophysical sources that are located far away in the Universe. The intergalactic magnetic fields bend the trajectories of the cosmic rays by an unknown and varying amount and so impede the pin-pointing of the cosmic accelerators. Even though the deflection is reduced with increas-ing energy, the difficulty remains, because at some point the cosmic rays are absorbed by the cosmic microwave background radiation limiting the depth of view to our galaxy. It is therefore planned to search for high-energy neutrinos that are expected to materialise in the same cosmic accelerators. As neutrinos carry no electric charge, their trajectories remain unperturbed despite the magnetic fields. The weak interaction of neutrinos with normal matter extends the panorama to the whole Universe. On the other hand, this same weak interaction neces-sitates an extremely large detector.

Following a sequence of pioneering projects, an international joint venture was initiated to build a large neutrino telescope: KM3NeT. The main goal of KM3NeT is to detect neutrinos from astrophysical sources that exhibit the most violent processes in the Universe. Also exotic particles, like those constitut-ing the dark matter or magnetic monopoles, could be detected with this telescope. As a first step towards the realisation of the KM3NeT neutrino telescope, the Antares collaboration has built a prototype

detector. This detector is now operational and data are taken routinely 24 hours per day. After comple-tion, KM3NeT will be the largest neutrino telescope in the world for the foreseeable future. The list of targets includes active galactic nuclei, micro quasars, supernova remnants and gamma-ray bursts.

Active galactic nuclei (AGN) are objects associated with the centres of far away galaxies. The amount of energy released by these objects exceeds that of any other steady source. The energy is thought to be pro-vided by the gravitational force of a super-massive black hole. In some cases, relativistic jets have been observed. A relativistic jet can best be imagined as a beamed outburst of matter moving at almost the speed of light. Many models predict neutrino pro-duction in these jets.

Micro quasars resemble AGN, but at a much smaller scale. They are believed to consist of a small black hole or a neutron star that accretes matter from a companion star. Unlike AGN, micro quasars can be found in our galaxy. Because they are much closer, they are easier to study. To some extent, they have become ‘laboratories’ for revealing the physical processes that produce superfast jets.

A supernova is a stellar explosion that creates an extremely luminous object. On average, supernovae occur once every 50 years in a galaxy the size of the Milky Way. The matter ejected in a supernova explosion will collide with the interstellar medium, thus creating a shock wave. Particles accelerated in these shock waves are believed to represent the bulk of the observed cosmic rays but no observa-tional evidence for this association has been found. The detection of neutrinos from supernova rem-nants could change this.

Gamma-ray bursts (GRB) are very short and intense flashes of gamma rays. These bursts occur at random times and at random places on the sky. GRBs were discovered by accident in the late 1960s by the U.S. Vela nuclear test satellites. As one of the most energetic phenomena since the Big Bang, GRBs are subject to many space- and ground-based observa-tions. The prompt emission of the gamma rays lasts for a short time, typically less than one minute. It is usually followed by a so-called afterglow. The afterglow is the emission of radiation at longer wavelengths. From the observation of the afterglow it has become possible to determine the redshift, and hence the distance to the GRB. It has been found that GRBs occur at cosmological distances. The short duration of the burst and the shape of the observed light spectra have led to the idea that a GRB is caused by a kind of ‘fireball’ explosion. The early phase of a GRB, however, cannot be studied directly

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as no light can escape. But many models predict the production of high-energy neutrinos that do escape the initial fireball. The detection of neutrinos from gamma-ray bursts will provide a unique image of the birth of relativistic jets. The neutrino signature of GRB events will be particularly clean due to the space and time correlation with the optical observa-tions provided by a network of satellites that detect the GRBs. The background is expected to be so small that a few events will be sufficient to claim discovery of a correlated neutrino signal.

A surprising but inevitable conclusion from a range of astronomical observations is that a large fraction of the Universe consists of unknown matter. As it is not luminous, this matter is referred to as dark matter. Although the existence of dark matter is supported by several pieces of observational evi-dence, its nature is completely unknown. It has been suggested that dark matter consists of a hypothetical particle that corresponds to one of the particles in a new theory (super-symmetry model). Theoretical support for this new theory comes mainly from the observation that it gracefully unifies the electroweak and strong forces at a single energy scale. The neutralino is the lightest super-symmetric particle. It is stable and the end-product of a decay chain of any massive super-symmetric particle which could have been created in the early Universe. The presence of neutralinos in the Universe could explain the dark matter mystery.

The underlying physics of some neutrino point sources might well have its origin in super-symmetry. If neutralinos are indeed the dominant constituent of dark matter, they are expected to be found in large quantities in the vicinity of galaxies. Because of their large mass and the small (but nonzero) interaction cross section with normal matter, neutralinos are expected to gradually lose kinetic energy and gravi-tate towards the centre of stars and galaxies. The accumulation of neutralinos in e.g. the Sun and their subsequent annihilation can produce a significant flux of high-energy neutrinos. In direct searches for dark matter, one tries to detect the (very small) recoil energy deposited by a passing dark matter particle. The advantage of indirect searches for dark matter based on the detection of cosmic neutrinos is that a directional correlation with a well defined source can be explored. The Sun is a good candidate to search for dark matter in this way because its mass is large and the distance to Earth is small. The neutrinos coming from the annihilation of neutralinos have much higher energies than those arising from the nuclear fusion reactions that are the main energy source of the Sun. The expected flux of high-energy neutrinos is such that it cannot be detected by the existing solar neutrino experiments. A large neutrino

telescope is needed to search for the neutrinos that are related to dark matter. The observation of a neu-tralino signal at the Large Hadron Collider (LHC) at CERN and with the KM3NeT neutrino telescope would represent a major breakthrough as it would establish a direct link between astrophysics (dark matter) and particle physics (super-symmetry).

The holy grail of particle physics is the unification of the fundamental forces of nature. The idea is that, at sufficiently high energies, the strong force and the electroweak force have the same strength. In this case, a unique force should remain that results from a single symmetry. At lower energies, this symmetry is spontaneously broken into the apparent strong and electroweak forces. It has been shown that the existence of magnetic monopoles appears as a generic prediction of this symme-try breaking. In practice however, no magnetic monopole has been found so far. The mass of a monopole is related to the energy scale at which the unification occurs and the strength of the gauge coupling. As the Universe expanded and cooled, phase transitions occurred that can be associated with the breakdown of symmetries. During these phase transitions, magnetic monopoles could have been created. The density of monopoles in the Universe that resulted from these phase transitions is uncertain. In fact, simple assumptions lead to an abundance of monopoles that exceeds the total mass of the Universe. Many mechanisms have been proposed to overcome this problem. In all cases, monopoles residing in the Universe will have been accelerated by large scale magnetic fields. An upper limit on the monopole flux can be estimated by assuming that all dark matter (see above) in the Universe can be attributed to monopoles. This upper limit results in a measurable rate provided that such monopoles can be detected efficiently in a very large volume. Due to the large size of the KM3NeT detector, the search for monopoles can be pursued with an unprecedented sensitivity.

The scientific case for studying neutrinos from the cosmos is compelling. The construction of a neu-trino telescope is, however, extremely challenging. Neutrinos are best known for their reluctance to be detected. In short, the study of cosmic neutrinos requires a massive telescope with a size of at least one cubic kilometre. A solution to make such a large mass sensitive to neutrinos is to build a three-dimen-sional array of very sensitive light sensors in the sea. Neutrinos can then be detected indirectly through the detection of the Cherenkov light emitted by charged particles emerging from a neutrino interaction. The transparency of the water makes it possible to distribute the light sensors in a cost-effective way.

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The main deliverable of this proposal is the data-fil-ter system for the KM3NeT neutrino telescope. This system optimises the signal detection efficiency by using a large farm of standard PCs and state-of-the-art software. As a result, the value-for-money ratio of the entire project will be improved. The unique feature of this proposal is that for a moderate fraction of the total cost of the KM3NeT project, a key com-ponent can be accomplished. The foreseen analyses will be focused on neutrino-point sources and gamma-ray bursts. The sensitivity of the KM3NeT telescope to these potential neutrino sources will be greatly enhanced with the data-filter system devel-oped in the framework of this proposal. Hence, it is expected that these analyses will yield results within the foreseen time lines. With these analyses, one can become the first person to witness the birth of a relativistic jet and to unravel the mystery of particle acceleration in the cosmos.

With this proposal, the principal investigator (PI) will play a major role in the scientific capitalization of the KM3NeT neutrino telescope. This will position the PI and his group at the forefront of astro-particle physics for the foreseeable future.

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Scientific motivation - detection of cosmic neutrinosThe prime objective of this proposal is the scientific capitalization of the next generation neutrino tel-escope: KM3NeT. This telescope will break new grounds in the study of various frontier questions in science such as those related to the origin of cosmic rays, the mechanism of astrophysical particle ac-celeration and the birth of relativistic jets. The study of cosmic neutrinos is a key component in the field of astro-particle physics, a new interdisciplinary re-search field with strong links to its two progenitors: astrophysics and accelerator-based particle physics. The common way to study astrophysical sources is by the detection of electromagnetic radiation. Photons are, however, not in every respect the most suitable probe for observational astronomy. For ex-ample, there are regions in the Universe from which photons cannot escape. Photons can also be absorbed on their way to Earth. An alternative is to study the cosmos by the detection of cosmic rays. Association of the observed cosmic rays and any known astro-physical source is easier said than done because the cosmic rays are either deflected by (inter-)galactic magnetic fields or absorbed by the cosmic micro-wave background radiation. This impasse has led to the consideration of a different cosmic messenger: the neutrino. Indeed, the neutrino has no charge and is therefore not deflected by magnetic fields. It is stable and interacts only weakly with matter. Thus it will travel in a straight line from the most remote places in the Universe to Earth unhampered by the cosmic microwave background radiation or (inter-)galactic magnetic fields.

Traditionally, the particle accelerator has been the work horse of high-energy physics, but there is a growing awareness that future experiments should also involve cosmic particles. This is not only motivated by the ever increasing size and cost of man-made accelerators: cosmic particles are known

to have energies that extend orders of magnitude beyond the reach of any future machine. Cosmic neutrinos have the potential of giving access to particle physics at the most extreme conditions.

The scientific case for studying cosmic neutrinos is compelling. Detection of these neutrinos, however, is extremely challenging. With this proposal, the sensitivity of the KM3NeT neutrino telescope will be enhanced significantly.

Neutrino point sourcesVarious cosmic neutrino sources have been proposed in the literature. Among these are Active Galactic Nuclei (AGN), micro quasars, and supernova remnants. AGN are objects associated with the centres of galaxies. The amount of energy released by these objects exceeds that of any other steady source. The energy is thought to be provided by the gravitational force of a super-massive (106–1010

solar masses) black hole. In some cases, relativistic

Figure 1: A TeV gamma ray image of the supernova remnant RX J1713.7-3946 as reported by the HESS experiment[1].

What is KM3NeT?KM3NeT (A km3 sized Neutrino Telescope) is a future deep-sea research infrastructure hosting a neutrino telescope with a volume of at least one cubic kilometre to be constructed in the Mediterranean Sea. The KM3NeT neutrino telescope is one of the major highways on the ASPERA roadmap. The European Strategic Forum for Research Infrastructure (ESFRI) has published a list of the most important large-scale infrastructures that should be built in the next decade. KM3NeT appears on this list confirming the strong level of support for this project Europe-wide. The design study for the infrastructure, funded by the EU FP6 framework, started in february 2006. The preparatory phase study of the infrastructure funded by the EU FP7 framework will start this year and finish in 2011. The construction phase will follow immediately and span five years. The total cost of KM3NeT is estimated at 220–250 M€.

According to the coordinator of the KM3NeT project, prof. dr. U.F. Katz:“The KM3NeT neutrino telescope will be unique in the world in its physics sensitivity and will provide access to scientific data that will propel research in different fields, including astronomy, dark matter searches, cosmic ray and high energy physics.”

Section 2: Project Proposal

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jets have been observed. Models exist that predict neutrino production in these jets[2]. Micro quasars are X-ray binary systems that are found in our galaxy. They are believed to consist of a small black hole or a neutron star that accretes matter from a companion star. To some extent, micro quasars resemble AGN but at a much smaller scale. A supernova is a stel-lar explosion that creates an extremely luminous object. On average, supernovae occur once every 50 years in a galaxy the size of the Milky Way. The matter ejected in a supernova explosion will collide with the interstellar medium, thus creating a shock wave. Particles accelerated in these shock waves are believed to represent the bulk of the observed cosmic rays but no observational evidence for this associa-tion has been found. The detection of neutrinos from supernova remnants could change this.

Recent observations from Cangaroo[3] and HESS[4] include TeV gamma rays from supernova remnants in the centre of our galaxy (see Fig. 1). It has been suggested that the observed high-energy gamma rays are produced by inverse Compton scattering. In this process, a high-energy electron exchanges energy with a low-energy photon. Due to the pres-ence of magnetic fields, the same electrons also emit synchrotron radiation. As a result, the spectrum will have a characteristic broadband feature. Some of the recently discovered TeV gamma ray sources, how-ever, do not have this broadband feature. It is gener-ally believed that in this case the TeV photons are the result of the two-photon decay of neutral pions that are produced in interactions of high-energy protons with photons. The neutral pions are naturally accom-panied by charged pions which predominantly decay to muon neutrinos. These TeV gamma-ray emitters are found near the galactic centre and might there-fore be the nearest sources of high-energy neutrinos.

An initial search for neutrino point sources away from the galactic centre resulted in a set of upper

limits[5]. In order to survey the galactic disk, includ-ing its centre, a neutrino telescope on the northern hemisphere (relatively close to the equator) is neces-sary. The detector that is the basis of the present proposal will be located in the Mediterranean Sea. From this position, one can observe the galactic cen-tre for about 70% of the time. An evaluation of the discovery potential of KM3NeT for known sources near the galactic centre is shown in Fig. 2.

Dark MatterA surprising but inevitable conclusion from a range of astronomical observations is that a large fraction of the Universe consists of unknown matter. As it is not luminous, this matter is referred to as dark matter. Evidence for the existence of dark matter

Figure 2: The number of point sources that can be detected with the KM3NeT neutrino telescope at the 99% confidence level as a function of time. The results are based on the measurements of the HESS experiment and the assumption that pions are at the origin of the observed high-energy gamma rays[6]. The blue (bottom) bars correspond to the scenario with an energy cut-off at about 10 TeV. The green (top)bars correspond to the additional sources that can be detected assuming no energy cut-off.

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Figure 3: Three pieces of observational evidence supporting the existence of dark matter in the Universe. From left to right: The discrepancy between the observed and the expected rotational light curves, a picture of gravitational lensing, and the measured temperature anisotropy in the cosmic microwave background radiation by the COBE and the WMAP satellites.

R (kpc)

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expectedfromluminous disk

observed

M33 rotation curve

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comes from various sources and will be briefly sum-marised here. The observed rotational light curves of (clusters of) stars in spiral galaxies do not comply with Newton’s law when all the mass of the galaxy is assumed to be in the luminous disk. By assuming a large amount of dark matter spread throughout these galaxies, this discrepancy can be resolved. The relative motion of galaxies belonging to one cluster and the thin arcs of light observed around the cluster centre due to gravitational lensing suggest that a large fraction of the mass of the cluster is dark as well. Measurements of the temperature anisotropies in the 2.7 K cosmic microwave background radiation indicate that 22 ± 4% of the total energy content of the Universe appears in the form of dark matter[8,9].

Although the existence of dark matter is supported by several pieces of observational evidence (see Fig. 3), the nature of dark matter is completely unknown. It has been suggested that the dark matter consists of a new hypothetical particle that corresponds to one of the particles in the super-symmetry model. In this model, a new symmetry between the two fundamental types of elementary particles (fermions and bosons) is introduced. Theoretical support for this new theory comes mainly from the observation that it gracefully unifies the electroweak and strong forces at a single energy scale. The neutralino is the lightest super-symmetric particle. It is stable and the end-product of a decay chain of any massive super-symmetric particle which could have been created in the early Universe. The presence of neutralinos in the Universe could explain the dark matter mystery.

The underlying physics of some neutrino point-sources might well have its origin in super-symmetry. If neutralinos are indeed the dominant constituent of dark matter, they are expected to be

Cosmic raysAfter the discovery of radioactivity by Henri Becquerel in 1896, it was generally believed that the atmospheric electricity (i.e. the ionisation of the air) was caused by radioactive elements in the ground or in the air. Then, in 1912, Victor Hess found that the ionisation rate at an altitude of 5000 meters was dramatically higher than that at the ground level. He concluded “The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above”. Today, this phenomenon is known as cosmic rays. Charged particles accelerated in the expanding shock waves of supernova remnants are believed to represent the bulk of the cosmic rays up to energies of several thousands of TeV. However, no association between the observed cosmic rays and known supernova remnants can be made because the (inter-)galactic magnetic fields blur the image of the sky. This scientific deadlock could come to an end by the detection of cosmic neutrinos. Interactions of the accelerated particles with the matter surrounding the source are expected to produce neutrinos via the decay of short lived particles (mainly pions). As the cross sections of these interactions are well-known, the fluxes of the high-energy neutrinos can be calculated based on the measured flux of cosmic rays and the assumed densities of photons and nuclei. These calculations indicate that a detectable cosmic neutrino signal is within reach. On their way to Earth, these neutrinos are not deflected by (inter-)galactic mag-netic fields. Detection of such a neutrino signal would provide the experimental evidence that supernova remnants are indeed the sought after cosmic accelerators.

found in large quantities in the vicinity of galax-ies. Because of their large mass and the small (but nonzero) interaction cross section with normal mat-ter, neutralinos are expected to gradually lose kinetic energy and gravitate towards the centre of stars and galaxies. The accumulation of neutralinos in e.g. the Sun and their subsequent annihilation can produce a significant flux of high-energy neutrinos.

In direct searches for dark matter, one tries to detect the (very small) recoil energy deposited by a passing dark matter particle. The DAMA[10] experiment has reported a so far unconfirmed and in fact disputed[11] signal. The advantage of indirect searches for dark matter based on the detection of cosmic neutrinos is that a directional correlation with a well defined source can be explored[12]. The Sun is a good candi-date to search for dark matter in this way because its mass is large and the distance to Earth is small. The neutrinos coming from the annihilation of neutral-inos have much higher energies (10–500 GeV) than those arising from the nuclear fusion reactions that are the main energy source of the Sun (1–10 MeV). The expected flux of high-energy neutrinos is such that it cannot be detected by the existing solar neutrino experiments. A large neutrino telescope is needed to search for the neutrinos that are related to dark matter.

Gamma-ray burstsGamma-ray bursts (GRB) are very short and intense flashes of MeV gamma rays. These bursts occur at random times and at random places on the sky (see Fig. 4). The prompt emission of the gamma rays lasts for a short time, typically less than one minute. It is usually followed by a so-called afterglow. The afterglow is the emission of radiation at other wavelengths. From the observation of the afterglow

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it has become possible to determine the redshift, and hence the distance to the GRB16. It has been found that GRBs occur at cosmological distances: the mean redshift is about 1.3, which corresponds to a distance of about 2.5 Gpc or 7 billion light years. For com-parison, the size of our galaxy is 30 kpc across.

The short duration of the bursts and the non-thermal character of the observed spectra have led to the idea that a GRB is caused by some kind of ‘fireball’ explosion[17]. By some unknown mechanism, a large amount of energy is deposited into a small object. This object is then heated up and becomes a kind of fireball. Unable to cool or to keep its energy confined, this object will release its heat by ejecting matter. The kinetic energy of this matter is such that it almost reaches the speed of light. The enormous amount of energy emitted by a single source made early models that assumed an isotropic explosion questionable. Nowadays, the outflow of matter from the fireball is assumed to be collimated, which reduces the total energy output[18]. The kinetic energy of the matter and the collimation of the outflow make up a rela-tivistic jet. It is the conversion of the kinetic energy of this jet into radiation that leads to the observed gamma rays. Whether this is due to internal shocks or external shocks is not yet clear. In the internal shock model[19], many shells are ejected by the same compact object. A later but faster shell can then collide with an earlier but slower shell. The external shocks[20] occur when a relativistic jet interacts with the ambient matter. The afterglow emission is a natural phenomenon in either model as the matter in the jets will gradually slow down anyway due to interactions with the interstellar matter. While the mechanism that initiates the fireball explosion is unknown, in some cases a supernova could be associ-ated with a GRB. This would indicate that the core collapse of a massive star is at the origin of a GRB.

Unfortunately, the start of the fireball and the early phase of the explosion cannot be studied directly as no electromagnetic radiation can escape. Present fireball models predict the acceleration of charged particles (mainly protons) preceding the observable light flash. Interactions of these protons with the surroundings produce high-energy neutrinos that can escape the fireball. The detection of neutrinos associ-ated with a GRB will thus uncover the invisible core. The electromagnetic afterglow can be detected on Earth provided that the accurate position of the GRB is known instantaneously. In that case, ground-based telescopes can be pointed in the right direction in time. For this purpose, satellite-based warning sys-tems have been developed that distribute messages around the world, containing the celestial positions of the GRBs. The Swift[22] satellite was launched in 2004 and is part of this system. It detects about 100 GRBs per year and distributes the corresponding warning messages within several seconds.

The neutrino signature of GRB events will be particularly clean due to the space and time correla-tion with the optical observations provided by the previously mentioned warning systems. The back-ground is expected to be so small that a few events will be sufficient to claim discovery of a correlated neutrino signal. Based on the model in reference[14],

Figure 4: Sky map of the gamma-ray bursts detected by the BATSE detector on board of the Compton Gamma Ray Observatory[21], launched in 1991. (The horizontal centre line coincides with the galactic disk.)

+90

–90

+180 –180

2704 BATSE Gamma-Ray Bursts

10-7 10-6 10-5 10-4

Fluence, 50–300 keV (ergs cm-2)

Relativistic jetsAnother field where the observation of cosmic neutrinos could lead to a breakthrough is related to relativistic jets. Relativistic jets are very powerful beams of plasma which emerge from the centres of some astrophysical objects. These jets can carry as much mass as the planet Jupiter and move at more than 99.9% of the speed of light. Relativistic jets have been observed in active galactic nuclei and micro quasars but it is not clear how they are produced. The formation of relativistic jets could be the key to explaining the production of gamma-ray bursts. Gamma-ray bursts were discovered in the late 1960s by the U.S. Vela nuclear test satellites. These satellites were built to detect gamma-radiation pulses emitted by possible secret nuclear weapons tests by the former U.S.S.R. after signing the nuclear test ban treaty in 1963. As one of the most energetic phenomena since the Big Bang, gamma-ray bursts are subject to many space- and ground-based observations. The early phase of gamma-ray bursts cannot be studied directly as no electromagnetic radiation can escape. Many models[13,14,15] predict the production of high-energy neutrinos that can escape the initial ‘fireball’. The detection of neutrinos from gamma-ray bursts will provide a unique image of the birth of relativistic jets.

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about six GRBs have occurred during the last decade that would have produced three (or more) detect-able events in the KM3NeT neutrino telescope. A comparison of the time profiles of the neutrino signal and the light curves will provide information on the creation and evolution of relativistic jets that are at the origin of all GRBs. Exotic particlesTheories aiming at the unification of the fundamental forces of nature are based on the hypothesis that at sufficiently high energies, the strong coupling and the electroweak coupling have the same magnitude. In this regime, a unique force should remain that results from a single symmetry. At lower energies, this symmetry is spontaneously broken into the apparent strong and electroweak symmetries as formulated in the Standard Model of particles and fields. It has been shown that the existence of magnetic monopoles appears as a generic prediction of this symmetry breaking[26,27]. The mass of such a monopole is related to the energy scale at which the unification occurs and the strength of the gauge coupling. It is commonly expected that the broken symmetry was restored in the early Universe. As the Universe expanded and cooled, phase transitions occurred that can be associated with the breakdown of symmetries. During these phase transitions, magnetic monopoles could have been created. The density of monopoles in the Universe that resulted from these phase transitions is uncertain. In fact, simple assumptions lead to an abundance of mo-nopoles that exceeds the total mass of the Universe. Many mechanisms have been proposed to overcome this problem[28,29,30]. In all cases, monopoles residing in the Universe will have been accelerated by large scale magnetic fields. An upper limit on the monopole flux can be estimated by assuming that all dark matter (see above) in the Universe can be attributed to monopoles. This upper limit results in a measurable rate provided that such monopoles can be detected efficiently in a very large volume[40].

Although a neutrino telescope is meant to detect the products of a neutrino interaction, it is also capable of detecting the passage of a magnetic monopole. When such a particle is in motion, its radial magnetic field will excite and ionise the atoms in the medium that it traverses[31]. Phenomena related to this are the direct emission of Cherenkov light and the produc-tion of knock-on electrons (δ-rays). Both effects will lead to a detectable signal in deep-sea neutrino telescopes. Detection of a magnetic monopole would be considered a major discovery. On the other hand, experimental limits on the flux of magnetic monopo-les will constrain or even reject some cosmological models and/or new theories. Due to the large size of the KM3NeT detector, the search for monopoles can be pursued with an unprecedented sensitivity.

Detection principleThe construction of a neutrino telescope is extremely challenging. Neutrinos are best known for their reluctance to be detected. In short, a systematic study of cosmic neutrinos requires a massive telescope with a size of at least one cubic kilometre. A solution to make such a large mass sensitive to neutrinos is to build a three-dimensional array of very sensitive light sensors in the sea[4]. Neutrinos can then be detected indirectly through the detection of the Cherenkov light emitted by charged particles emerging from a neutrino interaction (see Fig. 5). The transpar-ency of the water makes it possible to distribute the light sensors in a cost-effective way. The absorption length of the water (λabs) has been measured at selected sites in the Mediterranean Sea and was found to be about 50 metres.

The angular resolution of such a detector is limited by the lever arm between the light sensors and the measurement precision of their positions and the arrival times of the Cherenkov light. The mechani-cal structure that accommodates the light sensors

The quantum UniverseAt the interface between particle physics and astrophysics, several far-reaching questions persist which can be addressed with a neutrino telescope. These questions are motivated by the quest to explain the Universe in terms of quantum physics. Subtle asymmetries between particles and anti-particles –some of which have been observed experimentally– may explain the dominance of matter in the Universe. However, most of the matter in the Universe is ‘dark’: it appears not to consist of any known particle. But without dark matter, stars and galaxies would not have formed. The discovery that neutrinos have a non-zero mass could imply that they constitute the dark matter. But despite their omnipresence –there are almost as many neutrinos in the Universe as photons– their masses are simply too small to solve the dark matter mystery. An alternative candidate for dark matter is a new hypothetical particle: the neutralino. Neutralinos are weakly interacting with normal matter and therefore difficult to detect. But, if they exist, they will accumulate in massive celestial bodies, such as the Sun. There, they can annihilate and produce high-energy neutrinos. The observation of a neutralino signal at the Large Hadron Collider (LHC) at CERN[7] and with the KM3NeT neutrino telescope would represent a major breakthrough as it would establish a direct link between dark matter (astrophysics) and super-symmetry (particle physics).

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does not form a static system due to changing sea currents. Hence, their positions must be monitored continuously through acoustic triangulation. Of the three neutrino species that exist in nature, the muon neutrino yields the best angular resolution because the muon that emerges from a neutrino interac-tion has the longest range. The range of the muon increases linearly with its energy up to the so-called ‘critical energy’ of approximately 250 GeV. At this energy, the range is about one kilometre. Above this energy, Bremsstrahlung limits the increase of the muon range but does enhance the detectable signal. The light transmission properties of sea water com-bined with the presently feasible position (10 cm)

and time (1 ns) resolutions of the light sensors make it possible to reconstruct the direction of high-energy muons with an accuracy of about 0.2 degrees. This corresponds to about half of the apparent size of the Sun on the sky. Below the maximum penetration depth of daylight in the sea (about 1000 m), the light sensors can be operated 24 hours per day. Suitable light sensors are photo-multiplier tubes. Such photo-multiplier tubes housed in pressure resistant glass spheres form the basic building blocks of the detector (also referred to as optical modules). The huge mass needed for a neutrino to interact requires a large 3-dimensional array of such optical modules (about 10,000). The field of view of a neutrino telescope can in principle cover the full sky, but one must cope with the background from downward-going muons produced in cosmic ray interactions with the Earth’s atmosphere. The flux of these muons has been measured extensively and decreases with depth. At a depth of three kilometres, the flux of muons with energies in excess of 100 GeV is about 100 km–2s–1. A unique feature of a neutrino telescope is its ability to look downwards at neutrinos that traverse the Earth. The concept of a neutrino telescope is based on the detection of the products of a neutrino interaction that takes place in the vicinity of the detector. Hence, the sensitivity of a neutrino telescope depends on the interaction cross section of neutrinos and the effec-tive volume of the detector. This contrasts sharply with the traditional picture that the sensitivity of a telescope is only proportional to a surface area, e.g. that of a lens or a mirror. The equivalent expression for the surface area of a neutrino telescope will be given in the following. In general, the number of

NeutrinosIn a famous letter to colleagues, Wolfgang Pauli postulated in 1930 the existence of a new and invisible particle as a desperate remedy to account for the missing energy observed in radioactive decays. In the following years, Enrico Fermi took up Pauli’s idea and developed the theory of weak interactions. He assigned the name neutrino to the particle as a pun on neutrone, the Italian for neutron. (The neutron is the neutral companion of a proton; together they are the constituents of the nucleus of all atoms.) In his theory, neutrinos can interact with matter, but only very weakly. With the advent of nuclear reactors, a powerful source of neutrinos was at hand. This led to the first observation of neutrino interactions by Clyde Cowan and Frederick Reines in 1956. This result provided the experimental evidence for the existence of a neutrino.

In 1962, another kind of neutrino was discovered by detecting interactions of neutrinos produced with man-made particle accelerators. Experiments at the LEP collider at CERN, Geneva revealed in 1989 that there are three kinds of neutrinos. The first glimpse of the third kind of neutrino was caught in 2000 at Fermilab near Chicago. The three different kinds in which a neutrino may appear are usually referred to as neutrino flavours. Each neutrino is created with a well defined flavour (electron, muon or tau). However, in a phenomenon known as neutrino flavour oscillations, neutrinos are able to oscillate between the three available flavours while they propagate through space. The existence of flavour oscillations implies a non-zero neutrino mass. Despite their massive nature, it is not yet clear whether the neutrino and the antineu-trino are in fact the same particle, a hypothesis first proposed by Majorana in 1937.

muon

~ 1 km

1–2 λabs

wavefront

Figure 5: Schematic view of the production of Cherenkov light by a muon. (A muon is one of the possible products of a neutrino interaction.) According to the principle of Huygens, spherical light waves are produced along the muon trajectory. The lightwaves interfere because the muon travels faster through the water than the light. As a result, a sharp wave front is formed that can be detected with very sensitive light sensors.

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Figure 6: Schematic view of an interaction of a muon neutrino, νµ. A W boson is exchanged between the neutrino and a nucleon, transforming the neutrino into a muon. The angle θ is defined as the angle between the incident neutrino direction and the muon direction.

νμ θ

muon

other particlesnucleon

W

detected neutrinos per unit time as a function of the neutrino energy, E

ν, can be expressed as:

1: dN(Eν ) = Φ(E

ν )×σ(E

ν )×NA×ρ×Veff (Eν

)×dEν ,

where Φ(Eν ) is the incident neutrino flux, σ(E

ν ) the

neutrino cross section, NA Avogadro’s number, ρ the density of the medium (i.e. water), and Veff (Eν

) the effective detection volume. In this equation, the dependence of the cross section and the effec-tive volume on the kinematics of the interaction is implicitly integrated. The cross section is known to increase with the energy of the neutrino; up to about 10 TeV this increase is linear. The effective volume is defined as the volume in which a neutrino inter-action produces a detectable muon, and basically indicates the size of the telescope.

Equation 1 can be rewritten as:

2: dN(Eν ) = Φ(E

ν )×Aeff (Eν

)×dEν ,

where Aeff (Eν ) is defined as the neutrino effective

area. In this definition, the neutrino effective area includes both the neutrino cross section and the effective volume. As a result, the sensitivity of a neutrino telescope can be directly related to a surface area. Due to the small cross section for a neutrino in-teraction with matter, this area is much smaller than the geometrical cross section of the instrumented volume. As both the neutrino cross section and to some extent also the effective volume increase with the energy of the neutrino, so does the effective area. This is an important benefit as the signal-to-back-ground ratio is also expected to increase with energy. In conclusion, a huge instrumented volume (1 km3) is needed to achieve a reasonably large effective area (100 m2) at the highest energies.

As the neutrinos are detected indirectly, it is impor-tant to know the correlation between the direction of the incident neutrino and the muon (see Fig. 6). The neutrino interaction proceeds through the exchange of a W or Z boson, referred to as a charged and neu-tral current interaction, respectively. For a neutrino energy above 0.01 TeV, the interaction is dominantly deep inelastic, i.e. a nucleon in the target breaks up into low energy particles (mainly pions). A muon is produced in the charged current interaction of a muon neutrino. The energy of the muon (E

µ ) is on average

half of that of the neutrino. As long as the neutrino energy is below 10 TeV, the value of Q2 (–Q2 is the four-momentum transfer squared) is small compared to the square of the W and Z mass. The cross sec-tion is then proportional to the neutrino energy. The higher the neutrino energy, the smaller the value of Björken x at a given Q2 (x can be interpreted as the momentum fraction of the nucleon carried by the struck quark). Measurements at the HERA collider (DESY, Germany) determined the quark density in the proton down to very small values of x. The results show that this density increases strongly with decreasing x. Since Q2 ≃ 4E

νEµ sin2 (θ / 2) ≤ 2MNE

ν(MN is the nucleon mass) and 〈Eµ 〉 = ½E

ν , the

Neutrino telescopeThe neutrino is of great scientific interest because it can make an exceptional probe for places in the Universe that are otherwise concealed. An early fulfilment of this perspective has been the observation of neutrinos from the core of the Sun. Direct optical observation of the solar core is impossible due to the diffusion of the electromagnetic radiation by the huge amount of matter surrounding the core. While photons may require thousands of years to get to the outer layers of the Sun, neutrinos generated in nuclear fusion reactions escape the solar core unperturbed in a second or two. Neutrinos are also useful to probe astrophysical sources beyond our solar system. A prime example is the observation of neutrinos from a supernova. A supernova may briefly out-shine its entire host galaxy inferring that all its energy is released in the form of electromagnetic radiation. The observation of neutrinos from the supernova SN1987a revealed that most of the energy is actually released in the form of neutrinos and not light.

The next target in the list of neutrino sources is the centre of our galaxy which is known to host many powerful emitters of high-energy radiation. It is likely that neutrinos produced in the galactic centre will be observed for the first time by large Earth-based neutrino telescopes, such as KM3NeT, in the next decade.

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average scattering angle θ reduces with increasing neutrino energy. Both the enhanced contribution of small x and the effect of the W mass reduce the increase of the average Q2 with increasing neutrino energy. Above 1 TeV, a safe limit on the average scattering angle is:

3: 〈θ〉 ≤ 1.5 deg / √Eν [TeV] ,

i.e. the direction of the muon is closely related to that of the neutrino, which is essential for the neutrino telescope concept.

Even in the absence of daylight, a ubiquitous back-ground luminosity is present in deep-sea water due to decays of radioactive isotopes and bioluminescence. This background luminosity produces a relatively high count rate of random signals in the light sen-sors of the detector. Consequently, the data rate of a deep-sea neutrino telescope exceeds any data storage capacity by several orders of magnitude. The rare neutrino signal can be discriminated from the random background utilising the time-position correlations produced by a traversing particle. The main difficulty is the real-time filtering of the neutrino signal from the continuous random background. As a conse-quence, the effective area of a neutrino telescope depends also on the quality of the data filter.

Above a certain energy (1000 TeV), the neutrino cross section has grown to such an extent that the Earth becomes opaque. In this regime, the neutrino has a significant probability to interact with the Earth before it reaches the detector. Hence, ultra-high energy neutrinos can only be detected if the tel-escope is also sensitive to downward going muons. In that case, one has to cope with the abundance of atmospheric muons. The atmospheric muons,

however, are decay products of short lived particles (mainly pions and K mesons) that are produced in cosmic ray interactions with the Earth’s atmos-phere. The higher the energy of these particles, the longer they live and the farther they travel. It thus becomes more and more likely that these particles re-interact with the Earth’s atmosphere (or hit the Earth) before they decay. Consequently, the energy spectrum of atmospheric muons is steeper than the original cosmic ray spectrum, leading to an effective reduction of the background at high energy. The background is further reduced by limiting the field of view of the neutrino telescope to angles close to the

Is there an end to the cosmic ray spectrum?The observations of cosmic rays with energies in excess of 1020 eV are a mystery as they should have interacted with photons from the cosmic microwave background radiation before reaching the Earth. This is known as the GZK[23,24] cut-off. Some scenarios to explain the paradox involve very massive particles (about 1022 eV/c2) which can decay nearby and produce the observed cos-mic rays. Recent observations of the Pierre Auger observatory in Argentina support, however, the existence of the anticipated cut-off[25]. Moreover, a correlation of the highest-energy cosmic rays with nearby extragalactic objects has been reported a few months ago[41]. If all the high-energy cosmic rays from far away sources are absorbed, plenty of neutrinos are produced (every lost proton yields a neutrino). One of the unique features of the neutrino is that it is not sensitive to the GZK cut-off. This leads to a detectable neutrino flux on Earth, provided that the detector is large enough. KM3NeT will be the largest neutrino telescope in the world, thus providing the best chance to find back the missing cosmic rays.

Figure 7: Schematic view of the ‘All-data-to-shore’ concept and the link to the Swift satellite that is part of the GRB warning system. The deep-sea neutrino tel-escope can be operated from anywhere in the world. The neutrino sig-nal is filtered on shore by a farm of PCs and distributed real time.

GRBwarning

data transmissionsoftware �lter

real timeanalysis

remoteoperation

… .… .... …… .... .. ……

15

Cosmic neutrinosThe only cosmic neutrinos that have been detected so far are neutrinos from the Sun and from the supernova SN1987a. The detection of solar neutrinos not only confirmed the solar model[32], it unexpectedly led to the discovery of neutrino oscillations[33]. The neutrino signal from the supernova SN1987a was originally missed. It was only after the suggestion of the late J. Bahcall that a significant increase in the noise rate of the photo-multiplier tubes in the Kamiokande detector was noticed[34]. The neutrino signal appeared only a few minutes after a calibration run which would have wiped out the signal completely. A more detailed offline analysis of the data showed a dozen events that were coincident with the observed supernova explosion. This implied that most of the energy of the supernova explosion was released in the form of neutrinos and not light. It is as yet the one-and-only example of a time coincidence between a light signal and a neutrino signal from a single as-trophysical source. The cosmic neutrinos detected so far have energies of the order of a few MeV. They are not the subject of this proposal. The angular resolution of a neutrino telescope would then be limited to 10 degrees or so. With the advent of a new generation of high-energy neutrino telescopes, the study of cosmic neutrinos with an angular resolution of about 0.2 degrees becomes possible. The realisation of this perspective started with the successful commissioning and operation of the prototype detector built by the Antares collaboration.

horizon. Within this field of view, it will be possible to detect ultra-high energy cosmic neutrinos despite the background of atmospheric muons.

In the study of cosmic neutrinos, there is a small but inevitable background due to the neutrinos that come together with the atmospheric muons (the pions and K mesons decay into a muon and a neutrino). For point sources, this background scales with the square of the angular resolution of the telescope and with the measurement time. For transient sources like gamma-ray bursts, the net result is an almost background-free data sample. For steady sources, the number of neutrinos needed to claim a discovery depends on the location of the source on the sky and the energy of the neutrinos. For an observation period of 3 years, it typically ranges between 5–10 events for a confidence level of 99%.

Development planAs a first step towards a large neutrino telescope, the Antares collaboration has built a prototype detector. Major contributions of the principal investigator (PI) to this project include the novel ‘All-data-to-shore’ readout system (see Fig. 7). In this system, the rare neutrino signal is filtered on shore from the back-ground using a farm of commodity PCs and state-of-the-art software (the signal-to-noise ratio is about 10–8 at the primary light sensor). The software devel-opments have been realised by the PI. This software is now fully operational in the Antares experiment. This achievement is remarkable (few considered this possible) and changed the picture dramatically: the readout system has become a seminal part of the astro-particle physics programme with a neutrino telescope. It is obvious - but worth noting - that the faster the software data filter is, the more physics can be done with the same instrument.

The Antares neutrino telescope is the first neutrino telescope that uses the earlier mentioned GRB

warning systems in the manner for which they are meant[35]. As all raw data are sent to shore, they can be buffered before being processed. In case of a GRB alert, the data in memory and all data taken during and following the burst can be saved on disk and analysed offline. In this way, one can look before, during and after the GRB for a correlated neutrino signal. This offers a unique opportunity to study the early phase of a GRB with the best pos-sible sensitivity.

A further development made recently is a direction sensitive data filter. If the direction of the incident neutrino is known, one can in principle lower the detection threshold. This idea is virtually impossible to implement in hardware. A software solution has been implemented by the PI for the Antares detector. This achievement shows that one can track an astro-physical source and thus detect a neutrino signal with the best possible sensitivity.

The concept of a software based data filter has been established for a small neutrino telescope. For the much larger KM3NeT neutrino telescope, massive parallel computing will be necessary. The anticipated data throughput poses tough requirements on the performance of the system. Therefore, simulations have to be made to study the data traffic through the system. The rare neutrino signal needs to be filtered real-time. Hence, all processes in the system have to keep up with the high input rate. New pattern recognition algorithms need to be developed to make this possible. The results of these developments will determine the success of the entire project. Analysis of the data will start as soon as the first part of the KM3NeT detector is deployed. The foreseen analy-ses will be focused on neutrino-point sources and gamma-ray bursts. The sensitivity of the KM3NeT telescope to these potential neutrino sources will be greatly enhanced with the data-filter system devel-oped in the framework of this proposal. Hence, it is

16Figure 8: Time lines of the development plan of this proposal and the KM3NeT project.

expected that these analyses will yield results within the time lines of this proposal. With these analyses, one can become the first person to witness the birth of a relativistic jet and to unravel the mystery of particle acceleration in the cosmos.

Another key issue in neutrino astronomy is the accuracy with which the direction of the neutrino can be determined. Due to the nature of the Cherenkov light and the sparse distribution of light sensors, this problem is highly nonlinear. In the classical approach, only one solution is searched for using a general op-timisation procedure. Such a procedure could lead to one of the degenerate solutions that is inherent to the nonlinearity of the problem. The net result is a loss of efficiency. It turns out that the algorithms used in the data-filter software mentioned above can also be used to project the full parameter space of the problem onto a sub-space in which the problem is (almost) linear. All solutions can then be found by a systematic scan of a minimal set of possible projections. The final result is determined by the selection of the best solution from all solutions found. In addition, a per-sistent ambiguity can be treated properly by keeping all corresponding solutions. This approach has never been considered before but preliminary studies show that it is very promising.

With this proposal, the PI will play a major role in the scientific capitalization of the KM3NeT neutrino telescope. This will position the PI and his group at the forefront of astro-particle physics for the foreseeable future.

BudgetThe main deliverable of this proposal is the data-filter system for the KM3NeT neutrino telescope. With this system it is possible to make the envisaged scientific studies as soon as the telescope is opera-tional and obtain results within the foreseen time lines.

The total budget is 2.5 M€ for a period of five years. A breakdown of the budget is given in Table 1. The two post-docs will make the simulations and develop the required software. The two PhD students will make the physics performance studies and do the data analyses. The obtained results will be presented at (international) conferences and published in refereed journals. The item ‘data-filter system’ includes cost of prototyping, purchase of hardware (excluding VAT), transport and installation. The item ‘running costs’ includes power, cooling and maintenance, for an assumed period of 4 years. The required computing power has been evaluated for the complete KM3NeT detector, taking into account the results of the software developments which are part of this proposal. The item ‘miscellaneous’ covers the general running cost of this project. The overall costs of the data-filter system includes the purchase and the operation during the period covered by this proposal.

Table 1: Budget break-down in k€ for the entire period covered by this proposal.

Item description cost k€

i) personnelpost-doc A (4 years) 240post-doc B (4 years) 240PhD student A (4 years) 160PhD student B (4 years) 160travel 85overhead 160ii) data-filter systemcomputer farm 820Ethernet switch fabric 130running costs 420transport/installation 25iii) miscellaneousoffice equipment 50software licenses 10Total 2,500

ID Task Name1 KM3NeT FP6 design study2 Conceptual design3 Conceptual Design Report4 Technical design5 Technical Design Report6 KM3NeT FP7 preparatory phase7 Production modelling8 Market analysis9 Financial analysis

10 Human resources analysis11 Funding model12 This proposal13 Intended starting date14 Design & development15 Purchase & testing16 Installation17 Data analyses18 KM3NeT19 Construction20 Operation

30/11

25/05

16/02

01/07

'05 '06 '07 '08 '09 '10 '11 '12 '13 '14 '15 '16 '17 '18 '19 '20 '21

17

Reasons for the proposed investmentThe scientific case for studying neutrinos from the cosmos is compelling. Detection of these neutrinos, however, remains extremely challenging. Following a sequence of pioneering projects of which Antares was one, an international joint venture was initi-ated to build a large deep-sea infrastructure in the Mediterranean Sea. This infrastructure will accommo-date the next generation neutrino telescope. KM3NeT is supported through the 6th and 7th framework programmes of the EU. This will culminate in a tech-nical design report by the year 2009 and a Europe-wide funding model by 2010. The time lines of the KM3NeT project and the present proposal are shown in Fig. 8. The installation of the computing hardware proceeds in two steps to comply with the actual construction of the KM3NeT neutrino telescope.

The sheer size of the detector, the large number of optical modules, the unavoidable background and the rare neutrino signal makes the filtering of the data a major challenge. This proposal aims at optimis-ing the signal-detection efficiency by using a large farm of standard PCs and state-of-the-art software. The necessary software does not exist and is not commercially available. Although the project has a high risk (is it possible to process data at a rate of 100-1000 Gb/s and detect a signal at a 10–8 level?) the prospects are favourable. A proof of concept has been established in the framework of the Antares prototype project. At this point, we propose to capi-talize on our experience gained within the Antares project and set our ambitions to the next and highest level: build the hardware and develop the software

Galactic longitude-150 -100 -50 0 50 100 150

Gal

actic

latit

ude

-80

-60

-40

-20

0

20

40

60

80

ϒ180

ϒ90

Figure 9: Simulated sky map of neutrinos detected by the KM3NeT neutrino telescope over a ten year period. The simulation is based on the known diffuse background of atmospheric neutrinos and includes the measurements of the HESS experiment. Clusters of neutrinos are clearly visible in the galactic plane (horizontal line) and correspond to the HESS measurements.

for the data-filter system of the the largest neutrino telescope in the world. In view of the time lines shown in Fig. 8 –the construction of the KM3NeT neutrino telescope should start in 2011– this proposal is urgent. This planning makes it possible to profit maximally from the experience gained with the Antares project and to be ready in time for the opera-tion of the KM3NeT neutrino telescope.

The unique feature of this proposal is that for a moderate fraction of the total cost of the KM3NeT neutrino telescope, a key component can be ac-complished. The cost of this project includes the purchase of the hardware as well as the development of the software. These costs represent a significant fraction of the total budget. It should be noted, however, that the idea of transferring all data to shore flies in the face of conventional wisdom which often dictates that the volume of data be reduced as quickly as possible. The present proposal has in this sense no challenger. Hence there are no other budget requests for the same purpose. The data processing on shore, where all signals can be searched simulta-neously, is challenging and yet has the potential of great rewards.

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National importanceNeutrino astronomy commenced in the Netherlands in the year 2000 when Nikhef –the Dutch national institute for subatomic physics– joined Antares, the international effort to construct a prototype neutrino telescope. Within the Antares project, the PI has the full responsibility for the data acquisition system and the coordination of the physics working groups. Since 2005 the PI is the Deputy Spokesperson of Antares. The participation of Nikhef in Antares has been financed by the Dutch general scientific funding agency NWO through an investment grant of 3.6 M€, and the Dutch physics funding agency FOM through a six-year scientific programme (6.7 M€). The involve-ment of Nikhef in Antares has paved the way for the participation of the Dutch astro-particle physics com-munity in the KM3NeT project. Within the KM3NeT project the PI is the coordinator of the ‘information technology’ work package, which covers the comput-ing infrastructure described in this proposal.

In the ‘Strategic Plan for Astro-particle Physics in the Netherlands’[36], written in 2005, searches for neutrino point sources with Antares (as a first generation neu-trino telescope) and KM3NeT (as the next generation neutrino telescope) were selected as key projects for the development of this field in the Netherlands.

International positionThe KM3NeT neutrino telescope is one of the major ambitions of the European astro-particle physics community, as evidenced by the roadmap for astro-particle physics, which is presently being prepared by the FP6-funded ERA-NET project ASPERA[37]. ASPERA, in which FOM is one of the leading partners, aims at identifying and removing financial and organizational barriers that could prevent the realization of large-scale European infrastructures in the field of astro-particle physics. The KM3NeT neu-trino telescope is one of the major highways on the ASPERA roadmap. Recently the European Strategic Forum for Research Infrastructures (ESFRI) pub-lished a list of the most important large-scale infra-structures that should be built in the next decade. KM3NeT appears on this list confirming the strong level of support for the project Europe-wide.

More than 200 scientists from 9 different countries collaborate in the European KM3NeT project. In

2006 this collaboration received funding from the 6th

Framework Programme of the EU for a design study. About 9 M€ was awarded to the KM3NeT design study, 0.65 M€ was allocated to the Netherlands. The same collaboration submitted a proposal for a Preparatory Phase Study in the 7th Framework Programme of the EU, which was approved in August 2007. This type of support is only available for projects that appear on the ESFRI list. The KM3NeT collaboration requested 6.8 M€, of which 5.0 M€ was granted. About 10% is earmarked for the Netherlands.

Existing underground neutrino detectors, such as Super-Kamiokande[38], are optimised for the detec-tion of lower energy neutrinos, typically in the range 106–1010 eV. High-energy neutrino telescopes are radically different; their architecture is optimised to achieve a large detection volume and a good angular resolution. Worldwide, several large under-water or -ice neutrino telescope projects exist in various stag-es of development at the present time. The Amanda collaboration has built a 0.05 km3 size neutrino telescope in the ice cap of the South Pole. Due to the light scattering in the ice, the angular resolution of this telescope is limited to about 1 degree. The U.S. led IceCube collaboration has started building a large neutrino telescope around the Amanda detector[39].

The KM3NeT collaboration plans to build a neutrino telescope in the Mediterranean Sea. Many of the intricate technologies required for the operation of a large detector in the deep-sea were developed by the Antares project. The analysis of the first data taken has shown that the light scattering in the sea water is as small as expected and that the absorption length is large. This implies that the angular resolution of the neutrino telescope will be better than 0.2 degrees. In view of the competition with IceCube and the time needed to build such a large deep-sea infrastructure, this proposal is timely. The present proposal is expected to start mid 2009 and will take five years to completion. The KM3NeT neutrino telescope will complement IceCube in the sense that the telescope will cover a different part of the sky, most notably the centre of the galactic disk. The superior angular resolution of KM3NeT will yield the better back-ground rejection. After its completion, KM3NeT will be the most sensitive neutrino telescope in the world.

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References

1 http://www.mpi-hd.mpg.de/hfm/HESS2 F.W. Stecker and M.H. Salamon, Space Sci. Rev. 75 (1996) 341.3 http://icrhp9.icrr.u-tokyo.ac.jp/4 M.A. Markov, Proc. Of the Annual Int. Conf. On High Energy Physics, Rochester (1960).5 J. Ahrens et al., Phys. Rev. Lett. 92 (2004) 071102.6 A. Kappes, J. Hinton, C. Stegmann and F.A. Aharonian, J. Phys. Conf. Ser. 60 (2007) 243.7 http://www.cern.ch/8 J.C. Mather et al., Astrophys. J. 420 (1994) 439.9 M.R. Nolta et al., Astrophys. J. 608 (2004) 10.10 R. Barnabei et al., Riv. N. Cim. 26 (2003) 1.11 D.S. Akerib et al., Phys. Rev. Lett. 93 (2004) 211301.12 A. Achterberg et al., Astropart.Phys. 26 (2006) 129.13 E. Waxmann and J. Bahcall, Phys. Rev. Lett. 78 (1997) 2292.14 S. Razzaque et al., Phys. Rev. Lett. 90 (2003) 241103.15 A. Dar and A. De Rújula, astro-ph/0105094.16 J. van Paradijs et al., Nature 386 (1997) 686.17 M.J. Rees and P. Mészáros, Mon. Not. R. Astron. Soc. 258 (1992) L41.18 J.E. Rhoads, Astrophys. J. 487 (1992) L1.19 M.J. Rees and P. Mészáros, Astrophys. J. 430 (1994) L93.20 P. Mészáros and M.J. Rees, Astrophys. J. 405 (1993) 278.21 N. Gehrels et al., Scient. Am. 269 (1993) 68.22 http://heasarc.nasa.gov/docs/swift/swiftsc.html23 K. Greisen, Phys. Rev. Lett. (1966) 748.24 G.T. Zatsepin and V.A. Kuz’min, JETP Lett. (1966) 78.25 http://www.icrc2007.unam.mx/26 G. ’t Hooft, Nucl. Phys. B79 (1974) 276.27 A.M. Polyakov, JETP Lett. 20 (1974) 194.28 A.H. Guth and E.J. Weinberg, Nucl. Phys. B212 (1996) 321.29 T.W. Kephart and T.J. Weiler, Astropart. Phys. 4 (1996) 271.30 E. Huguet and P. Peter, Astropart. Phys. 12 (2000) 277.31 S.P. Ahlen, Phys. Rev. D17 (1978) 229.32 J.N. Bahcall and R. Davis, Science 191 (1976) 1494.33 Q. R. Ahmad et al., Phys. Rev. Lett. 89 (2002) 011301.34 K. Hirata et al., Phys. Rev. Lett. 58 (1987) 1490.35 M.C. Bouwhuis, PhD thesis UvA, Detection of Neutrinos from Gamma-Ray Bursts (2005).36 http://www.astroparticlephysics.nl/papers.php37 http://www.aspera-eu.org/38 Y. Fukuda et al., Phys. Rev. Let. 81 (1998) 1562.39 A. Achterberg et al., Astropart. Phys. 26 (2006) 155.40 B.A.P. van Rens, PhD thesis UvA, Detection of Magnetic Monopoles below the Cherenkov Limit (2006).41 The Pierre Auger collaboration, Science 318 (2007) 938.

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Section 3: Research Environment

The National institute for subatomic physics Nikhef –a joint venture of the Dutch funding agency FOM and four universities– coordinates experimental subatomic physics in the Netherlands. As such, Nikhef forms the home base for Dutch experimental research and detector developments in this field worldwide. Nikhef is host to about 225 persons, including about 60 fte permanent scientific staff, 60 fte PhD students and 15 fte post-docs. The technical support is provided through an electronics technology department (about 40 fte), a mechanical engineering department (about 12 fte), a mechanics workshop (about 25 fte), and a computer technology department (about 20 fte). In addition, there is a large ongoing GRID activity (about 10 fte) at Nikhef which has been recently awarded 29 M€ to the BIG GRID infrastructure proposal.

In the Nikhef ‘Strategic Plan 2007–2012’*, astro–particle physics was identified as the major research prospect next to LHC. The relative budget for astro–particle physics will grow to approximately 30% by the year 2015. The strategic plan has been evaluated recently by an international panel under auspices of the Dutch funding agency NWO. I quote from the report of this panel: “There is no doubt that Nikhef is one of the leading laboratories in experimental par-ticle physics in the world, with an outstanding record of achievement in detector and electronics design, construction and commissioning, physics analysis and advanced computing techniques, supported by a strong phenomenology group.” and “Nikhef has very quickly established itself in the relatively new field of astro-particle physics through its participation in the Antares project, where Nikhef made an immediate and major impact, the benefits of which are just now being realised.” In order to enable the astro-particle physics programme to develop so that Nikhef can become a leader in this emerging field, building upon its established reputation in particle physics and its achievement in Antares, the panel strongly endorsed the request for an increase in the budget. The overall assessment of the institute by this panel was excel-lent and has led to a funding increase of the institute.

Nikhef has a long-standing tradition to involve, wherever possible, Dutch industry in the research and development for new facilities and detectors. Examples are the production of magnet systems for the HERA collider at DESY, the development of very large (10 meter in diameter) superconducting magnets for the ATLAS detector at CERN and the MEDIPIX project. For the Antares project, the sub-marine power system was developed and produced

in the Netherlands. The fibre-optic data transmission hardware was developed in close collaboration with Dutch industry. Similar opportunities for Dutch industry will arise in the framework of KM3NeT.Neutrino astro–particle physics commenced in the Netherlands in the year 2000 when, under the leadership of the principal investigator, physicists from Nikhef joined the development and construc-tion efforts of Antares. These efforts have led to the first fully operational neutrino telescope in the Mediterranean Sea. Building on the success of Antares, it is now possible to consider the construc-tion of the next generation neutrino telescope that is 20–50 times more sensitive: KM3NeT. The challenge of this project is to develop techniques that have a substantially improved performance as compared to the presently employed techniques, but at a significantly reduced cost. Nikhef has put forward a number of innovative designs and has the ambition to build one of the assembly sites. In the longer term, Nikhef will become a major analysis centre for KM3NeT.

The present proposal will greatly profit from the excellent infrastructure at Nikhef. There will be strong support from the computer technology department which will enhance the chances of success of this proposal. There is already a team of scientists actively involved in the analyses of the data obtained with the prototype Antares detector. As these scientists are literally next door, there will be plenty of opportunity for fruitful exchanges of ideas. Last but not least, a significant Dutch contribution to the construction of the KM3NeT neutrino telescope is foreseen, for which an investment proposal has been submitted. As the construction efforts will be concentrated at Nikhef, good knowledge of the future detector is at hand. This knowledge is a key input to the optimisation of the data-filter system for the KM3NeT neutrino telescope, which is the prime deliverable of this proposal.

Whereas the scientific challenge of the present pro-posal forms the main reason for the funding request, there are additional strategic advantages. It permits the widening of the scientific scope of the institute. The proposed research provides a natural extension of the scientific programme of Nikhef and will form a bridge between particle physics and astrophysics. This proposal will link a variety of fields, including astronomy, dark matter searches, cosmic ray and high-energy physics, increasing the chances of a major discovery.

*K. Huyser et al. FOM publication FOM-07.0927 (2007).