ELSEVIER Nuclear Physics B (Proc. Suppl.) 118 (2003) 383-387

Neutrino Telescopes in the Mediterranean John Carr a

a Centre de Physique de Particules de Marseille, 163 Avenue de Luminy, Case 907, 13288 Marseille, Prance. E-mail: carrQcppm.in2p3.fr

This review presents the scientific objectives and status of Neutrino Telescope Projects under construction in the Mediterranean Sea. The science program of these projects covers: neutrino astronomy, dark matter searches and measurements of neutrino oscillations. The status of the two related water Cherenkov projects ANTARES and NEMO will be described together with the status of the NESTOR telescope. A comparison between underwater and under-ice neutrino telescopes will be given.

1. Introduction

Among the possible techniques to detect high energy neutrinos from outer space, the most widely exploited method is the detection, in large water of ice volumes, of Cherenkov light from the muons and hadrons produced in the inter- actions. Water Cherenkov detectors (e.g. IMB, Kamiokande, Super-Kamiokande and SNO) are the only detectors so far to have observed neu- trinos produced beyond the earth, these obser- vation being of lo6 to 10’ eV neutrinos pro-? duced in the sun and supernova 1987a. The present review concentrates of the large water detectors aimed at detection of neutrinos in the 10” to 1015 eV energy range. In the 1980s there were two pioneering underwater neutrino tele- scope projects: DUMAND and BAIKAL. The DUMAND project was cancelled in 1995 after problems related to the deep-sea environment while BAIKAL continued to make a successfully operation telescope in Lake Baikal in Siberia. The under-ice project AMANDA has constructed a neutrino telescope in the glacial ice at the South Pole, which has been operating since 1997 in var- ious forms. Prom the early 1990s a number of groups have launched projects to build neutrino telescopes in the Mediterranean Sea.

The major scientific objective of the neutrino telescopes is the discovery and understanding of

the sites of acceleration of high-energy particles in the universe. Since their original discovery by V. Hess one hundred years ago the origin of the high flux of charged cosmic ray arriving at the earth is unknown. Neutrinos give a unique possibility to trace cosmic rays back to their origin since being electric charge neutral they are unperturbed by magnetic fields and being weakly interacting they can pass through dense dust clouds which might surround the sources.

An important secondary objective of neutrino telescopes is the search for dark matter in the form of neutralinos. In supersymmetric theories with R-parity conservation, the relic neutralinos from in the big-bang would concentrate in mas- sive bodies such as the centres earth, sun and galaxy. In these sites neutralino annihilations and the subsequent decays of the resulting particles would yield neutrinos detectable in neutrino tele- scopes of the scale currently in operation and be- ing constructed.

Further objectives of some of the projects in- clude the measurement of oscillations with atmo- spheric neutrinos, which is possible with the same:

detectors, in the range of oscillation parameters indicated by the Super-Kamiokande experiment. Ancillary science possible with the experiments cover oceanography, seismology and biology for the sea-based experiments and various other en- vironmental science subjects for the ice and lake

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384 1 Carr/Nuclear Physics B (Proc. Suppl.) 118 (2003) 383-387

experiments.

2. Cosmic Sources of High Energy Neutri- nos

The origin of the bulk of the high-energy cosmic rays observed on Earth is at present largely un- known. It is expected that the majority of cosmic rays with energies below about 1018eV have their origin in our own galaxy while those at higher energies come from extragalactic sources. High- energy gamma rays have been observed from nu- merous sources and it would be natural to expect charged cosmic rays also to originate from these. However, for many sources observed in gamma rays, it is uncertain whether the primary acceler- ated particles are hadrons or electrons and the ob- servation of neutrinos would clearly demonstrate the existence of accelerated hadrons.

In the galaxy it is Supernova Remnants (SNR) which are most popularly predicted to be the source of charged cosmic rays. A supernova rem- nant comprises a shell of matter, emitted after a supernova explosion, which continues to ex- pand at speeds of typically a few tenths of the speed of light for thousands of years. The cat- alogue of Green lists over two hundred galactic supernova remnants of which several correspond to optically observed supernova (SN). A handful of SNRs have central pulsars and are known as plerions: the most famous being the Crab Neb- ula (SN1054). Some of these sources are known to be powerful emitters of Tev gamma rays but shell SNR, without a central pulsar, are less easily vis- ible in TeV gammas. Recently, there has been on observation of the SNR RX 51713.7-3946 by the Gamma Ray Cherenkov telescope CANGAROO [l] in which the measured energy spectrum of the gamma rays is interpreted as evidence for the ac- celeration of cosmic ray protons in the source. This data for the first evidence of a cosmic ray source is still controversial [2], however if inter- preted es such the expected rate of neutrinos is of the order of 40 events/km2 /year [3].

Active Galactic Nuclei (AGN) are also known sources of TeV gamma rays. These objects, where jets of matter are emitted from the galaxy nu- cleus, are possibly a stage in the evolution of

the majority of galaxies. The distribution of AGNs peaks at red shifts around 2, distances around 10 Gpcs, with the closest observed in TeV gamma around 100 Mpcs. Existing data on energy spectra of gamma ray observation can be explained using acceleration models with only electrons. Observations of neutrinos from AGNs would demonstrate the presence of hadronic ac- celeration also in these sources.

Microquasars are thought to have the structure of a small scale AGN. Since 1992 about a dozen microquasars have been observed in the galaxy [4]. Multi wavelength observations support the model of microquasars se black holes of a few so- lar masses surrounded by an accretion disk fed from a companion star. The episodes of emis- sion of high energy radiation, seen as separating blobs in radio telescope images, are explained as being due to instabilities in the accretion disk where the inner few hundred kilometres of ma terial falls into the central black hole, with some fraction of this material being ejected in back- to-back jets. Calculations of fluxes of neutrinos from microquasars give easily detectable fluxes in the neutrino telescopes in construction [5], e.g. in ANTARES 6.5 and 4.3 events/year from the mi- croquasars GX339-4 and SS433 respectively with a background from atmospheric neutrinos of 0.3 events/year in a lo cone around the source.

Gamma Ray Bursts (GRB) are energetic sources observed to emit short bursts of gam- mas in the energy range of a few hundred MeV with burst durations between looms and 100s. When it was operational the BATSE detector on the Compton Gamma Ray Observatory ob- served l-2 events per day. The distribution of the BATSE observed GRBs, is uniform in galactic co- ordinates giving an indication of extragalactic ori- gin. For about 20 GRBs with long burst duration, the redshift of the after glow has been observed and all are measured to be extragalactic. Many theories exist for the nature of gamma ray bursts and more data is needed to distinguish between them with neutrino observations again giving vi- tal information to distinguish between models.

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3. Search for Dark Matter

The present knowledge of the composition of matter in the universe comes from observations of galaxy clusters, cosmic microwave background radiation, supernovae type la and big bang nu- cleosynthesis. In the current picture [6], the total amount of matter is close to the critical density for a flat universe with Ototal - 1 and the fraction of matter is 30% with the rest being the, as yet, little understood dark energy. The matter contri- bution consists of baryonic matter with Rb - 0.04 and cold dark matter with RCDM - 0.26. A de- tailed review of searches for dark matter is given by L. Bergstrom in reference [7]. Neutrino Tele- scopes can perform indirect searches for Dark Matter, in the form of neutralinos concentrated at the centres of massive celestial objects, by searches for the neutrinos emanating from anni- hilation reactions.

While laboratory direct dark matter search ex- periments are sensitive to any particle capable of causing a nuclear recoil, the indirect searches possible with neutrino telescopes require a spe- cific model as to the nature of the dark mat- ter candidate in order to predict the expected rates in an experiment. Generally supersymmet- ric models are applied with the neutralino as the dark matter candidate particle and in the MSSM the neutralino-nucleon cross-sections can be pre- dicted for given model parameters. These models also predict the annihilation cross-sections of neu- tralinos and so the rates in the various indirect searches for dark matter. Two different searches are made for neutralino annihilations; the first, annihilations occurring in the galactic halo giv- ing gamma rays of unique energies in reaction like xx --f Zy, yy and the second, annihilations in re- gions of concentrations of neutralinos in massive bodies in reactions such as xx + WW, ff with W or f decaying to neutrinos. The searches for gamma ray lines from annihilations in the halo are performed in satellite and ground based gamma ray telescopes. In these experiments the gamma ray line energy would be directly related to the neutralino mass and give a very clear signature. The neutrino telescopes search for the neutrino decay products in the annihilations in massive

bodies. Concentration of neutralinos in massive bodies such as the Earth, Sun and Galactic Cen- tre would build up since the early universe where the neutralino dark matter would naturally be a fossil of the big bang similar to the 3K relic pho- tons. Given the matter density in the various bodies and the total dark matter content in the halo, calculations can be made as a function of MSSM parameters for the rates to be expected in the current and future experiments.

4. Operating Neutrino Telescopes

There are currently two operating neutrino telescopes: BAIKAL [8] at a depth of 1200 m in the water of Lake Baikal in Siberia and AMANDA [9] at a depth of 2000 m in the ice at the South Pole in Antarctica. These detectors are sensitive in the energy range - lOlo - 1015eV.

The BAIKAL detector was operating in 1993 with 36 optical modules and was finished in 1998 with 192 optical modules. Each optical module contains a 15-inch phototube, QUASAR-370, de- veloped especially for the experiment. The de- tector is located in the Southern part of Lake Baikal at a point where the lake has a depth of 1366m and the distance to the shore is 3.6 km. The light transmission properties of the lake wa ter vary greatly depending on the season due to sedimentation due to river in-flow. Typical light absorption lengths are 20m and light scattering lengths 15m. The optical modules are deployed on 8 strings arranged at the edges and centre of an equilateral heptagon supported from above by a rigid frame. The detector is deployed into the water using the platform of frozen surface ice dur- ing the winter months. The effective area of the detector is about 2000m2.

AMANDA was installed in stages in holes in the glacial ice made with a hot water drilling tech- nique. The first detector elements were deployed in 1993 at depths of 810m to 1000m; however, measurements of the ice transparency at those depths showed that the light scattering was un- acceptable for operation of a detector. Subse- quent strings were deployed at depths of 1500m to 2000m where the ice properties are better. In 1997 the AMANDA BlO detector had 300 opti-

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cal modules on 10 strings and the data so far published from AMANDA comes from this de- tector. Since then extra strings have been added with improved the signal readout technology. The present AMANDA II detector has 19 strings and about 700 optical modules with an effective area of 30,OOOm’. The signal readout on the new strings is performed by optical fibre links while the earlier strings have readout on twisted pair cables. The rise time of the signal pulses read out by the analogue optical links is improved to 7ns compared to that of loons on the twisted pair readout. These changes to the AMANDA detec- tor give a detected event rate increased by a factor 4 to 5, together with a very much larger angular acceptance [lo].

5. Mediterranean Neutrino Telescope Projects

A deep seawater telescope has significant ad- vantages over ice and lake-water experiments due to the better optical properties of the medium. However there are serious technological challenges to overcome to deploy and operate in the sea. The pioneer seawater project, DUMAND that worked from 1980 to 1995 to deploy a detector off the coast of Hawaii did not overcome these challenges and the project was cancelled. In contrast the projects AMANDA and BAIKAL which deploy from the solid glacial ice and frozen surface lake ice, respectively, have developed workable deploy- ment systems. The advantages of seawater neu- trino telescopes are significantly better angular resolution e.g. less than 0.3’ for ANTARES com- pared to 3O for AMANDA as well as more uniform efficiency due to the homogeneous medium. A disadvantage of a seawater detector is the higher optical background due to radioactive decay of 40K and light emission from living organisms: bi- oluminescence. These backgrounds can be over- come in the design of the detector by having a higher density of optical modules and by having high bandwidth data readout.

In the Mediterranean Sea there are three sites under evaluation for Neutrino Telescope projects. The most advanced project is that of the ANTARES collaboration which is building a

detector with initially 900 optical modules and effective area 50,000 m2 at a site off the south coast of Prance near Toulon. The NEMO collab- oration is exploring a site off Sicily. Since 1990 the ANTARES and NEMO collaborations have been working together on the detector at the Toulon site with the intention to chose the best site for a future larger telescope. The NESTOR collabora- tion intends to build a detector with 168 optical modules and around 20,000 m2 effective area at a site near Pylos off the coast of Greece.

The ANTARES collaboration started in 1996 to explore sites off the French coast. The site cho- sen is at location 42”50N 6OlOE with a depth of 2400m. The first phase of the ANTARES project has been to fully evaluate this site in terms of water quality, sedimentation rate and geological stability. The light absorption length at the site has been measured to be 45-60 m in the blue and 25-30 m in the ultra-violet, the scattering length for large angle scatters is greater than 100m and the loss of light transmission through the glass housings of the optical modules has been evalu- ated in measurements lasting 8 months to be less than 2% / year. Extensive studies of the biolu- minescence rate at the site have been carried out and lead to the conclusion that this background will give a dead time of less than 5% in the photo- multipliers, given the electronics design of the de- tector.

The design of the ANTARES detector array is to have optical modules suspended on individ- ual mooring lines, with readout via cables con- nected to the bottom of the lines. This technol- ogy is similar to the solution originally chosen by the DUMAND collaboration. As with DU- MAND, the ANTARES detector requires connec- tions made on the seabed by underwater vehicles. However, in the last 10 years the relevant under- water technology has advanced dramatically due to the needs of the offshore-oil industry, facilitat- ing the realization of the ANTARES instrumenta- tion. Currently a wide range of suitable deep-sea connectors is available and extensively used in in- dustry, including electro-optical connectors wet- mateable on the site. Many commercial underwa ter vehicles exist capable of making these connec- tions. The ANTARES readout design maximizes

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the reliability of the detector by dividing the sys- tem into independent sections such that there is no single active component in the sea whose fail- ure causes the loss of the whole detector. The detector signals are digitized in local electronics in the sea and than transmitted to the shore on high bandwidth optical links. On the shore, a computer farm makes the trigger decisions to de- cide which data is recorded to tape. A major as- pect of the ANTARES approach is the possibility to recover and repair all elements of the detector deployed in the sea.

The NESTOR detector is planned to be in- stalled at a depth of 3800 m at a site near Py- 10s in Greece. An important concept of the NESTOR project, and a significant difference with ANTARES, is to arrange the optical mod- ules on a tower structure with all internal con- nections made on the surface during deployment and to so avoid the need for submarine connec- tions. The towers have 12 hexagonal floors, of 16m radius, with photo-multipliers looking both upward and downward. Test deployments have been performed and many detector elements ex- ist, including a cable connection from the site to the shore.

The NEMO collaboration has taken measure- ments of several sites at depths - 3000m both north and east of Sicily. The preferred site is 1OOkm east of Cape Passer0 in Sicily and for this site many long-term site parameter measurements are underway. The NEMO team also has a test site at a depth - 2000m in the bay of Catania which is linked to the shore via a cable. This test site is intended to develop new technology for a future 1 km3 detector. Many of the NEMO col- laborators are part of the ANTARES collabora- tion and work on the construction of the present ANTARES detector south of Toulon. In order to choose the best site for a future detector the col- laborations have a common site exploration pro- gram to record site data on water transparency, sedimentation and bioluminescence at the Toulon and Cape Passer0 sites with the same instruments in order to control the systematic errors on the measurements.

6. Conclusions

The neutrino telescopes ANTARES and NESTOR being built in the Mediterranean will complement the sky coverage of the operating AMANDA detector of the south pole. Together these projects hold great promise to understand the origins of high energy cosmic radiation both in the local galaxy and beyond. Further, these neutrino telescope projects will enable cold dark matter searches in so-far unexplored region of the model phase space and give hope of discovering this missing component in the matter composi- tion of the universe.

The three Mediterranean projects ANTARES, NESTOR and NEMO are planning together to- wards a future km3 water neutrino telescope in the northern hemisphere to complement the km3 icecube project at the south pole.

REFERENCES

1. 2. 3.

4. 5. 6. 7.

8.

9.

R. Enomoto et al., Nature 416 (2002), 823. 0. Reimer and M. Pohl, astro-ph/0205256 J. Alvarez-Muiz and F. Halzen, astro- ph/0205408. I. F. Mirabel, astro-ph/0011315. C. Distefano et al., astro-ph/0202200. M.S. Turner, astro-ph/9904051. L. Bergstrom, Rep. Prog. Phys. 63 (2000) 793, hep-ph/0002126. LA. Belolaptikov et al., (BAIKAL Collabora- tion), Astro Part. Phys. 7 (1997) 263. E. Andres et al., (AMANDA Collaboration), Nature 410 (2001) 441.

10. R. Wischnewski et al., (AMANDA Collabo- ration), Proceedings of 27th ICRC, Hamburg 2001.

11. V.A. Balkanov et al., (BAIKAL Collabora- tion), Astro Part. Phys. 14 (2000) 61.