s. hundertmark- results from the amanda neutrino telescope and status of the icecube detector

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RESULTS FROM THE AMANDA NEUTRINO TELESCOPE

AND

STATUS OF THE ICECUBE DETECTOR

S. HUNDERTMARKfor the

Amanda CollaborationFysikum, Stockholms universitet, Roslagstullsbacken 21,

10691 Stockholm, Sweden

The Amanda neutrino telescope is installed in the 3 km thick Antarctic ice shield at the

South Pole. Various strategies are used to search for signals from extraterrestrial high energy

neutrino sources above the background caused by atmospheric neutrinos and muon bundles. A

selection of results from these searches is presented. The construction of the next generation

cubic-kilometer neutrino telescope IceCube has started. One string, carrying 60 optical

modules was successfully installed and is taking data.

1 Motivation and Operation Principle

Neutrinos are unique astronomical messengers. Once produced, they travel undisturbed bymatter or radiation fields along straight lines. This makes them ideal to locate the unknownsources of the high energy cosmic rays. In contrast to photons which can be produced both byaccelerating electrons or protons, neutrinos are linked to proton acceleration via the interactionof the accelerated charged particles with ambient matter or photons.

p + p(γ ) → p(n) + π and π → µ + ν

Typically, models predict a generic neutrino energy spectrum following E−2. The softer spectrumof the unavoidable atmospheric neutrinos enables the search for astrophysical neutrinos above across over energy.

Optical Cherenkov neutrino telescopes use the secondary muon that is produced in a charged-current interaction to detect the neutrinos. At high energies the angle between the neutrino andthe muon is becoming sufficiently small (0.7o at Eν =1 TeV) to use the detector as a telescope.



The advantage of the neutrino, to only interact weakly is also the biggest obstacle to its detec-tion. Huge volumes need to be monitored to be able to detect the feeble flux expected fromextraterrestrial neutrino sources. Running neutrino telescopes are deployed in volumes smallerthan 0.02 km3, but construction of one next generation detector covering 1 km3 has started. Thelarge range of muons, up to several tens of kilometers (> 20 km at PeV), considerably increases

the effective volume of these open detectors at high energies.Neutrino telescopes look downward, using the Earth as filter that can only be passed by

neutrinos. Muons coming from above are produced in atmospheric air showers and outnumberthe neutrino induced upward moving muons by several orders of magnitude. At energies aboveO(PeV) the Earth becomes opaque even to neutrinos and the signal has to be searched for atand above the horizon. Apart from muon tracks, cascades caused by electron- or tau-neutrinoscan be detected, leaving a specific signature in the detector. Their different topologies makeflavor identification possible, opening the field of particle physics to neutrino telescopes 1.

2 The Detector

Hot water was used to drill the 2 km deep holesfor the Amanda 2 detector in the 3 km thickAntarctic ice sheet. After the deployment thewater slowly re-freezes, providing mechanicalsupport to the detector. 677 optical modulesdistributed on 19 strings, are connected via ca-bles to the surface. The detector was installedduring the period 1996 to 2000. The equippedice volume can be approximated by a cylin-der of 500 m height and 200 m diameter. Thissparseness can be afforded as the absorption

length for photons of the relevant wavelength inthe ice is quite long, about 100 m. The signalsfrom the photo-multipliers are sent without fur-ther amplification or processing to the electron-ics housed at the surface. Figure 1 shows thedetector and a sketch of an optical module.Construction of the much larger IceCube ar-ray 3 has started in January 2005. One string,carrying 60 optical modules distributed along1 km of cable, was lowered down to a depth of

1500 m

2000 m

2500 m

1000 m

50 m

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view

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Figure 1: Schematic of the Amanda detector.The scale on the left hand side indicates the

depth below the ice surface.

2.5 km, extending above and below the current Amanda detector. Up to 16 strings per season

will follow until the detector is completed with a total of 4800 optical modules on 80 stringsin the year 2010. This detector will contain the Amanda array as its high density core. The

IceCube optical modules are more advanced than the Amanda optical modules. Equippedwith an on-board processor, the signal is digitized in the ice. The more complex hardwareallows for a large dynamic range of several thousand photo-electrons and a timing accuracy of a few nanoseconds.

3 Searches for a Diffuse Neutrino Flux and Point Sources

The combined neutrino flux from distributed unresolved sources gives rise to a diffuse flux of astrophysical neutrinos. This flux is larger than the flux from individual resolved sources, but thebackground rejection is more difficult than in point source searches. The backgrounds are muons



and neutrinos, products of atmospheric air showers caused by cosmic ray nucleons. Amanda

has searched for the diffuse flux of neutrinos with energies from 50 TeV up to above EeV. Herea preliminary measurement of the energy spectrum of atmospheric neutrinos, the result of thesearch for ultra-high energy (UHE) neutrinos and one multi-year point source search is presented.

3.1 Atmospheric Neutrinos

The measurement of atmospheric neutrinos isa mandatory analysis. It is needed to estab-lish Amandas working as a neutrino telescopeand can be used to ”calibrate” the detector.Previously, the smaller Amanda-B10 detectorwas used to measure the angular distribution of events, which was found to be consistent withexpectations from atmospheric neutrino simu-lations 4. From the year 2000 data sample, 570

atmospheric neutrino events were extracted,containing an estimated background contami-nation of 4 events. A neural net, trained onsimulated event samples was used to unfold theenergy spectrum of these events 5. Figure 2shows the extracted spectrum (preliminary),

10-9

10

-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

0 1 2 3 4 5 6

Figure 2: Atmospheric neutrino energy spec-trum unfolded from data taken with Amanda.

which is consistent with an E−2.7ν

energy dependence and previous measurements by Frejus 6.The line (marked ”limit”) in the 100–300 TeV energy interval is the 90 % confidence level limiton an additional neutrino flux following an E−2 energy dependence. Also shown are modelpredictions for the horizontal (upper) and the vertical (lower line) atmospheric neutrino flux 7.This is the first measured atmospheric neutrino spectrum in the range from 1–300 TeV.

3.2 UHE Analysis

At energies above PeV the Earth is becomingopaque to neutrinos and only downward to hor-izontally traveling neutrinos can be detected.The limited overburden above Amanda con-centrates the signal at the horizon. This signalhas to be searched for in the large backgroundof down-going muon bundles. The primarycosmic ray spectrum rapidly falls with energy

(∼

E

−3

); at UHE only a small fraction of theprimary energy is transfered to muons. There-fore, neutrino induced events deposit more en-ergy in the ice, which is converted to Cherenkovphotons, than muons induced by cosmic rayprimaries of the same energy. Analysis of datataken in 1997 shows that a background at thelevel of 109 cosmic ray events can be sufficientlyrejected to search for a diffuse UHE neutrinosignal. Simple selection criteria reduced thedata set to 3326 experimental events. A neural

NN2

e v e n t s / 1 3 1

d a y s

Experiment

w. CORSIKA

1

10

102

103

0 0.2 0.4 0.6 0.8 1

Figure 3: Neural net results for experimentaldata and prediction from the air shower simu-lation. Superimposed, neutrino induced eventsfrom an equally mixed all flavor neutrino flux

of E2Φ(Eν )=10−6GeV cm−2s−1sr−1.

net was trained to separate background from signal events. Figure 3 shows the distribution of the neural net output for the experimental events compared to a prediction from simulations of



2976±51 events. The experimental and simulated data show good agreement, both in absolutenumber of events and in shape. Also shown is the number of events expected for a neutrino flux(all flavors equally mixed) of E2Φ(Eν )=10−6GeV cm−2s−1sr−1. Selecting events in the rightmostbin and including systematic uncertainties of 40 % (mostly from the uncertainty in describing thephoton propagation in ice), a limit at 90 % confidence level of E2Φ(1015 eV<Eν <3×1018 eV)=

0.99×10−6

GeV cm−2

s−1

sr−1

is set8

.

3.3 Point Source Search

Extending a previously published analysis 9,four years of data (2000–2003) were combinedto a total live-time of 807 days. The selec-tion resulted in 3329 up-going neutrino candi-dates in good agreement with the expectationfrom atmospheric neutrino simulations (3438events). Figure 4 shows the distribution of

the arrival direction of the events, revealing nosignificant clustering which would correspondto a point source of extraterrestrial neutri-nos. Searching for a signal from a steady pointsource in a catalog of 33 predefined objects re-

24h 0h

°-90

°90

Figure 4: Sky-plot of the events selected from807 days of live-time taken during 2000–2003.

vealed no statistically significant deviation from the background expectation, as well. Currentlyan accurate estimation of the systematic error is performed.

4 Conclusions and Perspectives

The Amanda detector has searched the sky for fluxes of extraterrestrial high energy neutrinos

over an energy range extending to above EeV. No diffuse flux or point source could be identified,but the produced limits are excluding several models of astrophysical phenomena producingneutrinos.

The successful operation of the Amanda detector laid the ground work for the cubic-kilometer IceCube detector. The first string was successfully deployed in January 2005 anddata taking has started. Deployment will continue during the Antarctic summer seasons untilthe full detector is commissioned in 2010. Already next year the IceCube detector shouldnumber as many optical modules as the Amanda detector.

References

1. T. Han and D. Hooper, New J. Phys. 6 (2004) 1502. http://amanda.uci.edu 3. http://icecube.wisc.edu 4. J. Ahrens et al., Phys. Rev. D 66 (2002) 012005.5. K. Woschnagg et al., Nuc. Phys. B (Proceedings Supplements) 143 (2005) 343-350.6. K. Daum et al., Zeitschrift fur Physik C 66 (1995) 417.7. L. V. Volkova and G. T. Zatsepin, Sov. J Nucl. Phys. 31 (1980) 212. M. Honda et al.,

Phys. Rev. D 52 (1995) 4985.8. M. Ackermann et al., Astropart. Phys. 22 (2005) 339–353.9. M. Ackermann et al., Phys. Rev. D 71 (2005) 077102.

s. hundertmark- results from the amanda neutrino telescope and status of the icecube detector

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