Various aspects of the construction, operation and calibration of the
recently completed deep-sea ANTARES neutrino telescope are described. Some
first results obtained with a partial five line configuration are presented,
including depth dependence of the atmospheric muon rate, the search for
point-like cosmic neutrino sources and the search for dark matter annihilation
in the Sun.Comment: 8 pages, 16 figure

Coyle, Paschal

English

arXiv

The ANTARES neutrino telescope has been accomplished in May

2008. Located at a depth between 2100 and 2500m in the Mediterranean Sea, 40km off the Provencal coast, it comprises a large three-dimensional array of 885 Optical

Modules deployed on 12 vertical lines. The telescope is aimed to observe high energy cosmic neutrinos through the detection of the Cherenkov light produced by up-going

induced muons. The status of the experiment is briefly reviewed, and first results of atmospheric muons and neutrinos analysis will be discussed

Picot Clémente, N.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10701-9Colloquia: LaThuile10IL NUOVO CIMENTO Vol. 33 C, N. 5 Settembre-Ottobre 2010The ANTARES deep-sea neutrino telescope—Statusand first resultsN. Picot Cle´mente on behalf of the ANTARES CollaborationCentre de Physique des Particules de Marseille - Marseille, France(ricevuto il 14 Settembre 2010; pubblicato online il 10 Gennaio 2011)Summary. — The ANTARES neutrino telescope has been accomplished in May2008. Located at a depth between 2100 and 2500m in the Mediterranean Sea, 40 kmoff the Provencal coast, it comprises a large three-dimensional array of 885 OpticalModules deployed on 12 vertical lines. The telescope is aimed to observe high energycosmic neutrinos through the detection of the Cherenkov light produced by up-goinginduced muons. The status of the experiment is briefly reviewed, and first resultsof atmospheric muons and neutrinos analysis will be discussed.PACS 95.55.Vj – Neutrino, muon, pion, and other elementary particle detectors;cosmic ray detectors.PACS 96.50.Vg – Energetic particles.PACS 29.40.Ka – Cherenkov detectors.1. – IntroductionThe study of cosmic rays is one of the most important topics of astrophysics today. Itrepresents an important step toward the understanding of the Universe. Several gamma-ray sources have been detected in the last few years, however their detection cannot give,up to now, a complete understanding of gamma-ray production mechanisms. If they areproduced by hadronics mechanisms, and thus if nuclei are accelerated, then a high energyneutrino counterpart should be observed. Contrary to cosmic rays which are sensitiveto magnetic fields, and to photons which are easily absorbed, neutrinos can escape fromcompact objects, and travel over very large distances without being deflected by magneticfields, or absorbed by interstellar clouds. Nevertheless, because of the weak interactionbetween neutrino and matter, very large detectors are needed, and are often installed inhostile environments, where the construction represents a challenge.ANTARES is a neutrino deep-sea telescope, completed in May 2008, and designed forthe detection of high energy neutrinos emitted by Galactic (supernova remnants, micro-quasars, . . . ) and extragalactic (active galactic nuclei, gamma ray bursters, pulsars, . . . )sources. It is also aimed to search for neutrinos induced by dark matter annihilationwithin massive bodies. Moreover the large area of the detector offers the possibility forc© Societa` Italiana di Fisica 6162 N. PICOT CLE´MENTE on behalf of the ANTARES COLLABORATIONFig. 1. – Schematic description of the ANTARES detector and of a typical storey containing aLED beacon.the search for exotic particles such as magnetic monopoles, or nuclearites. ANTARESis the largest neutrino telescope operating in the northern hemisphere, and, by its loca-tion, contains the Galactic Centre in its field of view. It provides also a unique platformfor multidisplinary sciences as oceanography, sea biology, seismology, or environmentmonitoring.2. – The ANTARES detectorThe ANTARES neutrino telescope is located at 2475m depth in the MediterraneanSea, offshore from Toulon (France) covering an area of about 0.1 km2 [1]. The detectoris composed by 12 flexible lines of 450m length, which are fixed on the sea bed thanks toan anchor, and are kept taut by a buoy (see fig. 1). They are connected to a junction boxwhich is relied to the shore station with an electro-optical cable of about 40 km. Everyline comprises 25 storeys separated by 14.5m, whose each contains 3 optical modules.Parallel to an optical module, two Analog Ring Samplers [2] are used to digitize signalswith amplitude higher than 0.3 photoelectrons. In addition to photodetectors, somestoreys contain devices for in situ calibrations such as LED Beacons [3], and for acousticpositioning with hydrophones.The strategy of the ANTARES data acquisition is based on the “all-data-to-shore”concept [2]. This implementation leads to the transmission of all raw data above a giventhreshold to shore, where different triggers are applied for storage.THE ANTARES DEEP-SEA NEUTRINO TELESCOPE—STATUS AND FIRST RESULTS 633. – Status of the detector and in situ calibrationThe ANTARES observatory was built gradually until May 2008, giving rise to variousdetector layouts used for physics analysis: the first two lines were installed, and connectedin 2006, then 3 additional lines were plugged at the beginning of 2007, and in December2007 the detector reached a 10-line configuration. Finally, ANTARES was completed inMay 2008, with its final 12-line configuration.The telescope has an angular resolution of about 0.2o for high energy neutrinos(> 10TeV), it is determined by the intrinsic detector resolution, i.e. the timing res-olution and accuracy of the location of the optical modules. The time calibration isperformed for each line in dark room before their deployment, and is then done regularlyin situ after the immersion. The relative time calibration relies on several independentsystems [4]. The master clock system, by sending redundant signals to each storey, givesthe time signal propagation through cables to the shore station. Then to obtain timesoffsets between optical modules, which depend on the front-ends electronics and on thetime propagation inside phototubes, LED optical beacons have been installed every 5storeys on each line. The flashing light emitted by LED beacons allows to compute timedifferences between signals received by pairs of optical modules in order to extract theirtime offsets. Moreover, optical beacons provide the possibility to study optical propertiesof the sea water, as the light absorption length.Potassium-40 is a radioactive isotope naturally present in the sea water. Its β-decaygives rise to Cherenkov light produced by electrons, producing a detectable signal inANTARES. This light is then employed to perform crosscheck of time calibration, bystudying coincidences between optical modules of a same storey, and to monitor theevolution of the optical module gains, which are tuned if a significant drift is noticed.Finally, thanks to these two time calibration methods, a rms of 0.6 ns on the timingprecision is reached, in good agreement with the precision of the ANTARES designgoal.4. – Atmospheric muonsImportant backgrounds come from biological activities of micro and macro organisms,called bioluminescence, from the dark noise of photomultipliers, and from the 40K decay.The bioluminescence and the dark noise produce irregular hits in the detector, and canbe easily removed by searching for correlated hits, corresponding to the crossing of amuon. The light coming from the 40K decay can only illuminate optical modules of asame storey, and are not selected in the reconstruction algorithm when local coincidencesbetween neighbouring floors are chosen.However, the most of the background is the one produced by atmospheric downgoingmuon events. They are produced by high energy cosmic rays interacting with atomicnuclei of the upper atmosphere, with the production of kaons and pions, which decay intomuons. At the ANTARES depth, the muon flux exceeds by several orders of magnitudesthe atmospheric neutrino-induced muon flux, but can be extracted by regarding onlyupgoing reconstructed events. Despite they are an important background for neutrinodetection, atmospheric muons can be used to verify the detector response. Moreover adeficit of muons in the moon direction will give some important information about thepointing accuracy of the detector.A first study, recently published in [5], is based on the observation of coincidencesignals in adjacent storeys of the detector, yielding to a low energy threshold of 4GeV, in64 N. PICOT CLE´MENTE on behalf of the ANTARES COLLABORATIONFig. 2. – (Colour on-line) Depth dependence of the total atmospheric muon flux as measuredfrom coincidence signals on adjacent floors (red points). The measured flux dependence iscompared to results from Monte Carlo simulations performed with CORSIKA [7] (dash-dottedblack line), using the NSU model [8], and MUPAGE [6] (dashed blue line). The grey band showsthe normalisation uncertainty of data.order to measure the attenuation length of the muon flux. Figure 2 shows the coincidencerates measured in different storeys as a function of depth for atmospheric downgoingmuons. The measured depth dependence of the rates can be very well fitted with theexpected exponential fall-off of the muon intensity, and is compatible with Monte Carlosimulations performed with MUPAGE [6], and CORSIKA [7] with the NSU model of theprimary cosmic ray spectrum [8]. The results are also consistent with previous analysisperformed using a full track reconstruction and by converting the reconstructed zenithangle into an equivalent slant depth through the sea water [9].5. – ANTARES first neutrino eventsThe online reconstruction method isolates in real time upward going neutrino-inducedmuon candidates by applying a simple cut on the track fit quality. For 2007 and 2008(∼ 341 days of effective livetime), about 750 upgoing muons have been reconstructed inANTARES. These events, reconstructed on at least two strings, are shown fig. 3 as afunction of their elevation. From atmospheric neutrino simulations, based on the Bartolflux [10], and downgoing atmospheric muons simulations, which uses Corsika [8] with theNSU primary composition [8], one obtains very good agreements with the reconstructeddata, with an expected contamination of the neutrino sample less than 4% from mis-reconstructed atmospheric muons. The agreement with Monte Carlo expectations givesthe confidence in the simulation of the detector and of the event reconstruction for down-going reconstructed muons, confirming a correct understanding of the detector. For thesystematic errors the optical modules properties, and the uncertainties in the sea waterdescription are considered. They are larger for downgoing events because of the opticalmodules orientations (45o downward looking).THE ANTARES DEEP-SEA NEUTRINO TELESCOPE—STATUS AND FIRST RESULTS 65Fig. 3. – (Colour on-line) Elevation of reconstructed data (black crosses) from 2007 and 2008on at least two strings. The distribution is compared to muon (red line) and neutrino (blueline) simulations, respectively performed according to the NSU primary composition [8], andthe Bartol flux model [10].The 750 upgoing reconstructed events are then selected, and are presently used forthe search of cosmic point-like sources. A scrambled sky map of these events is shown infig. 4 in Galactic coordinates. The grey scale represents the exposure time of ANTARES,which can see the interesting center of the Galaxy. The scrambling, consisting in changingrandomly the time of the events, has been employed to avoid any human bias in theanalysis.Fig. 4. – (Colour on-line) Scrambled sky map of 750 neutrino candidates (blue crosses) takenfrom 2007 and 2008 data (∼ 341 days of effective livetime). The grey scale represents theexposure time of ANTARES, from permanent (in white) to null exposure (in dark grey).66 N. PICOT CLE´MENTE on behalf of the ANTARES COLLABORATION6. – ConclusionANTARES has been completed in May 2008. It is the largest operational neutrinotelescope of the northern hemisphere, taking data with a high duty cycle. The flux ofatmospheric muons has been measured and is found to be in agreement with MonteCarlo expectations. The detector is now well understood, and very good agreements areobtained with Monte Carlo simulations, performed for atmospheric muons and neutrinos.The first physics analysis for the 2007 and 2008 data are in their finalisation steps, andwill be presented in the next few months.REFERENCES[1] Carr J. for the ANTARES Collaboration, J. Phys. Conf. Ser., 136 (2008) 022047.[2] Aguilar J. A. et al., Nucl. Instrum. Methods A, 570 (2007) 107.[3] Ageron M. et al., Nucl. Instrum. Methods A, 578 (2007) 498.[4] Gomez P., Timing Calibration of the ANTARES Neutrino Telescope, ICRC2009,arXiv:0911.3052.[5] Aguilar J. A. et al., Astropart. Phys., 33 (2010) 86.[6] Carminati G., Margiotta A. and Spurio M., Comput. Phys. Commun., 179 (2008)915.[7] Heck D., Schatz G., Thouw T., Knapp J. and Capdevielle J. N., ForschungszentrumKarlsruhe Report, FZKA-6019 (1998).[8] Nikolsky S. I., Stamenov I. N. and Ushev S. Z., Sov. Phys. JETP, 60 (1984) 10; Zh.Eksp. Teor. Fiz., 87 (1984) 18.[9] Ageron M. et al., Astropart. Phys., 31 (2009) 277.[10] Barr G. et al., Phys. Rev. D, 39 (1989) 3532; Agrawal V. et al., Phys. Rev. D, 53(1996) 1314.

The ANTARES Deep-Sea Neutrino Telescope: Status and First Results

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