The ANTARES neutrino telescope was completed in May 2008

and its first results of the searches for point-like and diffuse neutrino fluxes are presented. The sensitivity reached for point-like searches for a live time of 295 days

and for declination lower than −50° is of 7.5×10−8 GeV−1 

cm−2 s−1. For the search of diffuse flux the observed number of events is found to be compatible with the

background expectation and a 90% CL upper limit of 5.3×10−8 GeV cm−2 s−1 sr−1 for a total live time of 334 days was set. The multi-messenger ANTARES program is also briefly described with particular emphasis on the neutrino alert system for the detection of transient source of neutrinos

Coniglione, R.

English

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DOI 10.1393/ncc/i2011-10857-2Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011First results of the ANTARES neutrino telescopeR. Coniglione on the behalf of the ANTARES CollaborationINFN, Laboratori Nazionali del Sud (LNS) - Via S. Sofia 62, 95123 Catania, Italy(ricevuto il 25 Febbraio 2011; pubblicato online il 29 Aprile 2011)Summary. — The ANTARES neutrino telescope was completed in May 2008and its first results of the searches for point-like and diffuse neutrino fluxes arepresented. The sensitivity reached for point-like searches for a live time of 295 daysand for declination lower than −50◦ is of 7.5×10−8 GeV−1 cm−2 s−1. For the searchof diffuse flux the observed number of events is found to be compatible with thebackground expectation and a 90% CL upper limit of 5.3× 10−8 GeVcm−2 s−1 sr−1for a total live time of 334 days was set. The multi-messenger ANTARES programis also briefly described with particular emphasis on the neutrino alert system forthe detection of transient source of neutrinos.PACS 95.85.Ry – Neutrino, muon, pion, and other elementary particles; cosmicrays.PACS 95.55.Vj – Neutrino, muon, pion, and other elementary particle detectors;cosmic ray detectors.1. – IntroductionANTARES is the first operational neutrino detector in the Mediterranean Sea [1]. Themain purpose of the detector is the observation of high-energy neutrinos that are expectedto be emitted by Galactic and Extragalactic sources where a high-energy accelerationmechanism is present. The properties of neutrino make it complementary to other probesas high-energy gamma and charged particles. In fact, neutrinos can escape from densematter surrounding the acceleration zone and, due to their neutral charge, they arenot deflected by magnetic fields making it possible to point back to the source with highprecision. Moreover, the neutrino hadronic origin will allow to distinguish unambiguouslybetween hadronic or leptonic mechanisms.The neutrino detection principle is based on the observation of Cherenkov light pro-duced in sea water from secondary charged leptons which originate in charged currentinteraction of neutrino with the water or rock near the instrumented volume. The longmuon range makes muon neutrinos privileged with respect to the other neutrino flavoursthat are detected with lower efficiency and poorer angular resolution. Measuring thephoto-sensor positions and the photon arrival time it is possible to reconstruct the neu-trino direction (the induced muon and the neutrino are almost collinear at the energy ofinterest of the telescope).c© Societa` Italiana di Fisica 103104 R. CONIGLIONE on the behalf of the ANTARES COLLABORATIONThe ANTARES detector consists of 12 flexible lines, each one hosting 75 photomulti-pliers contained in glass spheres (Optical Modules OM). The lines are anchored to the seabed at a distance of around 70m from each other and are kept vertically by a buoy. TheOMs are located along the line and arranged in 25 storeys of three OMs. The distancebetween storeys is 14.5m. Each storey contains also the corresponding electronics. Thedetector is installed about 40 km off the Toulon coast (France) (42◦48′ N, 6◦10′ E) at adepth of 2475m. The ANTARES detector was completed in May 2008 but thanks to itsmodularity has been taking data since the deployment of its first line [1].The ANTARES detector is the largest operating telescope looking at the Southernsky where the Galactic center and a large part of the Galactic plane are located. In thissky region high-energy gamma detectors, like HESS, have disclosed a large number ofTeV gamma sources that are very promising neutrino sources. The ANTARES detectoris complementary to the AMANDA/IceCube [2] detectors located at the South Pole.2. – Searches for point-like sources and diffuse fluxFirst results of searches for point-like sources are based on data collected in 2007 and2008 corresponding to an integrated live time of 295 days. In this period the detectorwas under deployment and it was in a configuration between 5 to 12 operational lines. Inthe analysis the rejection of atmospheric muons, that are originated by the interaction ofcosmic rays with the atmosphere, is based on a likelihood variable Λ deduced from thetrack reconstruction algorithm. The Λ variable is linked to the goodness of the fit andconsequently to the reconstructed track precision. Putting cut on the Λ parameter and onthe estimated fit error we can reject most of the atmospheric muons badly reconstructedas up-going. The number of data events selected as up-going was 2040 which is in goodagreement with the Monte Carlo expectation. Search of cosmic neutrinos from point-likesources was performed using an unbinned method following two different approaches.In the first approach, the full sky search, no assumption is made regarding the positionin the sky of the neutrino sources and a search for event clusters is performed. Nosignificant excess was found. The most significant cluster contains 8 events (only 3background events are expected) that corresponds to a significance level of 2σ. In thesecond approach a list of 24 candidate sources was fixed and a search in order to findstatistically significant excesses in that directions was carried on. For each source theobserved flux limit is reported in fig. 1 (red squares). The sensitivity (90% CL) is reportedas a function of the source declination in fig. 1 (red line): the value for the declinationswith full visibility is 7.5 × 10−8 GeV−1 cm−2 s−1. The flux limit reached for point-likesearch is the best one that has been achieved for the Southern sky.The search of flux of high-energy ν produced by astrophysical sources withoutany particular assumption on the source direction is one of the main aims of aneutrino telescope. The main existing predictions are based on the observed Cos-mic Ray (Waxman-Bahcall) and Cosmic Ray and gamma-ray diffuse flux observations(Mannheim-Protheroe-Rachen) [3]. In the analysis the most challenging issue is the sep-aration of atmospheric and astrophysical neutrinos. In ANTARES this goal has beenreached with the implementation of an original energy estimator based on the aver-age number of hit repetitions (R) in the OMs due to the different arrival times of di-rect (Cherenkov not scattered photons) and delayed photons (photons from secondaryelectromagnetic showers or scattered photons). High-energy muons (Eμ > 1TeV) areexpected to produce more electromagnetic showers and the consequence is that the per-centage of delayed photons with respect to the direct photons increases with the muonFIRST RESULTS OF THE ANTARES NEUTRINO TELESCOPE 105declination (deg)-80 -60 -40 -20 0 20 40 60 80 )-1 s-2limit on flux ( GeV cm-910-810-710-610-510-410 ANTARES 2007+8 (295 days) ANTARES sensitivityIceCube 40 375.5 days MACRO (2300 days)Super-K (2623 days) Amanda-II (1387 days)Fig. 1. – (Colour on-line) 90% CL flux limits as a function of the source declination. Publishedlimits from other experiments are also shown.energy. The separation between the diffuse flux signal from the atmospheric neutrinobackground is performed by means of a cut on the R variable. The value of the cut on Rhas been estimated on Monte Carlo following the Feldmann and Cousins method [4] evalu-ating the best signal-to-noise ratio (Model Rejection Factor minimization). The obtainedvalue is R = 1.31. In fig. 2 left are reported Monte Carlo expectations for signal andbackground neutrino energy spectra as a function of the true neutrino energy before andafter the selection of the condition R ≥ 1.31. The region where the 90% of the astrophys-ical neutrino signal is present is highlighted. After data unblinding, 9 events have beenfound in data. From Monte Carlo the number of expected background neutrino eventsis 10.7 ± 2. The corresponding upper limit is E2Φ90% = 5.3 × 10−8 GeVcm−2 s−1 sr−1in the energy range between 20TeV and 2.5PeV. This limit has been obtained for 334days of equivalent live time using at least 9 detection lines. In fig. 2 right this limit and,for comparison, limits from other experiments are reported [5].3. – Multi-messenger astronomyThe multi-messenger astronomy, which can give some help in the understanding of themost violent phenomena in the Universe, is one of the aims of the astrophysical commu-nity. In this context the ANTARES Collaboration has developed a program to improveits capability of revealing possible spatial and temporal correlation with other messengersas photons, cosmic rays and gravitational waves. Thanks to the almost background freesearch, the detection of neutrinos from transient sources is one of the most promisingprograms for a neutrino telescope. Two strategies are implemented in ANTARES. Inthe first one alerts are received from the Gamma-ray burst Coordinate Network (GCN).Gamma-ray satellites as Fermi, Swift and AGILE deliver in real time alerts to the GCNwhen gamma bursts are detected. The direction and detection time are then distributedto the other observatories. ANTARES is connected with the GCN since the end of2006. In the second strategy ANTARES sends alerts to optical telescopes when multipleneutrinos or high-energy neutrino events are detected. This alert system called TAToO(Telescopes and Antares Target of Opportunity) has been implemented in the ANTARES106 R. CONIGLIONE on the behalf of the ANTARES COLLABORATION/GeV)ν(E10log2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7events/bin-110110, no R cutνAtms 1.31≥, RνAtms , no R cutνSignal 1.31≥, RνSignal -1 sr-1 s-2 GeV cm-7 = 10testΦ 2EFig. 2. – Left: Monte Carlo expectation for signal and background neutrino energy spectra asa function of the true neutrino energy before and after the selection on the average numberof hit repetitions R. The atmospheric neutrinos are the conventional Bartol+RQPM promptand the signal is at level of E2Φ = 10−7 GeVcm−2 s−1 sr−1. Right: ANTARES upper limit(90% CL) for a E−2 diffuse high-energy muon-neutrino flux compared with upper limits fromother experiments. The predictions for transparent sources are also shown. The grey bandrepresents the variation from vertical to the horizontal direction of the atmospheric νμ flux, thecentral line the average flux.Collaboration since early 2009. The TAToO system can send alerts to a network of opti-cal telescopes in real time thanks to a fast on-line event reconstruction algorithm with apointing accuracy of about 1.4◦(1TeV)–0.6◦(100TeV). In order to improve the precisionof these alerts an additional reconstruction strategy is used off-line which provide anangular resolution of 0.7◦(1TeV)–0.2◦(100TeV). The good angular accuracy, the abilityto filter efficiently events and the time elapsed from the passage of a high-energy particlein the detector to the time in which the alert is sent of few tens of seconds make possi-ble the trigger of optical robotic telescopes such as TAROT and ROTSE for which thetypical field of view is 2◦ × 2◦. The optical follow-up strategy is composed by real timeobservations (for rapidly fading sources) and long-term follow-up (for SN search). Fromearly 2009 up to now 22 alerts have been recorded. These numbers are in agreementwith the expectations if the unavailability of the two telescopes is taken into account.Recently a program for coincident searches of high-energy neutrinos and gravitationalwaves has been undertaken in the ANTARES Collaboration. A coincidence, if observed,will give strong evidence that gravitational waves and high-energy neutrinos are reallydetected and originate from a common source. A joint search with the VIRGO/LIGOinterferometers is going on with a dedicated working group. The common sky view is ofaround 30%.REFERENCES[1] Ageron M. et al. (ANTARES Collaboration), Astropart. Phys., 31 (2009) 277.[2] IceCube web page http://icecube.wisc.edu; AMANDA web page http://amanda.uci.edu/.[3] Waxman E. and Bahcall J., Phys. Rev. D, 64 (2001) 023002-1; Mannheim K.,Protheroe R. J. and Rachen J. P., Phys. Rev. D, 63 (2000) 023003.[4] Feldman G. J. and Cousins R. D., Phys. Rev. D, 47 (1998) 3873.[5] Aguilar et al. (ANTARES Collaboration), Phys. Lett. B., 696 (2011) 16.

First results of the ANTARES neutrino telescope

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First results of the ANTARES neutrino telescope

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