aa r X i v : . [ a s t r o - ph ] O c t TH I NTERNATIONAL C OSMIC R AY C ONFERENCE
The ANTARES neutrino telescope: a status report
A. K
OUCHNER ON BEHALF OF THE A NTARES COLLABORATION Laboratoire AstroParticle and Cosmology10, rue A. Domon et L. Duquet, 75205 Paris Cedex 13, France. [email protected]
Abstract:
ANTARES is a large volume neutrino telescope currently under construction off La Seyne-sur-mer, France, at 2475m depth. Neutrino telescopes aim at detecting neutrinos as a new probe for asky study at energies greater than 1 TeV. The detection principle relies on the observation, using pho-tomultipliers, of the Cherenkov light emitted by charged leptons induced by neutrino interactions in thesurrounding detector medium. Since late January 2007, the ANTARES detector consists of 5 lines, com-prising 75 optical detectors each, connected to the shore via a 40 km long undersea cable. The datafrom these lines not only allow an extensive study of the detector properties but also the reconstruction ofdownward going cosmic ray muons and the search for the first upward going neutrino induced muons.Theoperation of these lines follows on from that of the ANTARES instrumentation line, which has provideddata for more than a year on the detector stability and the environmental conditions. The full 12 linedetector is planned to be fully operational early 2008.
Scientific motivations
One of the major aims of neutrino astronomy is tocontribute solving the fundamental question of theorigin of high energy cosmic rays (HECR). Neutri-nos can indeed escape from the core of the sourcesand travel with the speed of light through mag-netic fields and matter without being deflected orabsorbed. Therefore they can deliver direct infor-mation about the processes taking place in the coreof the production sites and reveal the existence ofundetected sources. At high energies, neutrinos areunmatched in their capabilities to probe the Uni-verse.High energy neutrinos are produced in a beamdump scenario in dense matter via pion decay,when the accelerated protons interact with ambientmatter or dense photon fields: p + A/γ → π ↓ γγ + π ± ↓ µ ± + ν µ ( ν µ ) ↓ e ± + ν e ( ν e ) + ν µ ( ν µ ) + N + ... Good candidates for high energy neutrino produc-tion are active galactic nuclei (AGN) where theaccretion of matter by a supermassive black hole may lead to relativistic ejecta [1]. Other potentialsources of extra-galactic high energy neutrinos aretransient sources like gamma ray bursters (GRB).As many models [2] for GRBs involves the col-lapsing of a star, acceleration of hadrons followsnaturally. The diffuse flux of high energy neu-trinos from GRBs is lower than the one expectedfrom AGNs, but the background can be dramati-cally reduced by requiring a spatial and temporalcoincidence with the short electromagnetic burstsdetected by a satellite.High Energy activity from our Galaxy has alsobeen reported by ground based gamma-ray tele-scopes. Many astrophysical sources [3] are can-didates to accelerate hadrons and subsequentlyproduce neutrinos. Such sources could only beobserved by a northern neutrino telescope likeAntares.Neutrino telescopes are also sensitive to signalsdue to the annihilation of neutralinos, gravitation-ally trapped inside the core of massive objects likethe Sun, the Earth or the Galactic centre [4].Finally, deep-sea neutrino telescopes enable re-searches in the fields of marine biology, oceanog-raphy and seismology.
NTARES FIRST RESULTS
Detection principle
The neutrino’s advantage, the weak coupling tomatter, is at the same time a big disadvantage.Huge volumes need to be monitored to compensatefor the feeble signal expected from the cosmic neu-trino sources. In this context, the water Cherenkovtechnique offers both a cheap and reliable option.The detection principle relies on the observation,using a 3 dimensional array of photodetectors,of the Cherenkov light emitted, in a transparentmedium, by charged leptons induced by charged-current neutrino interactions in the surrounding de-tector medium.Thanks to the large muon pathlength, the effec-tive detection volume in the muon channel is sub-stantially higher than for other neutrino flavours.The higher the neutrino energy the smaller the de-viation between the muon and the neutrino (typi-cally ∆ θ ≃ . o ( E ν (TeV)) . ), thus enabling to pointback to the source with a precision close to the oneachieved by gamma-ray telescopes. Muon trajecto-ries are reconstructed using the time and amplitudefrom the photodetector signals.The energy of the event is estimated thanks to theenergy deposited in the detector. Monte Carlo sim-ulations for sea water predict a muon energy esti-mation by a factor of 2-3.Cosmic particles penetrating the atmosphere un-dergo a cascade of many secondary particles.Among them, high energy muons can reach the de-tector and constitute a very intense source of back-ground. To suppress this background the detectorconcentrates on upward detection. As a result, thefield of view is restricted to one half of the celes-tial sky ( π sr). Severe quality cuts criteria are thenapplied to the reconstruction to remove remainingmis-reconstructed muons. Atmospheric neutrinosproduced in the atmospheric cascades can travelthrough the Earth and interact in the detector vicin-ity. To some extent this background is irreducible.Fortunately, the atmospheric neutrino flux shows adependency upon energy dN/dE ∝ E − . whilecosmic neutrinos are expected to exhibit a flux de-pendency dN/dE ∝ E − . An excess of eventsabove a certain energy can therefore be attributedto extraterrestrial neutrinos. Figure 1: Schematic layout of the future Antaresdetector. The full detector will consist of 12 linesconnected to a junction box (deployed in Decem-ber 2002) and operated from shore in remote modethrough an electro-optical cable. The Antares Detector
Antares is a large European collaboration cur-rently deploying a 2475 m depth detector km offLa-Seyne-sur-Mer (Var, French Riviera) at a loca-tion o ′ N, o ′ E . The site benefits from theclose infrastructures of the French sea science in-stitute IFREMER. The sea water properties havebeen extensively studied revealing low light scat-tering, mainly forward [5] and an average opticalbackground (induced by bacteria and K decays)of 70 kHz per detection channel.The final detector will consist of an array of 12flexible individual mooring lines separated fromeach other on the sea bed by 60-80 m. Fig-ure 1 shows a sketch of the detector. The linesare weighted to the sea bed and held nearly ver-tical by syntactic-foam buoys. Each line will beequipped with 75 photomultipliers [6] housed inglass spheres, referred to as optical modules (OM).The OMs are inclined by o with respect to thevertical axis to ensure maximum sensitivity to up-ward moving Cherenkov light fronts. Expectedperformances, in particular in the frame of pointsource searches are described in [7].
1. for a complete list of the antares members seehttp://antares.in2p3.fr0 TH I NTERNATIONAL C OSMIC R AY C ONFERENCE
Figure 2: Integrated number of effective days ofdata taking since March 2006 taking into accountall losses.The default readout mode [8] of the detector is thetransmission of the time and amplitude of any lightsignal above a threshold corresponding to 1/3 of aphoto-electron for each OM. Time measurementsare relative to a master reference clock signal dis-tributed to each storey from shore via an electro-optical cable. The grouping of three optical mod-ules in a storey allows local coincidences to bemade to eventually reduce the readout rate. In ad-dition the front end electronics [9] allows a moredetailed readout of the light signal than the stan-dard time and amplitude mode. With this detailedreadout it is possible to sample (up to 1 GHz) thefull waveform of the signal with 128 channels, en-abling special calibration studies of the electronics.
First results from deep-sea
A mini-instrumented line equipped with 3 OMs(MILOM) and mainly dedicated to study environ-mental parameters (sea current, salinity, pressure,temperature...) has been in operation since spring2005. The results of this line are presented in de-tails in [10]. Since the end of January 2007, thedetector consists of 5 operating detection lines. Atthis stage, Antares is the largest neutrino telescopeever built in the northen hemisphere. Data with2 lines have been taken since October 2006 andwith one line since March 2006. Figure 2 givesan indication of the data taking efficiency since theconnection of the first line, which has been con- Figure 3: The zenith angle distribution of datataken during Feb-May 2007 with a quality cutbased on the fit likelihood. This preliminary re-construction is based on the nominal positionsof the OMs. Alignement data, now available,will considerably improve the recontruction effi-ciency. While most of the tracks are reconstructedin the downward-going direction there is a steepfall around cos θ = − . as expected from the fluxof cosmic ray muons. Some upward going eventsare seen which are candidates for neutrino events.tinuously improving. In spring of 2007 two furtherlines were immersed and two more lines will be de-ployed in July. These latter four lines are plannedto be connected in September 2007. The detectoris expected to be complete early 2008.The line motions are monitored by acoustic devices(high frequency long base line LBL) and by incli-nometers regularly spread along the line, allowingredundancy. The system allows a location of eachOM with a precision close to 10 cm. Timing cali-bration is ensured by a network of laser and LEDbeacons [11]. According to the design specifica-tions, a precision mesurement of . ns is achievedwhich guaranties an angular resolution within ex-pectations ( < . o ).The existing 5 line data are dominated by down-ward going muon bundles, the present trigger ratebeing roughly Hz. The reconstruction programfits a single track to these events under the assump-tion that light is emitted under the Cherenkov an-gle w.r.t the muon path. The angular distributionobtained, after quality cuts, is shown in figure 3.As one can see, upward candidates are also present
NTARES FIRST RESULTS
Figure 4: Example of atmospheric neutrino induced muon candidate obtained with the 5 line detector. Eachplot shows a single line hit distribution as a function of time. The bottom-right drawing is a 3D display ofthe same event. The muon trajectory is reconstructed upgoing with a zenith angle . o away from vertical.in the reconstructed sample. One of these neutrinocandidates is displayed in figure 4. Conclusions
Great achievements have been made by the Antarescollaboration in the last year. The detector isworking in nominal mode with 5 lines and shouldbe complete early 2008. Upward neutrino candi-dates have been found that validate the conceptualmethod and the chosen techniques. Very excitingtimes have started with a detector looking for neu-trinos in a region of the celestial sky which hasnever been studied with such a level of sensitivity.
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