Expected performances of the future neutrino telescope ANTARES

ELSEVIER

Nuclear Physics B (Proc. Suppl.) 114 (2003) 211-216

SUPPLllJk'NTS www.elsevier.eom/loeate/npe

Expected performances of the future neutrino telescope ANTARES Juan Jos@ Hern£ndez-Rey a (for the ANTARES

collaboration)

~IFIC Instituto de Fisica Corpuscular C.S.I.C. - Universitat de Val@ncia Apdo. 22085, E-46071 Valencia, Spain -

A review is made of the estimated performances of the future ANTARES 0.1 km 2 undersea neutrino telescope, whose deployment is expected to be completed by 2004. Forecasts of its intrinsic capabilities in terms of muon effective area and angular and energy resolutions are given. Foreseeable sensitivities to diffuse fluxes, point sources and neutrinos coming from neutralino annihilation in the Sun are shown.

1. I n t r o d u c t i o n The scientific potential of detectors of cosmic neutrinos in the 0.1 TeV-106 TeV energy range and their technological feasibility have been extensively reviewed [1]. Several on-going projects and new initiatives for the construction of these so-called neutrino telescopes exist [2]. The ANTARES collaboration [3] aims to deploy a high-energy cosmic neutrino telescope in the Mediterranean sea, 37 km off-shore of La Seyne sur Mer, near Toulon, France. An extensive R&D programme has been carried out to prove the feasibility of such a detector and to measure the relevant environmental parameters of the selected site. The results of this R&D programme and the lay-out of the detector have been described in detail elsewhere [4]. In this paper, the expected performances of the future 0.1 km 2 ANTARES telescope as estimated by computer simulation are reviewed. In particular, the effective area and the angular and energy resolutions are given (section 2), as well as the expected sensitivities for diffuse fluxes, point sources and W I M P searches (section 3).

2. E x p e c t e d p e r f o r m a n c e s The ANTARES collaboration has performed an extensive study by Monte Carlo simulation of the response of the detector to several possible signals. The ingredients entering into the

simulation are the following [5]: Atmospheric muons and neutrinos have been generated in order to simulate the background. The knowledge of the former is presently limited by the available Monte Carlo statistics, whereas that of the latter by the theoretical uncertainty in the prompt neutrino signal from heavy-flavour production at high energies in the atmosphere. The propagation of neutrinos through the Earth and their interaction in the surroundings of the detector have been taken into account. The propagation of the muons produced in the charged-current interactions of the neutrinos with the matter of both the rock and the water and its associated energy loss have also been included. The transmission in water of the Cherenkov light produced by the muons and its associated particles was also taken care of. To this end, the absorption and scattering of light was simulated according to the experimentally measured parameters. A 60 kHz random background on every P M T due to the 4°K in the water and to the continous luminescence background has also been included [6]. The simulation of the detector itself took into account the known behaviour of the P M T s and of the frontend electronics [7]. The intrinsic detection capability of the telescope can be characterized by the muon effective area: the ratio of the number of reconstructed or selected muon events to the incoming muon flux. In Figure 1, the effective area for muons as a function of the parent neutrino energy is shown

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for an isotropic neutrino signal. Selection cuts based on the errors given by the fit have been applied in order to keep only well reconstructed tracks and to reject the atmospheric muon background (some analyses may require less stringent cuts, e.g. those of point-like transient sources at known positions in the sky). The upper curve (squares) includes all the selected events. As can be seen, the effective area increases with energy and it is above the geometrical area at the highest energies. An indication of the angular resolution of the selected events is given by the lower curves in the same plot (circles and triangles), which correspond to those selected events with an angle between the simulated and the reconstructed muon tracks smaller than 1° and 0.3 °, respectively.

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The angular resolution of the telescope is best seen in Figure 2, where the median of the distribution of the angular difference in space between the reconstructed muon track and 1) the true muon

track (solid line) or 2) the original parent neutrino (dashed line) are shown as a function of the neutrino energy. The angle between the neutrino and the produced muon dominates up to around 10 TeV. Above that energy, the instrumental resolution -which depends in particular on the effect of the scattering of light in water and on the overall timing resolution of the detector- is the limiting factor. At the highest energies, a resolution of 0.12 ° can be reached. Note that a good angular resolution helps to reject the background in the case of point-like sources and is therefore of utmost importance in those studies where the source position is relevant. The energy of the crossing muon can be estimated from the amount of light deposited in the PMTs. To this goal, an estimator based on the total number of hits and on the sum of the actual amplitudes recorded in each of the hit PMTs (normalized to the sum of amplitudes expected for a minimum ionizing particle) is built. Monte Carlo studies show that with this estimator the width of the distribution of the logarithm of the ratio of the reconstructed over the actual muon energy,

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decreases with energy from 100 GeV to 100 TeV and remains almost constant above this energy (see figure 3). The relative energy resolution, -~, improves from ~ 5 at 100 GeV to --~ 2.5 at 100 TeV and levels off at this energy. For muon energies below ,,,200 GeV, the energy of the muon can be measured from its range in the water. This can be used to make neutrino oscillation studies (not covered in this review) in the 10-100 GeV muon energy range. 3. E x p e c t e d s e n s i t i v i t i e s The effective area and the resolutions in reconstructed angle and energy given in the previous section define the overall capabilities of the detector for the observation of generic signals. It is the exact nature of a particular signal, however, that defines the strategy to be used for its detection and therefore the actual expected sensitivity. In this section, estimates of the sensitivities to several expected signals are given. 3.1. D i f f u s e f l u x e s A wide variety of predictions for the diffuse neutrino flux at high energy (>1 TeV) exists [8].

Upper limits to this flux can be inferred from the known diffuse cosmic and "y-ray fluxes. In particular, the Waxman-Bahcall upper limit (W&B), E 2 d e / d E = 4.5 x 10 - s GeV cm -2 s -1 sr -1 [10], is customarily taken as a reference. Nevertheless, its applicability is restricted to photohadronic sources with a particular spectral shape and which are transparent to the emission of UHE cosmic rays. More general types of sources may exist which can soften significantly this limit, as pointed out by Mannheim, Protheroe and Rachen (MPR) [11]. A cut on the reconstructed energy in ANTARES has been optimized in terms of the signal to background ratio in order to obtain the best sensitivity of the detector to the neutrino diffuse flux. The analysis indicates that ANTARES can exclude after one year of data taking a diffuse flux above 8.8 x 10 - s E -2 GeV -1 cm -2 s -1 sr -1 for energies greater than 105 GeV (see figure 4 and table 1). Although still over the Waxman-Bahcall limit, this sensitivity is below the M P R estimate [ll]. Note that recent theoretical considerations [12] identify scenarios in which cosmogenic neutrino fluxes are substantially higher than both the W&B and M P R estimates and within the reach of the ANTARES telescope. On the experimental side, the ANTARES sensitivity is up to one order of magnitude better than the present existing limits [13]. 3.2. P o i n t - l i k e s o u r c e s The search for point-like sources can be performed in a variety of ways depending, among other things, on the information assumed a p r / -

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o r / a b o u t the sources. A first general approach is to divide the visible sky in bins of right-ascension and declination (the grid method) or in cones around each of the detected muons (the cluster method) and look for a statistical excess above the expected background in any of the bins or cones. The size of the bins or cones is determined by requiring the same amount of background in each of them and an optimal signal to background ratio. The background includes both atmospheric muons and muons from atmospheric neutrinos and the signal is assumed to follow an E -~ spectrum. Since the background depends only on declination, the bins are of unequal size in this variable and of the same size in right ascension. Under the above requirements an optimal grid of 70 × 70 bins is obtained for the grid method and a cone of 0.7 ° half-width for the cluster method. Although the comparison is not straightforward, note that AMANDA B-10

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Figure 5. Upper limit for neutrino fluxes from point sources as a function of the source's declination, ~, for three different energy spectra. A concrete example of the ANTARES sensitivity to some promising point sources can be obtained using the recent estimates of neutrino fluxes coming from microquasars in the Galaxy [15]. Assuming the neutrino flux predictions given in that reference, ANTARES will be able to detect 6.5 and 4.3 events per year over a background of 0.3 events in a cone of 1° for the GX339-4 and SS433 microquasars, respectively. A most encouraging forecast. 3.3. W I M P s e a r c h If the lightest neutralino is the main component of the cold dark matter in the Universe, neutrali-

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nos in the galactic halo -with velocities of a few hundreds of kilometers per second- will be slowed down by scattering and accumulate by gravitation in the centre of nearby celestial objects as the Earth itself, the Sun and the Galactic Centre and produce neutrinos through their annihilation. ANTARES can perform an indirect search of neutralinos through the search of such neutrinos. The following assumptions have been made to estimate ANTARES sensitivity to neutralinos [16]. The usual supergravity inspired supersymmetry breaking terms in the SUSY Lagrangian are assumed. Electroweak supersymmetry breaking through radiative corrections has been assumed and mass parameters at the weak scale are computed using renormalization group equations. R-parity is assumed to be conserved to ensure the existence of an LSP. In this context, the 5 parameters that define SUSY models have been varied within the following ranges: 40 GeV < m0 < 3 TeV; 40 GeV < rnl/2 < 3 TeV; #>0 tanfl = 10, 45 and 50 (for A0 = 0); tanfl = 20 and 35 (for A0 = 0, +400 and ±800) A total of almost 90,000 points have been generated. A point is rejected if either it is experimentally excluded by LEP limits, by the experimental constraints on the b --* s(7 ) process or if the neutralino is not the LSP at that point. Moreover, among the remaining points only those which are in the theoretically favoured region: 0.03 < ~ x h 2 < 0.3, are kept. The neutralino annihilation is assumed to produce W + W - pairs and therefore, a hard energy spectrum for neutrinos is assumed. In figure 6 the curve of ANTARES sensitivity (90% C.L.) for observing annihilating neutralinos from the Sun after three years of data taking is shown. The lowest detectable muon flux is given as a function of neutralino mass. A muon threshold of 5 GeV has been applied. For comparison, the regions that can be excluded by EDELWEISS II [17] are also shown. The 90% C.L. upper limit on the spin-independent WIMP-nucleon cross-sections, r7Slira that can be set by EDELI ,

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WEISS II is used to divide the SUSY parameter space points in three classes. Triangles (light-grey region), squares (dark-grey region) and circles indicate respectively the SUSY points where the actual spin-independent cross-section is a) above the EDELWEISS II upper limit, b) below it, but above 10% of its value and c) below 10% of the upper limit. As can be seen, ANTARES can provide after three years of running useful limits on the indirect search for dark matter which can complement those obtained from direct searches.

4. Summary The ANTARES 0.1 km 2 neutrino detector is expected to be completed by the end of 2004. Monte Carlo simulations studies show that angular resolutions below 0.2 ° and relative energy resolutions around 2.5 can be reached for ener-

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gies above ,,,100 TeV. Muon effective areas range from 0.01 km 2 at neutrino energies of ,-,1 TeV to almost 0.06 km 2 at a few tens of PeV. After one year of running, ANTARES will be able to exclude a neutrino diffuse flux above E 2 dO/dE = 9 × 10 -s GeV cm -2 s -1 sr -1, still over the Waxman-Bahcall limit, but enough to be sensitive to other broader scenarios of neutrino diffuseflux generating sources. The sensitivity to point sources depends on their declination. For E -2 spectra, fluxes above 10-s c m - 2 S-1 can be excluded for the most favourable declinations in one year. Assuming recent predictions of neutrino fluxes from microquasars in the Galaxy, two such objects, GX339-4 and SS433, would produce in ANTARES 6.5 and 4.3 events per year over a background of 0.3 events. Finally, the indirect limits that can be set by ANTARES after three years of running on neutrinos coming from neutralino annihilation in the Sun are competitive with and, at any rate, complementary to those obtained by direct search experiments.

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