(3) IFIC (CSIC-Univ. de Valencia), apdo. 22085, E-46071 Valencia, Spain
The European collaboration ANTARES aims to operate a large neutrino telescope inthe Mediterranean Sea, 2400 m deep, 40 km from Toulon (France). Muon neutrinos are detected through themuon produced in charged current in- teractions inthe medium surrounding the detector. The Cherenkov light emitted by themuon is registered by a 3D photomultiplier array. Muonenergy can be inferred using 3 different methods based on the knowledge of the features of muonenergy losses. They result in an energy resolution of a factor ∼ 2 above 1 TeV. TheANTARES sensitivity todiffuseneutrinoflux models is obtained from an energy cut, rejecting most of the atmospheric neutrino background which has a softer spectrum. Fake upgoing events from downgoing atmospheric muons are rejected using dedicated variables. After 1 year of data taking, theANTARES sensitivity is E 2
Though not yet sufficiently sensitive, the presented first shower analysis using the initial 6 years of data taken with theANTARESneutrino telescope demonstrates the potential of ANTARESto independently confirm and com- plement the measurement of a high-energy astrophysical neutrinoflux, as performed by IceCube. In order to meet this important goal, several improve- ments of the analysis have been identified and are under way. Building on the gained experience, a second shower reconstruction strategy is developed. It improves on the angular resolution and increases the shower event selection efficiency, while continuing to provide the necessary strong suppression of the atmospheric muon background. Using the track reconstruction already em- ployed in our previous searches for a diffuseflux with muon neutrinos [15, 16], an analysis combining both track-like and shower-like events is in progress. With the addition of the remaining ANTARES data until the scheduled end of its operation time in 2017, this combined search is expected to reach a sensitivity at the level of theflux discovered by IceCube .
The main goal of neutrino telescope experiments such as AMANDA , IceCube , NT-200 in lake Baikal  andANTARES  is the observation of high energy neutrinos from non-terrestrial sources. These instruments detect Cherenkov light from the passage of relativistic charged particles produced inneutrino interactions inthe detector or its surrounding material. By measuring the photon arrival times at known positions, the particle trajectory can be re- constructed. The flight direction of these particles is nearly colinear with the incident neutrino direction for neutrinos above 10 TeV. The best angular resolution can be reached for ν µ , ¯ν µ charged current interactions where the measured particle is a muon that can travel several kilometres in water at TeV energies. Events induced by neutrinos from astrophysical sources must be distinguished from background which originates inthe Earth’s atmosphere. Whereas atmospheric neutrinos are considered a non-reducible background (at least without the use of their energy), atmo- spheric muons can in principle be excluded by simple geometrical considerations. To exclude the contribution from downward-going atmospheric muons, it is sufficient to identify upward-going events, which can only be produced by neutrinos. This distinction requires a reliable determination of the elevation angle, because downward-going at- mospheric muons outnumber upward-going atmospheric neutrinos by 5 to 6 orders of magnitude for typical neutrino telescope installation depths.
If arriving inthe detector an atmospheric muon, or a muon produced in a neutrino CC interac- tion with vertex far from the instrumented volume, induces a long sequence of hits characteristic of a long track. The track reconstruction algorithm used in off-line ANTARES analyses is called AAFit  and it is based on a likelihood fit that exploits a detailed parametrisation of the prob- ability density function for the time of the hits. The algorithm provides the track direction with its estimated angular uncertainty and a proxy for themuonenergy loss, which can be used to estimate the parent neutrinoenergy. Thereconstruction quality is determined by a parameter, referred to as Λ, which is based on the maximum value of the likelihood fit andthe number of degrees of freedom of the fit. The AAFit method is described in  andin this analysis it is mainly used to remove the largest fraction of atmospheric muons inthe data sample. These events are downward going and can be significantly suppressed by a combination of cuts based on the reconstructed track direction andthe Λ quality parameter, as described in .
with P shower = P (N, d, t res |shower) and P muon = P (N, d, t res |muon). These PDFs are based on the same Monte Carlo simulations mentioned in section 2 with an energy spectrum proportional to E −2 for the cosmic neutrinos that induce the showers. The likelihood function shown in equa- tion (14) was developed to achieve an optimal separation of the shower andmuon distributions. This likelihood parameter can be combined with the zenith angle, reconstructed by the established muon-track ﬁtting algo- rithm : On events that have been reconstructed as down-going a harder likelihood ratio cut can be applied. The distribution for this quantity plot- ted before and after the combined cut is shown in ﬁgure 8. This method further reduces the number of atmospheric muons by more than one order of magnitude. Even so, the majority of the remaining events consists still of misreconstructed atmospheric muons.
Theflux of very high-energy neutrinos produced in our Galaxy by the interaction of accelerated cosmic rays with the interstellar medium is not yet determined. The characterization of this flux will shed light on Galactic accelerator features, gas distribution morphology and Galactic cosmic ray transport. The central Galactic plane can be the site of an enhanced neutrino production, thus leading to anisotropies inthe extraterrestrial neutrino signal as measured by the IceCube Collaboration. TheANTARESneutrino telescope, located inthe Mediterranean Sea, offers a favourable view on this part of the sky, thereby allowing for a contribution tothe determination of this flux. The expected diffuse Galactic neutrino emission can be obtained linking a model of generation and propagation of cosmic rays with the morphology of the gas distribution inthe Milky Way. In this paper, the so-called “Gamma model” introduced recently to explain the high-energy gamma ray diffuse Galactic emission, is assumed as reference. Theneutrinoflux predicted by the “Gamma model” depends of the assumed primary cosmic ray spectrum cut-off. Considering a radially-dependent diffusion coefficient, this proposed scenario is able to account for the local cosmic ray measurements, as well as for the Galactic gamma ray observations. Nine years of ANTARES data are used in this work to search for a possible Galactic contribution according to this scenario. All flavour neutrino interactions are considered. No excess of events is observed and an upper limit is set on theneutrinoflux of 1.1 (1.2) times the prediction of the “Gamma model” assuming the primary cosmic ray spectrum cut-off at 5 (50) PeV. This limit excludes thediffuse Galactic neutrino emission as the major cause of the “spectral anomaly” between the two hemispheres measured by IceCube.
with single power law energy spectrum. Two possible spectral indexes are considered for the optimization of the event selection: Γ = 2.0 and Γ = 2.5. The most abundant background comes from penetrating atmospheric muons reaching the apparatus. The MUPAGE simulation code [20, 21] is used to produce samples of atmospheric muon bundles deep underwater. The Earth can be used as a shield to reduce the influence of these muons, by discarding events that are reconstructed as downward-going. Nonetheless, a certain amount of atmospheric muon events could still be mis-reconstructed as upward-going. A selection based on the event reconstruction quality parameters (see below) allows for a significant reduction of their amount.
ν , where E ν is theneutrinoenergy. The median of this angular error is 0.5 ± 0.1 degrees. For the subset of data in which the full 12-line detector was operational, the resolution is estimated to be 0.4 ± 0.1 degrees.
The systematic uncertainty on this quantity has been estimated by varying the time resolution of the OMs ∆ t inthe simulation. The allowed range of ∆ t is determined by requiring that the Λ distribution inthe resulting simulation be compatible with the observed atmospheric neutrino events. The best agreement between data and simulation is obtained for ∆ t = 2.5 ns. Hence, this value is used for all simulations in this analysis, in particular for extracting the central value of the allowed range of angular resolutions. A time resolution of 3.4 ns is found to be incompatible with theneutrinoflux model at the 2σ level, where the uncertainty is taken from Barr et al. (2006). This places an upper bound on the time resolution, which translates into a 1 σ systematic uncertainty on angular resolution of 0.1 degrees. This uncertainty incorporates, to first order, all effects which have a net result of degrading the time resolution, such as possible mis-alignments and inaccuracies inthe simulation of light propagation inthe water or the transit time distribution of the PMT. A similar analysis with analogous results has been performed using downgoing muon data instead of upgoing neutrino candidates.
several GeV) coming from the Sun could not be explained by other known astrophysical processes.
In this paper an indirect search for dark matter by looking for high-energy neutrinos coming from the Sun, using the 2007-2008 data recorded by theANTARESneutrino tele- scope, is reported. The layout of the paper is as follows. In Section 2 , the main features of theANTARESneutrino telescope andthereconstruction algorithm used in this work are described. In Section 3 , the Monte Carlo simulation of the WIMP signal, the background expected from atmospheric muons and neutrinos, andthe grid scan performed to explore the parameter space of the CMSSM and MSSM-7 models are reported. In Section 4 , the method used to optimise the selection of theneutrino events is described. Finally, the results obtained are discussed in Section 5 , where limits on theneutrinoflux are derived from the absence of a signal coming from the Sun’s direction. The corresponding limits on the spin-dependent andthe spin-independent WIMP-proton cross-sections are obtained and compared tothe predictions of the CMSSM and MSSM-7 theoretical models.
gives a ∼ 5 − 10% increase at E ν ∼ 10 5 − 10 8 GeV. With the nCTEQ15 PDF set, we can see the
effect of the updated PDF is about 3 − 44% at the same energy range by comparing the charm induced fluxes with nCTEQ15-proton PDFs andthe BERSS results. We also found that the result with nCTEQ15-nitrogen is less than for proton targets by ∼ 20 − 35% due tothe nuclear effect. The combined effect of these factors listed above result inthemuonneutrino fluxes that are 40 − 60% lower than the BERSS results. In Fig. 2, for completeness, we presented the tau
the background of atmospheric muons and neutrinos only (Aartsen et al. 2013c,b). The specific origin of these events is currently unknown. Some authors propose that at least part of theflux may have a galactic origin (Ahlers et al. 2015; Anchordoqui et al. 2014a,b; Bai et al. 2014; Fox et al. 2013; Padovani & Resconi 2014; Razzaque 2013), whereas others have focused on the extragalactic component (Cholis & Hooper 2013; Kalashev et al. 2013; Roulet et al. 2013; Stecker 2013). Meanwhile theANTARES experiment has proven the feasibility of the Cherenkov telescope technique in sea water (Adri´ an-Mart´ınez et al. 2012a, 2013b). While its instrumented volume is significantly smaller than that of IceCube, its geographical location provides a view of the Southern sky with significantly reduced background for neutrino energies below 100 TeV, and hence better sensitivity to many predicted Galactic sources of neutrinos in this part of the sky. The complementarity of the detectors with respect to Southern sky sources, due to their different geographical location, size and atmospheric muon background, allows for a gain in sensitivity by combining the analyses of data from both experiments in a joint search for point-like sources. The level of improvement depends on the details of the assumed astrophysical flux, in particular on itsenergy spectrum andthe existence of a possible high-energy cut-off. Theenergy spectra are not yet known and predictions vary widely depending on the source model.
2.2 Search above theAntares horizon
A search for coincident neutrino candidates de- tected above theAntares horizon was carried out by selecting downgoing events with β smaller than 1 ◦ . The cuts are optimized on a com- bination of Λ andthe number of hits used inthereconstruction, where a hit corresponds to a PMT signal above a given threshold. The number of hits can be considered as a proxy of themuon/neutrinoenergy. Indeed, downgo- ing atmospheric muons are less likely to produce a number of hits as large as that produced by very high-energy cosmic neutrinos. Anew, the selection criteria were optimized such that one event occurring within the signal time window of 1000 s and located inside the GW error box located above theAntares horizon would lead to a detection with a significance level of 3σ. The final set of cuts is chosen as the one maximizing the fraction of surviving signal events. The fi- nal sample is mostly composed of atmospheric muons with a total of 8.2 × 10 −2 background events expected above theAntares horizon within ±500 s. The median neutrinoenergy that would be detected by Antares for a E −2 signal spectrum is about a factor of 10 higher for this analysis compared tothe search below the hori- zon described in Section 2.1 .
Downgoing atmospheric muons were simulated with the program MUPAGE [12, 13] which provides parametrised muon bundles at the detector. Upgoing neutrinos were simulated according tothe parametrisation of the atmospheric ν µ flux from  intheenergy range from 10 GeV to 10 PeV. The Cherenkov light, produced inside or inthe vicinity of
the detector instrumented volume, was propagated taking into account light absorption and scattering in sea water . The angular acceptance, quantum efficiency and other characteristics of the PMTs were taken from  andthe overall geometry corresponded tothe layout of theANTARES detector . The optical noise was simulated from counting rates observed inthe data. At the same time, the definition of active and inactive channels has been applied from data runs as well. The generated statistics corresponds to an equivalent observation time of 100 years for atmospheric neutrinos and ten months for atmospheric muons.
equatorial coordinates RA=−46.8 ◦ and Dec=−64.9 ◦ and corresponds to a 2.2σ
background fluctuation. In addition, upper limits on theflux normalization of an
E −2 muonneutrinoenergy spectrum have been set for 50 pre-selected astrophys-
ical objects. Finally, motivated by an accumulation of 7 events relatively close tothe Galactic Centre inthe recently reported neutrino sample of the IceCube telescope, a search for point sources in a broad region around this accumulation has been carried out. No indication of a neutrino signal has been found inthe
A novel method to analyse the spatial distribution of neutrino candi- dates recorded with theANTARESneutrino telescope is introduced, searching for an excess of neutrinos in a region of arbitrary size and shape from any direction inthe sky. Techniques originating from the domains of machine learning, pattern recognition and image process- ing are used to purify the sample of neutrino candidates and for the analysis of the obtained skymap. In contrast to a dedicated search for a specific neutrino emission model, this approach is sensitive to a wide range of possible morphologies of potential sources of high-energy neu- trino emission. Theapplication of these methods toANTARES data yields a large-scale excess with a post-trial significance of 2.5σ. Applied to public data from IceCube inits IC40 configuration, an excess con- sistent with the results from ANTARES is observed with a post-trial significance of 2.1σ.
The bulk of primary cosmic rays arriving at the top of the atmosphere has been represented with five groups of nuclei: protons, He, the C, N and O group, the Mg and Si group and Fe, produced according to a power law E −2 over an energy range between 1 and 10 5 TeV/nucleon and zenith angles from 0 ◦ to 85 ◦ . A total number of showers larger than 10 10 has been simulated. All muons from showers reaching the sea level with energies larger than about 500 GeV are transported tothe detector using the program MUSIC , a 3-dimensional muon propagator accounting for the main processes of muonenergy loss. The properties of each muon hitting the can surface are registered for future processing. Each event has a weight accounting for the spectrum used inthe generation. This allows theapplication of a reweight- ing procedure at the analysis stage to account for chemical composition of the primary cosmic rays. Different hypotheses for the primary cosmic ray composition have been considered .
from the vertical axis of the detector (R sh < 300 m) and on the vertical distance above the
centre of the detector (|Z sh | < 250 m ) are applied.
RDF. A different shower reconstruction algorithm was originally developed for diffuseflux anal- yses . Among all available quality parameters provided by this reconstruction chain, a subset of five parameters that showed a high potential to separate atmospheric muon tracks from shower events was chosen as input in a Random Decision Forest (RDF ) classification. The distribution of the RDF parameter for cosmic neutrinos and atmospheric muons and neutrinos after applying the cuts prior tothe RDF cut is shown in Figure 12-left. Only shower events with RDF > 0.3 are used in this analysis.
A simple method for the determination of the atmospheric muon ﬂux andits dependence on depth intheANTARESneutrino telescope has been presented. The method is based on the mea- surement of photon coincidences between adjacent storeys and has a low energy detection threshold. The atmospheric muon ﬂux has been measured inthe depth range from 2030 to 2380 m with a step of 14.5 m using a combined data sample of 5 and 10 line detector conﬁgurations. The data have been corrected for the pres- ence of dead channels and unequal efﬁciencies of the PMTs, which were measured with a novel calibration technique using the natu- ral radioactivity of sea water. A reasonable agreement is found be- tween our data and Monte Carlo simulations.
Systematic uncertainties on the expected number of back- ground events inthe high energy region (R 1 . 31) include: (i) the contribution of prompt neutrinos, estimated as + − 1 0 . . 7 3 events. Inthe following, the largest value is conservatively used. (ii) The uncer- tainties from theneutrino ﬂux from charged meson decay as a function of theenergy. By changing the atmospheric neutrino spec- tral index by ± 0 . 1, both below and above ∼ 10 TeV (when the conventional neutrino ﬂux has spectral index one power steeper than that of the primary CR below and after the knee, respec- tively), the relative number of events for R 1 . 31 changes at most by ± 1 . 1, keeping inthe region R < 1 . 31 the number of MC events equal tothe number of data. The migration from the Bartol tothe Honda MC  produces a smaller effect. The uncertainties on the detector eﬃciency (including the angular acceptance of the optical module  , water absorption and scattering length, trigger sim- ulation andthe effect of PMT afterpulses) amount to 5% after the normalization tothe observed atmospheric ν μ background inthe test region.
The stochastic nature of GRBs, in addition with the complex dynamical evolution of the jet, makes it hard to reliably determine a bulk Lorentz factor. Inthe previous ANTARES search, as well as in several IceCube searches, a default value of Γ = 316 was used ( Adrián-Martínez et al. , 2013 ). In Fig. A.11(a) the stacking fluence obtained in this work is compared with the previous ANTARES estimation, both computed with one- zone modelling of NeuCosmA. However, in this work, a novel method for the estimation of Γ is presented, consisting into exploiting the observed correlation among Γ andthe burst’s isotropic luminosity, as found by Lü et al. 2012 and reported in Eq. ( 2 ). Nonetheless, such a correlation cannot be used straightforwardly in most of the cases, since it would require the knowledge of the redshift for each GRB of the sample. Unfortunately redshift is unknown in 90 per cent of the cases: in this situation, for each GRB with 𝑧 not measured, up to 1000 values of redshift are randomly extracted from a redshift distribution that follows that of long GRBs detected since 2005 by the Swift satellite (see Fig. 2 ). Then, from such 1000 values of 𝑧, 1000 values of bulk Lorentz factor are calculated through Eq. ( 2 ). By averaging the resulting 1000 values of Γ for an individual GRB, hΓi is obtained. The resulting cumulative neutrino fluence is shown in Fig. A.21(a) , where it is also compared with the expected neutrino fluence estimated by the previous ANTARES analysis ( Adrián-Martínez et al. , 2013 ). The two are observed at a comparable level, even though the latest analysis has more than twice more sources than the previous. This result is in fact a consequence of theneutrino modeling adopted: while past predictions tended to overestimate the expected flux by assuming standard values for model parameters, here an accurate modeling is realized by accounting for variations in these parameters reflecting the properties of observed GRBs. An example is given in Fig. A.21(b) , where the distribution of the hΓi values obtained for each burst is shown and compared with the standard value used inthe past. The obtained distribution peaks at a value lower than Γ = 316.