The ANTARES neutrino telescope is located on the bottom of the Mediterranean Sea, 40 km off the French coast. The detector is installed at a depth of 2.5 km and consists of a three-dimensional array of 885 photomultipliers tubes arranged on 12 detector lines. The ANTARES collaboration aims to detect high-energy neutrinos from extraterrestrial origin. Relativistic muons emerging from charged-current muon neutrino interactions in the detector surroundings produce a cone of Cerenkov light which allows the reconstruction of the original neutrino direction. The collaboration has implemented different methods to search for neutrino point sources in the data collected since 2007. Results obtained with these methods as well as the sensitivity of the telescope will be presented.

Astrophysical point source search with the ANTARES neutrino telescope

Salvatore Mangano (manganos@ific.uv.es)IFIC (Instituto de Física Corpuscular) CSIC – University of Valencia, Spain

on behalf of the ANTARES Collaboration

Neutrino2010, XXIV International Conference on Neutrino Physics and Astrophysics, Athens (Greece), 14-19 June 2010

Why astrophysical source search using neutrinos?The main advantages of the neutrinos with respect to other particles are that they are not deflected by magnetic fields and are weakly interacting so that they can travel long distances.The drawback is that the weak interactions of neutrinos imply that a very massive detector is required to observe a measurable flux from astrophysical sources.

Cosmic accelerator

reach the detector, not deflected

absorbed by matter

p deflected by magnetic fields, GZK effect

CMB

Earth

X

What is the detection technique?The Earth acts as a shield against all particle except neutrinos. The detection technique requires discrimination of upward going muonsagainst the much higher flux of downward going atmospheric muons.

Fixed source search:The information of the presence of a luminous source such as gamma rays can be used to constrain the search to specific regions of the sky. By specifying the direction of the source the chance of the background to imitate a signal is reduced.

Unbinned clustering method:The Expectation-Maximization (EM) algorithm is based on an analytically likelihood maximization. The signal density distribution is assumed to be a two-dimensional gaussian and the background density distribution is taken from the declination distribution of the data. The probability to have signal for a given background model is maximized. The free parameters are the two standard deviations of the two-dimensional gaussian probability density function and the expected number of events from the source.

Relativistic muons created in charged-current muon neutrino interactions in the detector and its surroundings emit Cerenkov light in a cone with fixed opening angle which allows the indirect detection of the interacting neutrino.

Is there a point-source in the sky that is producing clustering of events that cannot be explained by the backgrounds of atmospheric neutrinos and muons at 3-sigma level?

The figure shows the number of muons reconstructed with some selection criteria as a function of the angle of incidence (+1 means vertical downward going and -1 vertical upward going). Data from year 2007 and 2008 are used making a total of 341 days of detector lifetime.

With the direction and time of the upward going muons the following sky view is obtained (scrambled).

For both figures the angular resolution of the signal is injected according to a gaussian distribution with a sigma of 0.2 degrees. The algorithm fits correctly the sigma of the two-dimensional gaussian signal distribution when there exist enough injected signal events. Also the number of injected signal events is estimated correctly by the algorithm.

Distribution of the likelihood ratio forthe background only case and for the case where a number of signal events are present.

Discovery potential as a function of the average number of events emitted by a source for a hypothetical angular resolution of the detector given by a gaussian with a sigma of 0.2 or 3 degrees. All three algorithms are used for the same sample.

Two additional algorithms have been tested:1) The EM fixed sigma algorithm, where the sigma of the two dimensional gaussian distribution is fixed. This algorithm has only one fit parameter, namely the number of signal events.2) The Likelihood algorithm, which first calculates the angular distance between the fixed source search point and the location of all selected events in the sky. Then it fits this distribution with a signal and background density distribution using maximization technique. The signal distribution is the point spread function which is calculated using Monte Carlo and the background distribution is taken from the data.

Motivation and method

The likelihood ratio, i.e. the ratio of probability to have the hypothesis of mixed signal and background model over the probability of only background, is the criterion to confirm or reject the existence of a signal. The better the discrimination of these models, the better is the performance of the algorithm.

The angular resolution of the detector is taken from a realistic reconstruction (around one degree) as used for the 2008 data. The conclusion are similar to the one of gaussian angular resolution. A more sophisticated reconstruction algorithm (0.3 degree of angular resolution) will be used in the next analysis.

Data from 2007 with the 5-line detector are analyzed by two of the in the poster presented algorithm searching for signals from a selected list of sources.

Flux upper limit of ANTARES compared with other experiments

Algorithm to search point sources

Performance and results

Conclusion from this figure:The better the angular resolution the easier is to find point sources.EM with fixed sigma has better performance then EM with free sigma.The gain in performance is more pronounced when the angular resolution is better.EM with fixed sigma has similar performance as the Likelihood method.