Poster Excerpt 13: Search for ultra-high energy neutrinos from below and above the horizon with the ANTARES Telescope
(The following text has been submitted by Chiara Perrina (“La Sapienza” University and INFN, Roma, Italy) for the ANTARES Collaboration)
One of the main questions in Physics is the origin of high energy cosmic rays. In the last decade great progress has been made related to the energy spectrum and composition of cosmic rays, but their origin remains unknown. Neutrinos are complementary to cosmic rays and photons to explore the high-energy sky, as they can emerge from dense media. They can travel across cosmological distances without being deflected by magnetic fields nor absorbed by ambient matter and radiation. High-energy (> TeV) neutrinos are expected to be produced in a wide range of astrophysical objects, from galactic sources such as Supernovae Remnants or Microquasars to the most powerful extragalactic emitters such as Active Galactic Nuclei and Gamma-Ray Bursts . Neutrino Astronomy is challenged both by the weakness of neutrino interactions and by the feebleness of the expected fluxes of cosmic neutrinos. Neutrino telescopes require the instrumentation of large volumes of transparent medium (water or ice) with arrays of photosensors, in order to detect the Cherenkov light induced by the charged leptons produced in the neutrino interactions with matter in or around the detector. Muons are the most straightforward detection channel, but showers induced by electron neutrinos and tau neutrinos can also be detected. The timing, position and amplitude of the light pulses recorded by the photosensors allow the reconstruction of the muon trajectory, providing the arrival direction of the parent neutrino and an estimation of its energy. Neutrino telescopes are installed at great depths and optimized to detect up-going muons produced by neutrinos that have traversed the Earth, in order to limit the background from down-going atmospheric muons. Atmospheric neutrinos produced in cosmic-ray-induced air showers can also traverse the Earth and interact close to the detector, providing an irreducible source of background with energy spectrum . Neutrinos fluxes of astrophysical origin, which are expected to be harder (typically with ), can then be identified as an excess of events above a certain energy. The observations of neutrinos from astrophysical sources could trace the existence of hadronic processes inside the sources, and it could also reveal types of objects in the Universe yet unobserved in photons or cosmic rays.
2. Neutrino detection with the ANTARES Telescope
ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch) is the first undersea neutrino telescope and the only one currently operating . It is located at a depth of 2475 m in the Mediterranean Sea, 40 km off the French coast near Toulon. It consists of a matrix of 885 photomultiplier tubes (PMTs) arranged into 12 strings anchored to the sea bed and maintained vertical by buoys, connected to a junction box which distributes the electrical power and transmits the data to shore through an electro-cable (Figure 1). Acoustic devices and inclinometers regularly spread along the strings allow to accurately monitor the position and orientation of the PMTs , and time calibration is performed by means of an in situ array of laser and LED beacons . The PMTs are orientated at 45° downwards in order to maximize the sensitivity to Cherenkov light from upcoming muons. The median angular resolution achieved for muon tracks is < 0.5° allowing good performance in the searches for neutrino point sources. Its location in the Northern Hemisphere allows for surveying a large part of the Galactic Plane, including the Galactic Centre, thus complementing the sky coverage of the IceCube detector installed at the South Pole.
3. Searches for point-like sources of cosmic neutrinos from below and above the horizon
The aim of point-like sources searches is detecting significant excesses of events from particular spots (or small regions) of the sky. These searches have been performed  on a data sample covering the period from February 2007 until the end of December 2012, for a total live time of 1339 days. The final neutrino sample has been obtained after tight cuts on the reconstruction quality parameter called L, on the estimated angular resolution (< 1°) and on the zenith angle (). It contains 5516 neutrino candidates, with a predicted atmospheric neutrino purity of around 90%. The median uncertainty on the reconstructed neutrino direction, assuming an energy spectrum, is (0.4 ± 0.1)°. Two different searches have been performed:
- a time-integrated full-sky search looking for an excess of events over the atmospheric neutrino background in the declination range [-90°, +48°];
- a candidate-list search looking for events in the directions of a predefined list of 51 candidate sources of interest which are known gamma-ray emitters and potential sites for hadronic acceleration.
In both searches, no significant excess over the background has been found and upper limits derived (Figure 2).
Until now the point-like source search has been restricted to the Southern Hemisphere: only up-going events have been studied in order to reject the muon background from cosmic-ray-induced showers. The development of a strategy for the identification of down-going neutrino events is going on with the aim to extend the field of view of ANTARES and increase the energy threshold of the search. There are three important variables in such analysis: the energy, the direction and the track reconstruction quality (Λ) of the event. The separation signal/background can be obtained (Figure 3) by looking for high energy, mostly horizontal () and well reconstructed events.
Figure 2 90% CL flux upper limits and sensitivities on the muon neutrino flux for six years of ANTARES data . IceCube results are also shown for comparison. The light-blue markers show the upper limit for any point source located in the ANTARES visible sky in declination bands of 1°. The solid blue (red) line indicates the ANTARES (IceCube) sensitivity for a point-source with an E?2 spectrum as a function of the declination. The blue (red) squares represent the upper limits for the ANTARES (IceCube) candidate sources. The dashed dark blue (red) line indicates the ANTARES (IceCube) sensitivity for a point-source and for neutrino energies lower than 100 TeV, which shows that the IceCube sensitivity for sources in the Southern hemisphere is mostly due to events of higher energy. The IceCube results were derived from .
Figure 3: Zenith angle Lambda and energy distribution for MC signal (blue) and background events (red).
- K. Becker. High-energy neutrinos in the context of multimessenger physics. Phys. Rept. 458, 173 , 2008. http://arxiv.org/abs/0710.1557.
- Ageron M. et al. (ANTARES Collaboration). ANTARES: the first undersea neutrino telescope. Nucl. Instrum. Meth. A 656, 11-38 , 2011. http://arxiv.org/abs/1104.1607.
- Adrian-Martinez, M. Ageron, J.A. Aguilar, I. Al Samarai, A. Albert, et al. The Positioning System of the ANTARES Neutrino Telescope. JINST, 7:T08002, 2012. http://arxiv.org/abs/1202.3894.
- A. Aguilar et al. Time Calibration of the ANTARES Neutrino Telescope. Astropart. Phys., 34:539-549, 2011. http://arxiv.org/abs/1012.2204.
- Adrian-Martinez et al. Searches for Point-like and extended neutrino sources close to the Galactic Centre using the ANTARES neutrino Telescope. JCAP05 (2014) 0001. http://arxiv.org/abs/1402.6182.
- G. Aartsen et al. Search for time-independent neutrino emission from astrophysical sources with 3 years of IceCube data. ApJ 779 132, 2013. http://arxiv.org/abs/1307.6669.
 L is defined as , where is the number of degrees of freedom of the track, the number of compatible solutions and L is the likelihood of residual time between the theoretical and the experimental arrival time of the photons on the PMTs.