Muon induced electromagnetic shower reconstruction in the ANTARES neutrino telescope

1. ICRC2011, 32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011 Muon induced electromagnetic shower reconstruction in the ANTARES neutrino telescope The primary goal of the ANTARES telescope is the detection of high energy cosmic muon neutrinos. The neutrinos are identified by the upward going muons that are produced in charged current neutrino interactions in the vicinity of the detector. The Cerenkov light produced by the muons in the detection volume is measured accurately by an array of photosensors. Muons that are going downward are background for neutrino searches. These muons are the decay products of cosmic-ray collisions in the Earth's atmosphere. The energy loss in water of a muon with an energy above 1 TeV is characterized by discrete bursts of Cerenkov light originating mostly from pair production and bremsstrahlung (electromagnetic showers). This poster presents a method to identify and count electromagnetic showers produced by the muons. The method can be used to select a sample of highest energy muons with the ANTARES detector. Salvatore Mangano (manganos@ific.uv.es) IFIC (Instituto de Física Corpuscular) CSIC – University of Valencia, Spain on behalf of the ANTARES Collaboration Introduction Figure 1 Figure 2 ANTARES The ANTARES neutrino telescope (see Figure 1) is located at a depth of 2475 m in the Mediterranean Sea, near the French coast. Its main objective is the observation of extraterrestrial neutrinos. Relativistic charged leptons produced by neutrino interactions in and around the detector produce Cerenkov light in the sea water. This light is detected by an array of photomultiplier tubes, allowing the muon direction to be reconstructed. Although the ANTARES detector is optimised for upward going particle detection, the most abundant signal is due to atmospheric downward going muons produced in the particle showers induced by the interactions of cosmic-rays in the atmosphere. In order to reduce this background the Earth is used as a filter, restricting the search for cosmic neutrinos to sources in the Southern sky. Energy loss The processes contributing to the energy loss of a muon in water include ionization, e+e- pair production, bremsstrahlung, and photonuclear interactions. Below about 1 TeV, the muon energy loss is dominated by the continuous ionization process. Above about 1 TeV, the muon energy loss is characterised by large energy fluctuations and discrete bursts originating from pair production and bremsstrahlung (electromagnetic showers). The average muon energy loss per unit track length due to these electromagnetic showers increases linearly with the energy of the muon allowing its energy to be determined (see Figure 2). Counting electromagnetic showers along muon tracks gives an estimate of the muon energy and can help in designing a better energy reconstruction algorithm. Algorithm for shower reconstruction Figure 3 Method The shower reconstruction algorithm relies on the identification of increased photon emission along the muon trajectory. Algorithm The shower identification algorithm proceeds as follows. First, the muon trajectory must be determined. This is done using a standard tracking algorithm. The fitted trajectory is used to compare the measured hit times to the expected time of direct Cerenkov photons. Their hit times are categorized into three groups: 1. Extremely late or extremely early hits are assumed to be background hits and are rejected. 2. Early hits contain mostly muon Cerenkov photons whose emission positions ζiCK (see Figure 3) along the muon track are given by 3. Late hits contain the largest fraction of hits due to shower photons. These shower photons may not necessarily be emitted at the Cerenkov angle from the muon track. Therefore the emission angle is left as a free parameter and, with the photon emission taking place at ζi (see Figure 3), the hit time is given by This equation has two distinct solutions, ζi+ and ζi-. All calculated ζiCK , ζi+ and ζi- positions along the muon track are collected in a one dimensional histogram. The shower position is identified by the localised increase of the number of emitted photons along the reconstructed muon trajectory, identified by a peak in the histogram. An example of the application of this procedure to data can be seen in Figure 4. Two excesses are visible that can be attributed to the two reconstructed showers. Figure 4 A muon emits Cerenkov photons continously on its trajectory in water with sometimes a discrete electromagnetic shower (upper panel). The emission points of the photons from the muon trajectory are calculated from the measured 3-dimensional position and arrival time in the PMTs. These emission points are presented by an unidimensional histogram with its Z-axis aligned with the muon axes. In the histogram the peaks identify the electromagnetic shower position (lower panel). Selection and results Figure 6 Selection The selection and performance of the shower identification algorithm has been studied and validated with a sample of simulated atmospheric multi-muons (Corsika). The shower induced photon emission along the muon trajectory results in localised peaks as seen in Figure 4. The task to identify a shower is then reduced to a one dimensional peak finding algorithm whose result can be characterised by three parameters, namely the center, width and height of the identified peak. The analysis has been tuned to select showers with a high level of purity, possibly at the expense of efficiency. Results The main result of the shower reconstruction algorithm is the shower multiplicity per atmospheric muon event. The atmospheric muon events are usually muons in a bundle with an average multiplicity of around 3.3. Figure 5 shows the event rate as a function of the number of generated showers with shower photons detected on at least five different storeys. Also shown is the number of reconstructed showers selected with a given set of cuts. The average shower reconstruction efficiency over all shower energies is around 4%. The reconstruction algorithm starts to be efficient for shower energies above 300 GeV. Figure 6 shows the number of generated showers with shower photons detected on at least five different storeys per atmospheric muon event as a function of the muon energy. The muon energy refers to the muon with the largest energy in the bundle. The number of generated shower and reconstructed shower increases as a function of the muon energy. Figure 7 shows the energy distribution of all muons as well as the one with at least one reconstructed shower. The muons have an average energy of 1.2 TeV, whereas muons with at least one identified shower have on average 2.5 times higher energy. Figure 5 Figure 7 Outlook The aim of this poster has been more to demonstrate the capability to detect electromagnetic showers than to make precise measurements. A more elaborated analysis is needed, in order to use the number of electromagnetic showers as a robust estimate of the muon energy. Moreover, the method discussed here for selecting showers emitted by downward going muons could be used also for upward going muons with the main purpose of selecting the highest energy upward going muons.