[Paper Review] Skymap for atmospheric muons at TeV energies measured in deep-sea neutrino telescope ANTARES

S. Mangano|arXiv (Cornell University)|Aug 6, 2009

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This paper presents the first skymap of down-going atmospheric muons at TeV energies using one year of data from the ANTARES deep-sea neutrino telescope, with five of twelve lines operational. Using a linear fit reconstruction and exposure-corrected right ascension normalization, it identifies a non-uniform distribution in right ascension due to non-uniform data-taking, which is corrected to reveal potential cosmic-ray anisotropies with significance calculated via Poisson statistics.

ABSTRACT

Recently different experiments mention to have observed a large scale cosmic-ray anisotropy at TeV energies, e.g. Milagro, Tibet and Super-Kamiokande. For these energies the cosmic-rays are expected to be nearly isotropic. Any measurements of cosmic-rays anisotropy could bring some information about propagation and origin of cosmic-rays. Though the primary aim of the ANTARES neutrino telescope is the detection of high energy cosmic neutrinos, the detector measures mainly down-doing muons, which are decay products of cosmic-rays collisions in the Earth's atmosphere. This proceeding describes an anlaysis method for the first year measurement of down-going atmospheric muons at TeV energies in the ANTARES experiment, when five out of the final number of twelve lines were taking data.

Motivation & Objective

• To search for large-scale anisotropies in the flux of atmospheric muons at TeV energies using data from the ANTARES neutrino telescope.
• To assess the impact of non-uniform data-taking and background fluctuations on the observed sky distribution of muons.
• To develop and apply an exposure-corrected normalization method to the right ascension distribution to isolate potential cosmic-ray anisotropies.
• To validate the detector's capability for high-statistics muon reconstruction and future anisotropy sensitivity.

Proposed method

• Data from 10.7 million muon events collected between February 2007 and December 2007 using five of twelve ANTARES detection lines were analyzed.
• Muon tracks were reconstructed using a linear fit algorithm based on Cherenkov photon arrival times and photomultiplier positions, selecting only tracks seen on at least two lines and six floors.
• The detector's local coordinates (zenith and azimuth) were converted to equatorial coordinates (right ascension and declination) using the time of track reconstruction.
• The right ascension distribution was corrected for exposure non-uniformity by modeling the time-dependent data-taking efficiency and background interruptions.
• Poisson statistics were used to calculate the probability of observing a given number of events in each sky bin, with significance defined as S = -log₁₀(P) for each bin.
• The analysis focused on declination bands, estimating expected background ν_b from average event rates and comparing to observed counts n_obs.

Experimental results

Research questions

• RQ1 Is there a measurable large-scale anisotropy in the flux of TeV-energy atmospheric muons as seen by ANTARES?

Key findings

• The raw right ascension distribution of muon events showed non-uniformities due to non-uniform data-taking and background rate variations, with exposure fluctuations matching the observed modulation.
• After exposure correction, the right ascension distribution became uniform within statistical expectations, indicating that non-uniform data-taking was the dominant source of apparent anisotropy.
• The significance of any potential cosmic-ray signal in individual sky bins was quantified using Poisson-based significance S = -log₁₀(P), with P calculated from observed vs. expected event counts.
• The analysis demonstrated that the detector's muon reconstruction and exposure modeling are robust enough to enable future searches for cosmic-ray anisotropies with higher statistics.
• The method developed is applicable to future ANTARES data with full detector configuration, where increased statistics will improve sensitivity to anisotropies.
• The results confirm that atmospheric muon data from ANTARES can be used to probe primary cosmic-ray anisotropies at TeV energies, provided exposure effects are properly corrected.