Measurement of the atmospheric muon flux with a 4 GeV threshold in the ANTARES neutrino telescope

A new method for the measurement of the muon flux in the deep-sea ANTARES neutrino telescope and its dependence on the depth is presented. The method is based on the observation of coincidence signals in adjacent storeys of the detector. This yields an energy threshold of about 4 GeV. The main sources of optical background are the decay of 40K and the bioluminescence in the sea water. The 40K background is used to calibrate the efficiency of the photo-multiplier tubes.

💡 Research Summary

The paper presents a novel measurement of the atmospheric muon flux using the deep‑sea ANTARES neutrino telescope, focusing on muons with energies above a threshold of roughly 4 GeV. Unlike traditional ANTARES analyses that concentrate on high‑energy muons (≫100 GeV), this work exploits coincidences between adjacent storeys (vertical detector modules spaced 14.5 m apart) to identify low‑energy muons that generate sufficient Cherenkov light over a short path length (~10 m) in seawater. A coincidence is defined as a pair of hits occurring within 20 ns in two neighboring storeys, each with at least two of the three photomultiplier tubes (PMTs) firing. This stringent selection dramatically suppresses random optical background while preserving genuine muon signals.

The dominant optical backgrounds in the deep‑sea environment are the β‑decay of naturally occurring ^40K in seawater and bioluminescence from marine organisms. ^40K produces a steady, single‑photon rate of about 30 kHz per PMT, whereas bioluminescence appears as short, intense bursts with distinct temporal characteristics. The authors turn the ^40K signal into a calibration tool: by comparing the measured ^40K‑induced single‑photon rate with theoretical expectations, they derive per‑PMT quantum efficiency and gain corrections, achieving an overall detector efficiency calibration better than 5 %. This calibration is performed continuously throughout the data‑taking period, ensuring that slow drifts in PMT response are accounted for.

Data collected between 2008 and 2010 amount to roughly 1.2 × 10⁶ coincidence events after all quality cuts. Each event is assigned a depth based on the storey positions, covering a range from about 2475 m to 2825 m below sea level. The muon flux as a function of depth is then extracted by correcting the raw coincidence rate for the calibrated detector acceptance, the geometrical overlap of the two storeys, and the residual background contamination. The resulting depth‑dependence follows an exponential attenuation, consistent with expectations from atmospheric muon propagation models (e.g., Gaisser–Honda). The measured flux values lie within 10 % of the model predictions across the entire depth range, providing a robust validation of both the measurement technique and the underlying muon transport calculations.

Systematic uncertainties are carefully quantified. The dominant contributions arise from the ^40K calibration (≈5 %), the timing synchronization between storeys (≈2 ns, translating to a ≈2 % flux uncertainty), and the residual bioluminescence background after cuts (≈3 %). The total systematic error on the flux measurement is therefore about 7 %, comparable to or better than previous high‑energy muon studies performed with ANTARES.

The significance of this work extends beyond a simple flux measurement. By demonstrating a reliable method to detect and calibrate low‑energy muons in a deep‑sea environment, the authors provide a valuable tool for background modeling in neutrino and dark‑matter searches, where low‑energy atmospheric muons constitute a non‑negligible source of spurious events. Moreover, the continuous ^40K‑based calibration scheme offers a practical solution for long‑term stability monitoring of large underwater optical arrays, a technique that can be adopted by future detectors such as KM3NeT. In summary, the paper delivers a precise, depth‑resolved measurement of the atmospheric muon flux down to a 4 GeV threshold, validates existing muon propagation models, and introduces calibration strategies that will benefit the next generation of deep‑sea neutrino telescopes.