The identification of the cosmic-ray light nuclei with the GAPS experiment

GAPS (General AntiParticle Spectrometer) is a balloon-borne large-acceptance experiment designed to detect low-energy (< 0.25 GeV/n) cosmic-ray antinuclei during three ∼35-day Antarctic flights, the first of these planned for the 2024-2025 austral summer. The GAPS experiment, currently in preparation for the first flight, consists of a tracker equipped with large-area lithium-drifted silicon detectors and surrounded by a large-acceptance time-of-flight system made of plastic scintillators. This design has been optimized to perform a novel antiparticle identification technique based on an antinucleus capture and the subsequent exotic atom formation and decay, allowing more active target material and a larger geometrical acceptance since no magnet is required. Although detecting the cosmic-ray antinuclei as an indirect dark-matter signature is the primary goal of GAPS, many low-energy cosmic-ray nuclei will also be recorded. Nuclei do not form exotic atoms in the GAPS detectors, and their detection is based on the measurements of the ionization energy depositions, evaluation of the kinetic energy, and the stopping depth relative to the measured velocity. An algorithm was developed to fit the slow-down of particles and antiparticles tracked inside GAPS. The quantities fitted by this algorithm, together with the measured velocity and energy deposition information, allow the identification of protons, deuterons, and helium nuclei and the measurement of their spectra in a low-energy range (< 0.25 GeV/n). The results of this analysis, based on detailed Monte Carlo simulation studies, will be presented in this contribution.