Study of cosmic-ray proton and deuteron solar modulation and its relevence for indirect dark-matter searches with the PAMELA and GAPS experiments

Cosmic rays (CRs) are electrically charged particles accelerated by astrophysical powerful sources. CRs consists mainly of protons and helium nuclei and in minimal part of other nuclei and electrons. Among these, there are the deuterons, i.e. the deuterium nuclei, which are produced as a secondary component from interactions of CRs with the interstellar medium. During their voyage in the Galaxy, CRs experience energy losses, fragmentations in secondary products, trajectory deflections by galactic magnetic fields and generally a change of the spectral features. Before reaching the Earth, CRs propagate through the Heliosphere, the space region permeated by the solar magnetic field embedded in the turbulent solar wind (SW) flowing from the Sun. The interaction with the heliospheric magnetic field (HMF) and the SW makes CR fluxes decrease with respect to the intensity of their energy spectra outside the Heliosphere. A time dependence of the CR fluxes is also induced by the different phases of the 11-year cycle of solar activity and of the 22-year cycle of the HMF. The latter also induces polarity-dependent drift motions, which are followed by CRs of opposite charges in the opposite direction. With the current CR detector sensitivity, all these effects of CR solar modulation are measurable up to about 50 GeV. The charge-sign dependence introduced by the drift motions in the Heliosphere is relevant for the studies of the galactic antinuclei, which are rare CR components considered promising probes for indirect searches for dark matter (DM). Especially the antideuterons, according to a variety of DM models, could be produced by DM annihilation or decay with a flux of orders of magnitude above the astrophysical background of secondary antideuterons for energies below a few GeV/n, where the solar modulation effects are significant. In this work, a new measurement of the time-dependent galactic CR proton and deuteron fluxes between 50 and 800 MeV/n are presented. This analysis was conducted on the data collected by the PAMELA experiment from July 2006 to September 2014. This period is particularly relevant for solar-modulation studies because it covers almost a whole solar cycle and includes a change of solar magnetic polarity in 2013 as well. A set of selections was developed to extract clean samples of galactic protons and deuterons from the data maintaining a high selection efficiency. As a result, nine yearly rigidity spectra from 2006 to 2014 have been measured between 0.6 and 2.8 GV for both galactic protons and deuterons. The obtained deuteron fluxes were compared with those obtained from a numerical 3D model of cosmic-ray propagation in the Heliosphere. Such comparison can be useful to constrain the modulation parameters of the model for the different heliospheric conditions. The deuteron-to-proton flux ratios were also calculated and can be used to study as the solar modulation effects depend on the different local-interstellar-spectrum shapes as well as from the charge-to-mass ratio dependence of the principal propagation mechanisms in the Heliosphere. All these results can help to improve the accuracy of the solar modulation modelling of the cosmic-ray deuteron fluxes, which is paramount to properly model the solar-modulation effects on the expected energy spectra of antideuterons, reducing the current related uncertainty. A preliminary study of the deuteron acceptance achievable with the GAPS experiment was also performed using simulated data. Although the GAPS apparatus has been designed to perform a novel exotic-atom formation and decay technique to detect CR antinuclei, particle detection is possible at low energy. An analysis technique was developed to identify the deuteron Bragg peaks occurring in the GAPS detectors and separate in this way deuterons from the larger background of protons. The obtained results about deuteron acceptance and proton power rejection are provided in the thesis.