Cosmic rays as a probe of new physics

Principal Investigator: Riccardo Munini


  • Astroparticle Physics


Cosmic rays are a sample of solar, galactic, and extragalactic matter, which include the nuclei of all elements and their isotopes, electrons, and antiparticles. They are associated with the most energetic events and active objects in the Universe, such as supernovae explosions, pulsars, relativistic jets, and active galactic nuclei, even if unambiguous evidence that these are the sources is still to be found.

Besides relevant astrophysical information about sources and the Galactic environment, cosmic rays may shed light on the nature of dark matter and the existence of relic antimatter. Particularly significant for these studies is cosmic-ray antimatter. Measurements of cosmic-ray antiparticle spectra have shown intriguing features that may indicate contributions from the annihilation or decay of dark matter particles. However, uncertainties on the secondary production of antiparticles, by the interaction of cosmic rays with interstellar matter, and their propagation in the Galaxy and the heliosphere affect a comprehensive understanding of their origin. Heavier cosmic-ray antinuclei, such as antideuterons and antihelium, are also predicted to be produced as secondaries. Yet, their spectra at low energies (below a few GeV/n) are expected to be orders of magnitude lower than those of antideuterons and antihelium produced by plausible models of dark matter annihilation or decay. However, at low energies, these particles are significantly affected by the propagation in the heliosphere. We propose to study the relevance of these measurements along with their phenomenological and experimental challenges. Other aspects of the interplay between dark matter and cosmic ray physics will be explored as well.

Status of project and perspectives:

Status of project: In recent years, through the cooperation of experimental and theoretical groups fostered by the IFPU environment, it was possible to obtain several of the set goals of this project. The foremost activity was an IFPU focus week program held in person in November 2021. Specifically, we compared experimental data from different experiments and numerical models. The analysis of the energy spectra of the protons of PAMELA, AMS02, and BESS-POLARII allowed the calibration of a numerical model describing the propagation of cosmic rays in the Heliosphere for various epochs of solar activity. The calibration process was performed with a multi-parameter optimization technique, developing a Markov Chain. The model made it possible to reproduce the experimental data with an accuracy of a few percent. Then, the obtained parameters were used for an inverse process to estimate the best Local Interstellar Spectrum (LIS) of antiprotons, thus developing an independent procedure to estimate the LIS than the usual approaches based on galactic propagation codes like GALPROP. Through a Markov Chain, the best spectral shape for the antiproton LIS was found.

Our proposed future objectives include:

Cosmic-ray propagation in the heliosphere: Our project aims to fine-tune a state-of-the-art numerical model for cosmic-ray propagation through the Heliosphere using different cosmic-ray data sets from balloon- and space-born experiments. The group plans to estimate the uncertainties related to each propagation parameter and, thus, the global uncertainty on the energy spectra associated with solar modulation. Furthermore, the group will continue to improve the study of systematic uncertainties on the antiproton LIS, including those related to the experimental measurements. Differing from the approaches used by other groups, it will allow for a comprehensive study of the experimental uncertainties of the various data sets. Finally, comparing the estimated antiproton LIS with theoretical prediction will allow competitive interpretation of the data in search of possible signals of new physics. Additionally, estimations on energy spectra of heavier antinuclei species, e.g., antideuterons and antihelium nuclei, will be drawn in anticipation of forthcoming experimental results expected by the GAPS experiment.

Tests on Particle Dark Matter models: Tests of the interaction between dark matter particles and ordinary matter have been mostly performed in definite regimes motivated by either a given particle dark matter scenario or a given framework for the generation of dark matter in the early Universe; this include, e.g., Weakly Interacting Massive Particles (WIMPs) or axions as dark matter candidates. Extending direct detection searches to a more generic set of dark matter couplings and masses is non-trivial since direct detection experiments, to avoid background contaminants, need to highly optimized on specific signatures and energy windows. In the latest years it has been realized that cosmic rays, as well as messengers, having an energetic spanning many orders of magnitude, may serve as an efficient probe of the interaction between dark matter and ordinary matter in a much more model independent particle setups. We plan to extend these studies considering, e.g., the interaction of dark matter in the Milky Way halo with galactic cosmic rays, or possible effects on dark matter at the highly energetic astrophysical sources wher most probably cosmic rays are accelerated.

Cosmic-ray propagation in galactic environments: The propagation of galactic cosmic rays is usually treated as a diffusive-convective process, sketching the random walk of charged particles in magnetic field irregularities. Turbulence in a galaxy with sizeable star formation rate, such as the Milky Way, is usually assumed to be injected in the system as an astrophysical feedback, such as from supernova explosions; on the other hand, recent analyses – mostly in connection to spectral features in local cosmic ray fluxes – have shown that streaming instabilities of CR themselves may be an additional and relevant source of turbulence in the Galaxy. Self-generation of magnetic turbulence may be associated also to exotic sources of high energy charged particles, such as annihilation or decay of dark matter. We will consider the relevance of this and other related processes for the Milky Way as well as for smaller/older dark matter dominated galaxies.

Collaborations and meetings: This project benefits from information exchange with other groups facilitated by an institute like IFPU. The one-week IFPU workshop was of great importance for the exchange of ideas and different points of view on various issues. Future meetings like Team Research programs are planned and expected to further the collaboration between groups inside and outside IFPU.