Measurements of cosmic ray antiprotons with PAMELA and studies of propagation models

Studying the acceleration and propagation mechanisms of Galactic cosmic rays can provide information regarding astrophysical sources, the properties of our Galaxy, and possible exotic sources such as dark matter. To understand cosmic ray acceleration and propagation mechanisms, accurate measurements of different cosmic ray elements over a wide energy range are needed. The PAMELA experiment is a satellite-borne apparatus which allows different cosmic ray species to be identified over background. Measurements of the cosmic ray antiproton flux and the antiproton-to-proton flux ratio from 1.5 GeV to 180 GeV are presented in this thesis. Compared to previous experiments, PAMELA extends the energy range of antiproton measurements and provides significantly higher statistics. The derived antiproton flux and antiproton-to-proton flux ratio are consistent with previous measurements and generally considered to be produced as secondary products when cosmic ray protons and helium nuclei interact with the interstellar medium. To constrain cosmic ray acceleration and propagation models, the antiproton data measured by PAMELA were further used together with the proton spectrum reported by PAMELA, as well as the B/C data provided by other experiments. Statistical tools were interfaced with the cosmic ray propagation package GALPROP to perform the constraining analyses. Diffusion models with a linear diffusion coefficient and modified diffusion models with a low energy dependence of the diffusion coefficient were studied in the $\chi^{2}$ study. Uncertainties on the parameters and the goodness of fit of each model were given. Some models are further studied using the Bayesian inference. Posterior means and errors of the parameters base on our prior knowledge on them were obtained in the Bayesian framework. This method also allowed us to understand the correlation between parameters and compare models.

💡 Research Summary

This thesis presents a comprehensive analysis of cosmic‑ray antiproton measurements obtained with the PAMELA satellite experiment and uses these data to constrain Galactic cosmic‑ray propagation models. PAMELA collected more than five thousand antiproton events in the kinetic energy range 1.5 GeV–180 GeV, extending both the upper energy limit and the statistical precision beyond previous balloon‑borne and space‑borne missions. The measured antiproton flux and the antiproton‑to‑proton (𝑝¯/𝑝) ratio are in good agreement with earlier results from BESS, CAPRICE, and AMS‑02, and they show no unexpected excess at high energies that would hint at exotic primary sources. The data are consistent with the standard picture in which antiprotons are secondary particles produced when primary cosmic‑ray protons and helium nuclei interact with the interstellar medium (ISM), generating pions that decay into antiprotons.

To translate these observations into constraints on propagation physics, the author coupled the GALPROP numerical code with statistical inference tools. Two families of diffusion models were examined: (i) a conventional linear diffusion coefficient D(R)=D₀(R/R₀)^δ, and (ii) a modified low‑energy diffusion model in which the rigidity dependence flattens below a few GV, D(R)=D₀(R/R₀)^{δ₁}