Secondary antiprotons and propagation of cosmic rays in the Galaxy and heliosphere
I. V. Moskalenko, A. W. Strong, J. F. Ormes, M. S. Potgieter
TL;DR
This paper addresses how cosmic-ray secondary antiprotons probe Galactic propagation and heliospheric modulation. Using the GALPROP code, the authors test multiple transport models against a wide array of CR data, including protons, helium, positrons, electrons, gamma rays, and antiprotons, and demonstrate that no simple model suffices. They identify a best-fit scenario—a diffusion-coefficient break with convection (DC) and, for primaries, an injection-spectrum break—that broadly reproduces the data, while predicting how proton and antiproton fluxes vary with solar activity and magnetic polarity. The study underlines the importance of accurate antiproton cross sections and heliospheric modulation modeling for interpreting secondary backgrounds and for constraining exotic signals, and it outlines future directions and measurements (e.g., PAMELA, GLAST) to refine propagation models.
Abstract
High-energy collisions of cosmic-ray nuclei with interstellar gas are believed to be the mechanism producing the majority of cosmic ray antiprotons. Due to the kinematics of the process they are created with a nonzero momentum; the characteristic spectral shape with a maximum at ~2 GeV and a sharp decrease towards lower energies makes antiprotons a unique probe of models for particle propagation in the Galaxy and modulation in the heliosphere. On the other hand, accurate calculation of the secondary antiproton flux provides a ``background'' for searches for exotic signals from the annihilation of supersymmetric particles and primordial black hole evaporation. Recently new data with large statistics on both low and high energy antiproton fluxes have become available which allow such tests to be performed. We use our propagation code GALPROP to calculate interstellar cosmic-ray propagation for a variety of models. We show that there is no simple model capable of accurately describing the whole variety of data: boron/carbon and sub-iron/iron ratios, spectra of protons, helium, antiprotons, positrons, electrons, and diffuse gamma rays. We find that only a model with a break in the diffusion coefficient plus convection can reproduce measurements of cosmic-ray species, and the reproduction of primaries (p, He) can be further improved by introducing a break in the primary injection spectra. For our best-fit model we make predictions of proton and antiproton fluxes near the Earth for different modulation levels and magnetic polarity using a steady-state drift model of propagation in the heliosphere.