Outstanding Problems in Particle Astrophysics

Abstract

The general features of the cosmic-ray spectrum have been known for a long time. Although the basic approaches to understanding cosmic-ray propagation and acceleration have also been well understood for many years, there are several questions of great interest that motivate the current intense experimental activity in the field. If the energy-dependence of the secondary to primary ratio of galactic cosmic rays is as steep as observed, why is the flux of PeV particles so nearly isotropic? Can all antiprotons and positrons be explained as secondaries or is there some contribution from exotic sources? What is the maximum energy of cosmic accelerators? Is the "knee" of the cosmic-ray spectrum an effect of propagation or does it perhaps reflect the upper limit of galactic acceleration processes? Are gamma-ray burst sources (GRBs) and/or active galactic nuclei (AGN) accelerators of ultra-high-energy cosmic rays (UHECR) as well as sources of high-energy photons? Are GRBs and/or AGNs also sources of high-energy neutrinos? If there are indeed particles with energies greater than the cutoff expected from propagation through the microwave background radiation, what are their sources? The purpose of this lecture is to introduce the main topics of the School and to relate the theoretical questions to the experiments that can answer them.

Highest energy cosmic-rays and results from the HiRes experiment

“…Because a change in the CR composition below 10 18 eV may be an important indicator of a transition between a galactic and extragalactic cosmic ray flux (or at the least a change in the nature of the CR source itself), it is of great importance to confirm this result. While a new experiment with an energy range covering from below 10 17 eV to 10 20 eV would be the ideal and has been proposed [49], the previous Fly's Eye experiment [46] stereo measurements of the elongation rate and fluctuations in the energy range 10 17 to 10 19 eV are of interest, even though they have significantly worse Xmax resolution (45 gm/cm 2 ). There are quantitative differences between the Fly's Eye and the HiRes and HiRes-MIA result.…”

“…A change in primary composition from heavy to light over some energy interval would produce an elongation rate larger than the 50-60 gm/cm 2 for an unchanging composition, while a change from light to heavy would produce a smaller or even negative elongation rate. Because of this relative insensitivity to hadronic models and to systematic errors affecting absolute X max determination, the measurement of the elongation rate has been the first approach to determining composition [46]. In many models, we anticipate a transition from a galactic to an extragactic cosmic ray flux somewhere between 10 17 and 10 19 eV [47].…”

Millicharged particles from the heavens: single- and multiple-scattering signatures

“…The origin JHEP11(2021)099 of these features remains unclear, and it is an active research topic (see, for instance, refs. [62,63]).…”

Millicharged Particles from the Heavens: Single- and Multiple-Scattering Signatures

“…The origin of these features remains unclear, and it is an active research topic (see, for instance, Refs. [52,53]).…”