AUTHOR=Alves Batista Rafael , Biteau Jonathan , Bustamante Mauricio , Dolag Klaus , Engel Ralph , Fang Ke , Kampert Karl-Heinz , Kostunin Dmitriy , Mostafa Miguel , Murase Kohta , Oikonomou Foteini , Olinto Angela V. , Panasyuk Mikhail I. , Sigl Guenter , Taylor Andrew M. , Unger Michael TITLE=Open Questions in Cosmic-Ray Research at Ultrahigh Energies JOURNAL=Frontiers in Astronomy and Space Sciences VOLUME=6 YEAR=2019 URL=https://www.frontiersin.org/journals/astronomy-and-space-sciences/articles/10.3389/fspas.2019.00023 DOI=10.3389/fspas.2019.00023 ISSN=2296-987X ABSTRACT=

We review open questions and prospects for progress in ultrahigh-energy cosmic ray (UHECR) research, based on a series of discussions that took place during the “The High-Energy Universe: Gamma-Ray, Neutrino, and Cosmic-ray Astronomy” MIAPP workshop in 2018. Specifically, we overview open questions on the origin of the bulk of UHECRs, the UHECR mass composition, the origin of the end of the cosmic-ray spectrum, the transition from Galactic to extragalactic cosmic rays, the effect of magnetic fields on the trajectories of UHECRs, anisotropy expectations for specific astrophysical scenarios, hadronic interactions, and prospects for discovering neutral particles as well as new physics at ultrahigh energies. We also briefly overview upcoming and proposed UHECR experiments and discuss their projected science reach.