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Front. Astron. Space Sci. | doi: 10.3389/fspas.2019.00023

Open questions in cosmic-ray research at ultrahigh energies

 Rafael A. Batista1, Jonathan Biteau2,  Mauricio Bustamante3, Klaus Dolag4, Ralph Engel5,  Foteini Oikonomou6*, Ke Fang7*,  Karl-Heinz Kampert8,  Dmitriy Kostunin9,  Miguel A. Mostafa10,  Kohta Murase10, Günter Sigl11, Angela Olinto12,  Andrew Taylor9, Michael Unger5 and  Mikhail Panasyuk13, 14
  • 1University of São Paulo, Brazil
  • 2UMR8608 Institut de Physique Nucléaire d'Orsay (IPNO), France
  • 3Niels Bohr Institute, Faculty of Natural and Life Sciences, University of Copenhagen, Denmark
  • 4Max Planck Institute for Astrophysics, Germany
  • 5Karlsruhe Institute of Technology (KIT), Germany
  • 6European Southern Observatory, Germany
  • 7Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, United States
  • 8University of Wuppertal, Germany
  • 9Deutsches Elektronen-Synchrotron, Helmholtz Association of German Research Centers (HZ), Germany
  • 10Pennsylvania State University, United States
  • 11Universität Hamburg, Germany
  • 12University of Chicago, United States
  • 13Lomonosov Moscow State University, Russia
  • 14Scientific Research Institute of Nuclear Physics Skobeltsyna, Lomonosov Moscow State University, Russia

We review open questions and prospects for progress in ultrahigh-energy cosmicray (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 thetrajectories of UHECRs, anisotropy expectations for specific astrophysical scenarios, hadronicinteractions, and prospects for discovering neutral particles as well as new physics at ultrahighenergies. We also briefly overview upcoming and proposed UHECR experiments and discusstheir projected science reach.

Keywords: Ultrahigh energy cosmic rays, ultrahigh energy neutrinos, extrensive-air-shower detectors, Intergalactic magnetic field, Mass composition of cosmic rays, hadronic interactions at ultrahigh energies, ultrahigh-energy-cosmic-ray anisotropy

Received: 15 Jan 2019; Accepted: 21 Mar 2019.

Edited by:

Frank F. Deppisch, University College London, United Kingdom

Reviewed by:

Francis Halzen, University of Wisconsin-Madison, United States
Soebur Razzaque, University of Johannesburg, South Africa  

Copyright: © 2019 Batista, Biteau, Bustamante, Dolag, Engel, Oikonomou, Fang, Kampert, Kostunin, Mostafa, Murase, Sigl, Olinto, Taylor, Unger and Panasyuk. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence:
Dr. Foteini Oikonomou, European Southern Observatory, Garching bei München, Germany,
Dr. Ke Fang, Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, 94025, California, United States,