UHECR theory and phenomenology: Summary and outlook

Department of Astronomy & Astrophysics, Kavli Institute for Cosmological Physics, Enrico Fermi Institute, The University of Chicago, Chicago, Illinois 60637, USA

^a e-mail: olinto@kicp.uchicago.edu

Abstract

Theorists and phenomenologists have contributed significantly to the development of the field of ultrahigh energy cosmic rays (UHECRs). Great progress has been achieved in modeling hadronic interactions, developing precise propagation codes, understanding the role of different backgrounds and magnetic fields in the propagation of ultrahigh energy protons and nuclei, predicting the flux of secondary neutrinos and photons, modeling astrophysical sources and their acceleration mechanisms, developing new techniques to test anisotropies in the sky distribution, proposing new physics phenomena that can be tested at ultrahigh energy, and sharpening the distinction between astrophysical interpretations of unexpected trends (such as the composition at the highest energies) and new physics at play in hadronic interactions at energies well beyond the reach of terrestrial laboratories. Better developed models when combined with recent data have framed current open questions. 1. Is the spectral feature at the highest energies the GZK cutoff or the effect of E_max? 2. Is the composition of primaries changing at the highest energies or are new interactions responsible for the change in behavior of extensive air showers? 3. At what energy and sensitivity will sources be observed? 4. At what energies cosmic rays transition from being Galactic to becoming extragalactic? And the most basic question remains, 5. what are the sources (and the acceleration mechanism) of ultrahigh energy cosmic rays? To answer these questions more observations are needed. Chief among the theorists’ wish list is the increase in statistics at the highest energies and the second wish is for full sky coverage. These efforts should lead to the localization of a source (or sources) in the sky which would revolutionize the field. Another avenue for major progress would be the detection of neutrino and photon secondaries, especially at ultrahigh energies. More immediate progress can be reached with a better establishment of an absolute energy scale which has a direct impact on many models. Another fertile area is the ongoing tests of hadronic interaction models with the LHC. More precise measurements of airshower properties may also enable the distinction between primary composition and hadronic physics effects on a wide energy range. Joint anisotropy and composition analysis between the two leading experiments (Auger and TA) would also provide a much needed consistency between the two hemispheres. This meeting marks the beginning of a great effort in this direction.