# Open Questions in Cosmic-Ray Research at Ultrahigh Energies

## TL;DR

This paper reviews key open questions and future prospects in ultrahigh-energy cosmic ray research, covering origins, composition, propagation, and potential new physics, based on discussions from a 2018 workshop.

## Contribution

It provides a comprehensive overview of unresolved issues and upcoming experimental efforts in UHECR research, highlighting areas for future investigation.

## Key findings

- Identification of key open questions in UHECR origin and composition.
- Discussion of the impact of magnetic fields on cosmic-ray trajectories.
- Overview of upcoming experiments and their potential to advance the field.

## 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 present upcoming and proposed UHECR experiments and discuss their projected science reach.