---
abstract: 'In this lecture I outline some of the underlying physics issues associated with accelerators plausibly capable of explaining the UHECRs up to ZeV energies. I concentrate on the mechanisms and their constraints, but provide a brief background on observations and the constraints they supply, as well.'
author:
- 'T.W. JONES'
title: |
    ACCELERATION OF ULTRA HIGH ENERGY COSMIC RAYS:\
    COSMIC ZEVATRONS?
---

Introduction
============

Strong observational evidence now exists for the existence of cosmic rays (CRs) at energies approaching 1 ZeV (=$10^{21}$eV). Their origins remain a mystery and a serious theoretical challenge. In this lecture I will outline some of the key issues that constrain theories for production of these “Ultra High Energy Cosmic Rays” (UHECRs). I will focus on so-called “bottom-up” scenarios, which accelerate charged particles, usually protons, from low energies. These hypothetical accelerators have been dubbed “Zevatrons”, since they must achieve ZeV energies. Other lecturers at this School have addressed the alternative “top-down” scenarios, which explain UHECRs as decay products of fossil Grand Unification defects, or by “new” physics (e.g., Stecker & Sánchez). The new physics approaches also include explanations that reduce constraints on long range UHECR propagation by allowing violation of Lorentz invariance, so those 