A new limit on the Ultra-High-Energy Cosmic-Ray flux with the Westerbork Synthesis Radio Telescope

TL;DR

This paper establishes a new method to set upper limits on ultra-high-energy cosmic-ray flux by analyzing radio emissions from particle showers in the Moon's surface, utilizing the Westerbork Synthesis Radio Telescope.

Contribution

It demonstrates that surface-emitted Cherenkov radiation is independent of shower depth, enabling new cosmic-ray flux limits from lunar radio observations.

Findings

Set a new upper limit on ultra-high-energy cosmic-ray flux.

Show that surface Cherenkov radiation is depth-independent.

Utilize lunar radio observations to constrain cosmic-ray models.

Abstract

A particle cascade (shower) in a dielectric, for example as initiated by an ultra-high energy cosmic ray, will have an excess of electrons which will emit coherent \v{C}erenkov radiation, known as the Askaryan effect. In this work we study the case in which such a particle shower occurs in a medium just below its surface. We show, for the first time, that the radiation transmitted through the surface is independent of the depth of the shower below the surface when observed from far away, apart from trivial absorption effects. As a direct application we use the recent results of the NuMoon project, where a limit on the neutrino flux for energies above $10^{22}$\,eV was set using the Westerbork Synthesis Radio Telescope by measuring pulsed radio emission from the Moon, to set a limit on the flux of ultra-high-energy cosmic rays.