Observation of the Askaryan Effect: Coherent Microwave Cherenkov Emission from Charge Asymmetry in High Energy Particle Cascades

TL;DR

This paper provides the first direct experimental evidence of the Askaryan effect, demonstrating coherent microwave Cherenkov emission from charge asymmetry in high energy particle cascades, supporting future cosmic ray and neutrino detection efforts.

Contribution

It presents the first direct measurement confirming the Askaryan effect in high energy particle showers, validating theoretical predictions of coherent radio emission from charge excess.

Findings

Detected strong, coherent radio pulses from particle showers.

Observed 100% linear polarization consistent with Cherenkov radiation.

Electric field strength matches expected charge distribution profile.

Abstract

We present the first direct experimental evidence for the charge excess in high energy particle showers predicted nearly 40 years ago by Askaryan. We directed bremsstrahlung photons from picosecond pulses of 28.5 GeV electrons at the SLAC Final Focus Test Beam facility into a 3.5 ton silica sand target, producing electromagnetic showers several meters long. A series of antennas spanning 0.3 to 6 GHz were used to detect strong, sub-nanosecond radio frequency pulses produced whenever a shower was present. The measured electric field strengths are consistent with a completely coherent radiation process. The pulses show 100% linear polarization, consistent with the expectations of Cherenkov radiation. The field strength versus depth closely follows the expected particle number density profile of the cascade, consistent with emission from excess charge distributed along the shower. These measurements therefore provide strong support for experiments designed to detect high energy cosmic rays and neutrinos via coherent radio emission from their cascades.