Study of Light Backgrounds from Relativistic Electrons in Air Light-Guides

TL;DR

This paper investigates the responses of various gas mixtures to relativistic electrons to inform the design of Cherenkov and scintillation detectors for the MOLLER experiment, aiming to accurately measure backgrounds in electron scattering measurements.

Contribution

It introduces a gas-filled detector to separately measure Cherenkov and scintillation responses, providing experimental data and comparisons to simulations for optimal detector design.

Findings

Gas mixtures show distinct Cherenkov and scintillation responses.

Experimental results align with simulation predictions.

Implications for reducing background in MOLLER detector design.

Abstract

The MOLLER experiment proposed at the Thomas Jefferson National Accelerator Facility plans a precision low energy determination of the weak mixing angle via the measurement of the parity-violating asymmetry in the scattering of high energy longitudinally polarized electrons from electrons bound in a liquid hydrogen target (M{\o}ller scattering). A relative measure of the scattering rate is planned to be obtained by intercepting the M{\o}ller scattered electrons with a circular array of thin fused silica tiles attached to air light guides, which facilitate the transport of Cherenkov photons generated within the tiles to photomultiplier tubes (PMTs). The scattered flux will also pass through the light guides of downstream tiles, generating additional Cherenkov as well as scintillation light and is a potential background. In order to estimate the rate of these backgrounds, a gas-filled tube detector was designed and deployed in an electron beam at the MAMI facility at Johannes Gutenberg University, Mainz, Germany. Described in this paper is the design of a detector to measure separately the scintillation and Cherenkov responses of gas mixtures from relativistic electrons, the results of studies of several gas mixtures with comparisons to simulations, and conclusions about the implications for the design of the MOLLER detector apparatus.