First Measurement of Charged Current Cross Sections at HERA with Longitudinally Polarised Positrons

TL;DR

This paper reports the first measurement of how the charged current cross section at HERA depends on the longitudinal polarisation of positrons, confirming the Standard Model predictions at high momentum transfer.

Contribution

It provides the first experimental measurement of the polarisation dependence of charged current cross sections at high Q^2 at HERA.

Findings

Cross section dependence on positron polarisation matches Standard Model predictions.

First measurement of polarisation effects in charged current interactions at high Q^2.

Results support the electroweak theory within experimental uncertainties.

Abstract

Data taken with positrons of different longitudinal polarisation states in collision with unpolarised protons at HERA are used to measure the total cross sections of the charged current process, e^+ p \to \bar{\nu}X, for negative four-momentum transfer squared Q^2 > 400 GeV^2 and inelasticity y<0.9. Together with the corresponding cross section obtained from the previously published unpolarised data, the polarisation dependence of the charged current cross section is measured for the first time at high Q^2 and found to be in agreement with the Standard Model prediction.