Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

H1 Collaboration

TL;DR

This work reports the first measurement of charged current cross sections in longitudinally polarised positron-proton collisions at HERA, selecting events in the high-Q^2 region (Q^2>400 GeV^2, y<0.9) and comparing two polarisation states. By combining with unpolarised data and exploiting the linear dependence of the CC cross section on beam polarisation, the authors test the V-A structure of CC interactions at large space-like Q^2 and constrain right-handed charged currents. The results show the polarisation dependence agrees with the Standard Model, with an extrapolated cross section at P_e=-1 consistent with zero and a 95% CL upper limit of 1.9 pb, reinforcing the SM prediction and setting bounds on new chiral currents. The analysis relies on detailed detector calibration, MC simulations, and careful control of systematic uncertainties, demonstrating robust tests of electroweak interactions in a previously unexplored kinematic regime.

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 \barνX, 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.

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

TL;DR

This work reports the first measurement of charged current cross sections in longitudinally polarised positron-proton collisions at HERA, selecting events in the high-Q^2 region (Q^2>400 GeV^2, y<0.9) and comparing two polarisation states. By combining with unpolarised data and exploiting the linear dependence of the CC cross section on beam polarisation, the authors test the V-A structure of CC interactions at large space-like Q^2 and constrain right-handed charged currents. The results show the polarisation dependence agrees with the Standard Model, with an extrapolated cross section at P_e=-1 consistent with zero and a 95% CL upper limit of 1.9 pb, reinforcing the SM prediction and setting bounds on new chiral currents. The analysis relies on detailed detector calibration, MC simulations, and careful control of systematic uncertainties, demonstrating robust tests of electroweak interactions in a previously unexplored kinematic regime.

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 \barνX, 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.

Paper Structure

This paper contains 6 sections, 4 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Charged Current Cross Section
  3. Experimental Technique
  4. Measurement Procedure
  5. Results
  6. Summary

Figures (2)

  • Figure 1: Distributions of $P_{T,h}$ (a,c) and $E_h-p_{z,h}$ (b,d) for the selected events in the right handed (R) and left handed (L) data sets. The Monte Carlo (MC) contributions from the charged current (CC) process and the $ep$ background (bkg) processes are shown as open histograms with the latter contribution alone being shown as shaded histograms.
  • Figure 2: The dependence of the $e^+p$ CC cross section on the lepton beam polarisation $P_e$. The inner and outer error bars represent respectively the statistical and total errors. The uncertainties on the polarisation measurement are smaller than the symbol size. The data are compared to the Standard Model prediction based on the H1 PDF $2000$ parametrisation (dark shaded band). The light shaded band corresponds to the resulting one-sigma contour of a linear fit to the data shown as the central line.