Measurement of Neutral and Charged Current Cross-Sections in Positron-Proton Collisions at Large Momentum Transfer

TL;DR

This study extends neutral and charged current deep-inelastic scattering measurements in $e^+p$ collisions at HERA to $Q^2$ up to $3\times10^4\ \mathrm{GeV^2}$ and high $x$, enabling stringent tests of perturbative QCD via DGLAP evolution and electroweak effects. Using a comprehensive detector calibration, multiple kinematic reconstruction methods, and detailed MC simulations, the analysis yields precise measurements of $F_2(x,Q^2)$, NC and CC cross-sections, and the proton's high-$x$ quark densities. The results are in agreement with the Standard Model, show visible $\gamma-Z^0$ interference at high $Q^2$, and provide a space-like determination of the $W$ propagator mass consistent with time-like precision, thereby reinforcing the consistency of QCD and electroweak theory in the proton's structure. The work also demonstrates the feasibility of extracting valence quark densities from $e^+p$ DIS at high $x$, contributing valuable constraints to global PDF fits and future ep collider analyses.

Abstract

The inclusive single and double differential cross-sections for neutral and charged current processes with four-momentum transfer squared Q^2 between 150 and 30,000 GeV2 and with Bjorken x between 0.0032 and 0.65 are measured in e^+ p collisions. The data were taken with the H1 detector at HERA between 1994 and 1997, and they correspond to an integrated luminosity of 35.6 pb^-1. The Q^2 evolution of the parton densities of the proton is tested, yielding no significant deviation from the prediction of perturbative QCD. The proton structure function F_2(x,Q^2) is determined. An extraction of the u and d quark distributions at high x is presented. At high Q^2 electroweak effects of the heavy bosons Z0 and W are observed and found to be consistent with Standard Model expectation.

Figures (17)

• Figure 1: (a) Distribution of $V_{ap}/V_{p}$ for tagged $\gamma p$ events passing the CC selection except for the $V_{ap}/V_{p}$ cut. The data (points) are compared to the Monte Carlo (MC) simulation (histogram) of the $\gamma p$ background. (b) Distribution of $V_{ap}/V_{p}$ for the CC event sample. The data (points) are compared to the simulation (histogram) which includes the CC and the background (${\rm bg} \equiv$$\gamma p$ + NC) events. A cut $V_{ap}/V_{p}<0.15$ is applied in the CC selection. The simulation is normalized to the integrated $ep$ luminosity. The shaded error bands represent the systematic uncertainty of the background simulation.
• Figure 2: Distribution of polar angle of the scattered positron. The data (points) are compared to the simulation (histogram) which is normalized to the integrated $ep$ luminosity.
• Figure 3: Comparison of the electromagnetic energy scale as determined by different calibration methods. Shown is $\langle{\delta E_e^{\prime}/E_e^{\prime}} \rangle$, the mean fractional energy shift of the different methods from the absolute energy scale. The shaded error band shows the systematic uncertainty on the energy scale quoted on this measurement, which varies from $0.7$ to $3\%$, depending on the position in the detector.
• Figure 4: Energy spectrum of the scattered positron at (a) $Q^2 > 150$${\rm GeV}^2$ , and (b) $Q^2 > 5000$${\rm GeV}^2$ . The data (points) are compared to the simulation (histogram) which is normalized to the integrated $ep$ luminosity.
• Figure 5: (a) Distribution of the fraction of $y_h$ contributed by the tracks ($y_{tracks}$), the LAr ($y_{LAr}$) and the SPACAL calorimeters ($y_{Spacal}$), and the fractional contribution of the subtracted noise ($y_{noise}$). (b) Distribution of $y_h/y_e$ for $y_e>0.1$. (c) Distribution of $P_{T,h}/P_{T,e}$ for the complete NC sample, (d) for the sub-sample at $P_{T,e} > 50$${\rm GeV}$ . The data (points) are compared to the simulation (histogram) which is normalized to the integrated $ep$ luminosity.