Deeply Virtual Compton Scattering and its Beam Charge Asymmetry in e^\pm p Collisions at HERA

TL;DR

This study measures elastic deeply virtual Compton scattering in $e^{\pm}p$ collisions at HERA with the H1 detector, spanning $6.5< Q^2<80$ GeV$^2$, $30< W<140$ GeV, and $|t|<1$ GeV$^2$, using $306$ pb$^{-1}$ of data. By exploiting BH–DVCS interference, the authors extract the beam-charge asymmetry, enabling the first collider measurement of the real-to-imaginary amplitude ratio $\rho$ and a dispersion-relation cross-check with the energy-dependence parameter $\delta(Q^2)$. The results show a hard-Process-like energy rise with $\delta \approx 0.63$ and a $t$-slope $b\approx5.41$ GeV$^{-2}$, with the average transverse parton size $\sqrt{\langle r_T^2\rangle} \approx 0.64$ fm at $x\sim10^{-3}$; inelastic DVCS has a much smaller slope $b_{inel} \approx1.53$ GeV$^{-2}$. The measured beam-charge asymmetry and $\rho$ agree with predictions from generalized parton distributions and dipole models, supporting a consistent picture of parton correlations and transverse structure in the proton probed by DVCS at low $x$.

Abstract

A measurement of elastic deeply virtual Compton scattering gamma* p -> gamma p using e^+ p and e^- p collision data recorded with the H1 detector at HERA is presented. The analysed data sample corresponds to an integrated luminosity of 306 pb^-1, almost equally shared between both beam charges. The cross section is measured as a function of the virtuality Q^2 of the exchanged photon and the centre-of-mass energy W of the gamma* p system in the kinematic domain 6.5 < Q^2 < 80 GeV^2, 30 < W < 140 GeV and |t| < 1 GeV^2, where t denotes the squared momentum transfer at the proton vertex. The cross section is determined differentially in t for different Q^2 and W values and exponential t-slope parameters are derived. Using e^+ p and e^- p data samples, a beam charge asymmetry is extracted for the first time in the low Bjorken x kinematic domain. The observed asymmetry is attributed to the interference between Bethe-Heitler and deeply virtual Compton scattering processes. Experimental results are discussed in the context of two different models, one based on generalised parton distributions and one based on the dipole approach.

Figures (6)

• Figure 1: Distributions of the energy (a) and polar angle (b) of the scattered electron, the energy (c) and polar angle (d) of the photon, the $\phi$ azimuthal angle between the plane of incoming and outgoing lepton and the plane of virtual and real photon bel (e) and the proton four momentum transfer squared $|t|$ (f). The data correspond to the full $e^{\pm}p$ sample and are compared to Monte Carlo expectations for elastic DVCS, elastic and inelastic BH and inelastic DVCS. All Monte Carlo simulations are normalised according to the luminosity of the data. The open histogram shows the total prediction and the shaded band its estimated uncertainty.
• Figure 2: The DVCS cross section $\gamma^\ast p \rightarrow \gamma p$ as a function of $Q^2$ at $W=82$ GeV (a) and as a function of $W$ at $Q^2=10$ GeV$^2$ (b). The results from the previous H1 dvcsh1a and ZEUS dvcszeusb publications based on HERA I data are also displayed. ZEUS measurements are propagated from $W=104$ GeV to $82$ GeV using a $W$ dependence $W^{0.52}$. The inner error bars represent the statistical errors, the outer error bars the statistical and systematic errors added in quadrature. The dashed line represents the prediction of the GPD model muller and the solid line the prediction of the dipole model gregory.
• Figure 3: The DVCS cross section $\gamma^\ast p \rightarrow \gamma p$ as a function of $W$ at three values of $Q^2$ (a). The solid lines represent the results of fits of the form $W^\delta$. The fitted values of $\delta(Q^2)$ are shown in (b) together with the values obtained using HERA I data dvcsh1a. The inner error bars represent the statistical errors, the outer error bars the statistical and systematic errors added in quadrature.
• Figure 4: The fitted $t$-slope parameters $b(Q^2)$ are shown in (a) together with the $t$-slope parameters from the previous H1 dvcsh1a and ZEUS dvcszeusb publications based on HERA I data. In (b) the fitted $t$-slope parameters $b(W)$ are shown. The inner error bars represent the statistical errors and the outer error bars the statistical and systematic errors added in quadrature. The dashed line represents the prediction of the GPD model muller and the solid line the prediction of the dipole model gregory.
• Figure 5: The inelastic DVCS cross section differential in $t$ at $W=82$ GeV and $Q^2=10$ GeV$^2$ and for events with $1.4 \lesssim M_Y \lesssim 10$ GeV. The inner error bars represent the statistical errors, the outer error bars the statistical and systematic errors added in quadrature.