Measurement of Deeply Virtual Compton Scattering and its t-dependence at HERA

TL;DR

This study measures elastic DVCS in $ep$ collisions at HERA to access generalized parton distributions and the proton's transverse structure. The analysis determines the $W$-dependence with $\delta\approx0.74$ at $Q^2\approx8$ GeV$^2$ and an exponential $t$-slope with $b\approx5.45$ GeV$^{-2}$, corresponding to a transverse size of about $0.65$ fm. Interpreting the results through GPDs shows sizable skewing consistent with skewed evolution, while a competing dipole model with saturation describes the data under geometric scaling with $\tau=Q^2/Q_s^2(x)$. The findings constrain the transverse parton distributions and support saturation-inspired descriptions in exclusive processes at high energy.

Abstract

A measurement of elastic deeply virtual Compton scattering gamma* p -> gamma p using e-p collision data recorded with the H1 detector at HERA is presented. The analysed data sample corresponds to an integrated luminosity of 145 pb^-1. 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. The measurements are compared to a NLO QCD calculation based on generalised parton distributions. In the context of the dipole approach, the geometric scaling property of the DVCS cross section is studied for different values of t.

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 electron-photon invariant mass (e) and the proton four momentum transfer squared $|t|$ (f). The data are compared with Monte Carlo expectations for elastic DVCS, elastic and inelastic BH and inelastic DVCS (labelled DISS. p). 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 as a function of $Q^2$ at $W=82$ GeV (a) and as a function of $W$ at $Q^2=8$ GeV$^2$ (b). The results from the previous H1 and ZEUS publications dvcsh1dvcszeus based on HERA I data are also displayed. The inner error bars represent the statistical errors, the outer error bars the statistical and systematic errors added in quadrature.
• Figure 3: The DVCS cross section 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). The inner error bars represent the statistical errors, the outer error bars the statistical and systematic errors added in quadrature.
• Figure 4: The DVCS cross section, differential in $t$, for three values of $Q^2$ expressed at $W=82$ GeV (a) and for three values of $W$ at $Q^2=10$ GeV$^2$ (b). The solid lines in (a) and (b) represent the results of fits of the form $e^{-b|t|}$. The fitted $t$-slope parameters $b(Q^2)$ are shown in (c) together with the $t$-slope parameters from the previous H1 publication dvcsh1. The dashed curve in (c) represents the result of a fit to the $b(Q^2)$ values using a phenomenological function as described in the text. In (d) the fitted $t$-slope parameters $b(W)$ are shown. The dashed line in (d) corresponds to the average value $b=5.45$ GeV$^{-2}$, obtained from a fit to the complete data sample of the present measurement. The inner error bars represent the statistical errors and the outer error bars the statistical and systematic errors added in quadrature.
• Figure 5: The observables $S$ and $R$ (see text), shown as a function of $Q^2$ in (a) and (b), respectively. The results from the previous H1 publication dvcsh1 based on HERA I data are also displayed. The inner error bars represent the statistical errors, the outer error bars the statistical and systematic errors added in quadrature. The dashed curves show the predictions of the GPD model freund2cteq. In (b), the dotted curve shows the prediction of a GPD model based on an approximation where only the kinematical part of the skewing effects are taken into account (see text).