A measurement of the Q^2, W and t dependences of deeply virtual Compton scattering at HERA

TL;DR

This ZEUS study presents exclusive DVCS cross sections in $ep$ collisions across $1.5<Q^2<100$ GeV$^2$ and $40<W<170$ GeV, plus a first direct $d\sigma/dt$ measurement using events with the scattered proton detected in the LPS. By combining MC modelling (FFS DVCS with ALLM97 for $F_2$) and careful background subtraction of the Bethe–Heitler process, the analysis finds a steep $Q^2$-dependence, $\sigma \propto Q^{-2n}$ with $n\approx1.54$, and a rising $W$-dependence, $\sigma \propto W^{\delta}$ with $\delta\approx0.52$, largely independent of $Q^2$. The $d\sigma/dt$ slope, $b\approx4.5$ GeV$^{-2}$, supports a hard DVCS mechanism even at low $Q^2$ and constrains generalized parton distributions and the gluon density in the proton. Overall, the results reinforce DVCS as a clean probe of the proton’s transverse structure and provide quantitative inputs for GPD-based descriptions.

Abstract

Deeply virtual Compton scattering has been measured in e^+p collisions at HERA with the ZEUS detector using an integrated luminosity of 61.1 pb^-1. Cross sections are presented as a function of the photon virtuality, Q^2, and photon-proton centre-of-mass energy, W, for a wide region of the phase space, Q^2>~1.5 GeV^2 and 40<W<170 GeV. A subsample of events in which the scattered proton is measured in the leading proton spectrometer, corresponding to an integrated luminosity of 31.3 pb^-1, is used for the first direct measurement of the differential cross section as a function of t, where t is the square of the four-momentum transfer at the proton vertex.

Figures (5)

• Figure 1: Distribution of (a) $W$, (b) $Q^2$ in the inclusive sample and of (c) $x_L$ and (d) $|t|$ in the LPS sample, for the $e$-sample (top), the $\gamma$-sample (middle) and the $\gamma$-sample after BH background and proton dissociation subtraction (bottom). Also shown are the expectations of the MC normalised to the luminosity of the data and the contribution from exclusive dilepton production ($e^+ e^-$).
• Figure 2: (a) The DVCS cross section, $\sigma^{\gamma ^*p\rightarrow \gamma p}$, as a function of $Q^2$. The solid line is the result of a fit of the form $\sim Q^{-2n}$. (b) The DVCS cross section, $\sigma^{\gamma ^*p\rightarrow \gamma p}$, as a function of $W$. The solid line is the result of a fit of the form $\sim W^\delta$. The inner error bars represent the statistical uncertainty while the outer error bars the statistical and systematic uncertainties added in quadrature.
• Figure 3: The DVCS cross section, $\sigma^{\gamma ^*p\rightarrow \gamma p}$, as a function of $~W$ for $Q^2=2.4\;{\text{Ge}\text{V}}^2$ (dots) shown together with previous ZEUS measurements (squares) pl:b573:46. Also shown at higher $Q^2$ are the new measurements at $W=155$ GeV (dots). The solid lines are the results of a fit of the form $\sigma^{\gamma ^*p\rightarrow \gamma p} \propto W^\delta$. The values of $\delta$ and their statistical uncertainties are given in the figure. The inner error bars represent the statistical uncertainty while the outer error bars the statistical and systematic uncertainties added in quadrature.
• Figure 4: The DVCS differential cross section, $d\sigma^{\gamma ^*p\rightarrow \gamma p}/dt$, as a function of $|t|$. The solid line is the result of a fit of the form $\sim e^{-b|t|}$. The inner error bars represent the statistical uncertainty while the outer error bars the statistical and systematic uncertainties added in quadrature.
• Figure 5: A compilation of the values of the slope b as a function of $Q^2+M^2$ for various exclusive processes including the present DVCS measurement. The inner error bars represent the statistical uncertainty while the outer error bars the statistical and systematic uncertainties added in quadrature.