Observation of exclusive DVCS in polarized electron beam asymmetry measurements

Abstract

We report the first results of the beam spin asymmetry measured in the reaction e + p -> e + p + gamma at a beam energy of 4.25 GeV. A large asymmetry with a sin(phi) modulation is observed, as predicted for the interference term of Deeply Virtual Compton Scattering and the Bethe-Heitler process. The amplitude of this modulation is alpha = 0.202 +/- 0.028. In leading-order and leading-twist pQCD, the alpha is directly proportional to the imaginary part of the DVCS amplitude.

Figures (4)

• Figure 1: Feynman diagrams for DVCS and Bethe-Heitler processes contributing to the amplitude of $ep~\rightarrow~ep\gamma$ scattering.
• Figure 2: Missing mass squared distribution of the detected (ep) system for a) $ep~\rightarrow~ep\gamma$ and b) $ep~\rightarrow~ep\pi^0~$. In each plot the solid line is the fit to the sum of a Gaussian and the third order polynomial distribution. The dashed curve corresponds to the Gaussian function and the dotted curve represents the polynomial function.
• Figure 3: Missing mass squared distribution for the reaction $ep~\rightarrow~epX~$. Events are integrated in the range of $\phi$ from 70$^\circ$ to 110$^\circ$. The curves are described in the text.
• Figure 4: $\phi$ dependence of the beam spin asymmetry A. The dark shaded region is the range of the fitted function $A(\phi)$ defined by the statistical errors of parameters $\alpha$ and $\beta$, the light shaded region includes systematic uncertainties added linearly to the statistical uncertainties. The curves are model calculations according to Refs. [6,11] and are discussed in the text.