Observation of Exclusive Deeply Virtual Compton Scattering in Polarized Electron Beam Asymmetry Measurements

We report the first results of the beam-spin asymmetry measured in the reaction $\stackrel{\ensuremath{\rightarrow}}{e}p\ensuremath{\rightarrow}\mathrm{ep}\ensuremath{\gamma}$ at a beam energy of 4.25 GeV. A large asymmetry with a $\mathrm{sin}\ensuremath{\varphi}$ modulation is observed, as predicted for the interference term of deeply virtual compton scattering (DVCS) and the Bethe-Heitler process. The amplitude of this modulation is $\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.202\ifmmode\pm\else\textpm\fi{}0.028$. In leading-order and leading-twist perturbative QCD, the $\ensuremath{\alpha}$ is directly proportional to the imaginary part of the DVCS amplitude.