Transverse Extension of Partons in the Proton probed by Deeply Virtual Compton Scattering

Abstract

We report on the first measurement of exclusive single-photon muoproduction on the proton by COMPASS using 160 GeV/$c$ polarized $\hat{I}{¼}^{+}$ and $\hat{I}{¼}^{-}$ beams of the CERN SPS impinging on a liquid hydrogen target. We determine the dependence of the average of the measured $\hat{I}{¼}^{+}$ and $\hat{I}{¼}^{-}$ cross sections for deeply virtual Compton scattering on the squared four-momentum transfer $t$ from the initial to the final final proton. The slope $B$ of the $t$-dependence is fitted with a single exponential function, which yields $B=(4.3\pm 0.6_{\text{stat}}\_{-0.3}^{+0.1}{|}_{\text{sys}})(\text{GeV}/c{)}^{-2}$. This result can be converted into an average transverse extension of partons in the proton, $\sqrt{⟨r_{\perp }^2⟩}=(0.58\pm 0.04_{\text{stat}}\_{-0.02}^{+0.01}{|}_{\text{sys}})\text{fm}$. For this measurement, the average virtuality of the photon mediating the interaction is $⟨Q^2⟩=1.8(\text{GeV/}c{)}^2$ and the average value of the Bjorken variable is $⟨x_{\text{Bj}}⟩=0.056$.

13 pages, 5 figures