Transverse extension of partons in the proton probed by DVCS at COMPASS

We present the first result on exclusive single-photon muoproduction on the proton in the range of intermediate values of the Bjorken variable xBj , which was recently published by COMPASS collaboration. The measurements were performed using 160GeV/c polarised μ + and μ − beams of the CERN SPS scattering off a liquid hydrogen target. We determine the dependence of the average of the measured μ + and μ − cross sections for deeply virtual Compton scattering on the squared four-momentum transfer t from the initial to the final proton. The slope B of the t-dependence is fitted with a single exponential function, which yields . This result can be converted into an average transverse extension of partons in the proton, . For this measurement, the average virtuality of the photon mediating the interaction is ⟨Q 2⟩ = 1.8 (GeV/c)2 and the average value of the Bjorken variable is ⟨xBj ⟩ = 0.056. We also compare COMPASS result to earlier estimates at very small xBj from HERA.