Transverse extension of partons in the proton probed in the sea-quark range by measuring the DVCS cross section

We report on the first measurement of exclusive single-photon muoproduction on the proton by COMPASS using 160 GeV/c polarised μ⁺ and μ⁻ beams of the CERN SPS impinging on 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 B=(4.3±0.6_stat|sys−0.3+0.1)(GeV/c)⁻². This result can be converted into a transverse extension of partons in the proton, 〈r_⊥ ²〉=(0.58±0.04_stat|sys−0.02+0.01±0.04_model)fm. For this measurement, the average virtuality of the photon mediating the interaction is 〈Q²〉=1.8(GeV/c)² and the average value of the Bjorken variable is 〈x_Bj〉=0.056.