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 〈 Q 2 〉 = 1.8 ( GeV/ c ) 2 and the average value of the Bjorken variable is 〈 x Bj 〉 = 0.056 .