Measurement of the cross section for hard exclusive π0 muoproduction on the proton

We report on a measurement of hard exclusive pi(0) muoproduction on the proton by COMPASS using 160GeV/cpolarised mu+ and mu- beams of the CERN SPS impinging on a liquid hydrogen target. From the average of the measured mu+ and mu- cross sections, the virtual-photon proton cross section is determined as a function of the squared four-momentum transfer between initial and final proton in the range 0.08 (GeV/c)(2) <vertical bar t vertical bar < 0.64 (GeV/c)(2). The average kinematics of the measurement are Q(2) = 2.0 (GeV/c)(2), nu = 12.8 GeV, x(Bj) = 0.093and -t = 0.256 (GeV/c)(2). Fitting the azimuthal dependence reveals a combined contribution by transversely and longitudinally polarised photons of (8.2 +/- 0.9(stat-1.2)(+1.2)vertical bar(sys)) nb/(GeV/c)(2), as well as transverse-transverse and longitudinal-transverse interference contributions of (-6.1 +/- 1.3(stat- 0.7)(+0.7) vertical bar sys) nb/(GeV/c)(2) and (1.5 +/- 0.5(stat-0.2)(+ 0.3) vertical bar(sys)) nb/(GeV/c)(2), respectively. Our results provide important input for modelling Generalised Parton Distributions. In the context of the phenomenological Goloskokov-Kroll model, the statistically significant transversetransverse interference contribution constitutes clear experimental evidence for the chiral-odd GPD ET. (C) 2020 The Author(s). Published by Elsevier B.V.