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

Abstract We report on a measurement of hard exclusive π 0 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. From the average of the measured μ + and μ − 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 | t | 0.64 ( GeV/ c ) 2 . The average kinematics of the measurement are 〈 Q 2 〉 = 2.0 ( GeV / c ) 2 , 〈 ν 〉 = 12.8 GeV , 〈 x B j 〉 = 0.093 and 〈 − 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 | 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 | sys ) nb / ( GeV / c ) 2 and ( 1.5 ± 0.5 stat − 0.2 + 0.3 | 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 transverse-transverse interference contribution constitutes clear experimental evidence for the chiral-odd GPD E ‾ T .