Exclusive leptoproduction of real photons on a longitudinally polarised hydrogen target

Polarisation asymmetries are measured for the hard exclusive leptoproduction of real photons from a longitudinally polarised hydrogen target. These asymmetries arise from the deeply virtual Compton scattering and Bethe-Heitler processes. From the data are extracted two asymmetries in the azimuthal distribution of produced real photons about the direction of the exchanged virtual photon: A(UL) with respect to the target polarisation and A(LL) with respect to the product of the beam and target polarisations. Results for both asymmetries are compared to the predictions from a generalised parton distribution model. The sin phi and cos(0 phi) amplitudes observed respectively for the A(UL) and A(LL) asymmetries are compatible with the sizeable predictions from the model. Unexpectedly, a sin(2 phi) modulation in the A(UL) asymmetry with a magnitude similar to that of the sin phi modulation is observed.