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: \( {\mathcal{A}_{\text{UL}}} \) with respect to the target polarisation and \( {\mathcal{A}_{\text{LL}}} \) 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 φ and cos(0φ) amplitudes observed respectively for the \( {\mathcal{A}_{\text{UL}}} \) and \( {\mathcal{A}_{\text{LL}}} \) asymmetries are compatible with the sizeable predictions from the model. Unexpectedly, a sin(2φ) modulation in the \( {\mathcal{A}_{\text{UL}}} \) asymmetry with a magnitude similar to that of the sin φ modulation is observed.