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.