Measurement of deeply virtual Compton scattering off He 4 with the CEBAF Large Acceptance Spectrometer at Jefferson Lab

Dupre, R.; Hattawy, M.; Baltzell, N. A.; Bultmann, S.; De Vita, R.; El Alaoui, A.; El Fassi, L.; Egiyan, H.; Girod, F. X.; Guidal, M.; Hafidi, K.; Jenkins, D.; Liuti, S.; Perrin, Y.; Stepanyan, S.; Torayev, B.; Voutier, E.; Amaryan, M. J.; Armstrong, W. R.; Atac, H.; Ayerbe Gayoso, C.; Barion, L.; Battaglieri, M.; Bedlinskiy, I.; Benmokhtar, F.; Bianconi, A.; Biselli, A. S.; Bondi, M.; Bossu, F.; Boiarinov, S.; Briscoe, W. J.; Bulumulla, D.; Burkert, V.; Carman, D. S.; Carvajal, J. C.; Caudron, M.; Celentano, A.; Chatagnon, P.; Chesnokov, V.; Chetry, T.; Ciullo, G.; Clary, B. A.; Cole, P. L.; Contalbrigo, M.; Costantini, G.; Crede, V.; D'Angelo, A.; Dashyan, N.; Defurne, M.; Deur, A.; Diehl, S.; Djalali, C.; Ehrhart, M.; Elouadrhiri, L.; Eugenio, P.; Fegan, S.; Filippi, A.; Forest, T. A.; Ghandilyan, Y.; Gilfoyle, G. P.; Gothe, R. W.; Griffioen, K. A.; Hakobyan, H.; Hayward, T. B.; Hicks, K.; Hobart, A.; Holtrop, M.; Ilieva, Y.; Ireland, D. G.; Isupov, E. L.; H. S., Jo; Joo, K.; Joosten, S.; Keller, D.; Khachatryan, G.; Khanal, A.; Khandaker, M.; Kim, A.; Kim, W.; Kripko, A.; Kubarovsky, V.; Kuhn, S. E.; Lanza, L.; Livingston, K.; Kabir, M. L.; Leali, M.; Lenisa, P.; Macgregor, I. J. D.; Marchand, D.; Markov, N.; Mascagna, V.; Mayer, M.; Mckinnon, B.; Mirazita, M.; Mokeev, V. I.; Neupane, K.; Niccolai, S.; O'Connell, T. R.; Osipenko, M.; Paolone, M.; Pappalardo, L. L.; Paremuzyan, R.; Pasyuk, E.; Payette, D.; Phelps, W.; Pivnyuk, N.; Pogorelko, O.; Poudel, J.; Prok, Y.; Ripani, M.; Ritman, J.; Rizzo, A.; Rosner, G.; Rossi, P.; Rowley, J.; Sabatie, F.; Salgado, C.; Schmidt, A.; Schumacher, R.; Sergeyeva, V.; Sharabian, Y.; Shrestha, U.; Sokhan, D.; Soto, O.; Sparveris, N.; Strakovsky, I. I.; Strauch, S.; Tyler, N.; Ungaro, M.; Venturelli, L.; Voskanyan, H.; Vossen, A.; Watts, D.; Wei, K.; Wei, X.; Weinstein, L. B.; Wishart, R.; Wood, M. H.; Yale, B.; Zachariou, N.; Zhang, J.

doi:10.1103/PhysRevC.104.025203

We report on the measurement of the beam spin asymmetry in the deeply virtual Compton scattering off He4 using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 6 GeV longitudinally polarized electron beam incident on a pressurized He4 gaseous target. We detail the method used to ensure the exclusivity of the measured reactions, in particular the upgrade of CLAS with a radial time projection chamber to detect the low-energy recoiling He4 nuclei and an inner calorimeter to extend the photon detection acceptance at forward angles. Our results confirm the theoretically predicted enhancement of the coherent (eHe4→e′He4′γ′) beam spin asymmetries compared to those observed on the free proton, while the incoherent (eHe4→e′p′γ′X′) asymmetries exhibit a 30% suppression. From the coherent data, we were able to extract, in a model-independent way, the real and imaginary parts of the only He4 Compton form factor, HA, leading the way toward 3D imaging of the partonic structure of nuclei.