First Measurement of Timelike Compton Scattering

Chatagnon, P.; Niccolai, S.; Stepanyan, S.; Amaryan, M. J.; Angelini, G.; Armstrong, W. R.; Atac, H.; Ayerbe Gayoso, C.; Baltzell, N. A.; Barion, L.; Bashkanov, M.; Battaglieri, M.; Bedlinskiy, I.; Benmokhtar, F.; Bianconi, A.; Biondo, L.; Biselli, A. S.; Bondi, M.; Bossu, F.; Boiarinov, S.; Briscoe, W. J.; Brooks, W. K.; Bulumulla, D.; Burkert, V. D.; Carman, D. S.; Carvajal, J. C.; Caudron, M.; Celentano, A.; Chetry, T.; Ciullo, G.; Clark, L.; Cole, P. L.; Contalbrigo, M.; Costantini, G.; Crede, V.; D'Angelo, A.; Dashyan, N.; Defurne, M.; De Vita, R.; Deur, A.; Diehl, S.; Djalali, C.; Dupre, R.; Egiyan, H.; Ehrhart, M.; El Alaoui, A.; El Fassi, L.; Elouadrhiri, L.; Fegan, S.; Fersch, R.; Filippi, A.; Gavalian, G.; Ghandilyan, Y.; Gilfoyle, G. P.; Girod, F. X.; Glazier, D. I.; Golubenko, A. A.; Gothe, R. W.; Gotra, Y.; Griffioen, K. A.; Guidal, M.; Guo, L.; Hakobyan, H.; Hattawy, M.; Hayward, T. B.; Heddle, D.; Hobart, A.; Holtrop, M.; Hyde, C. E.; Ilieva, Y.; Ireland, D. G.; Isupov, E. L.; H. S., Jo; Joo, K.; Kabir, M. L.; Keller, D.; Khachatryan, G.; Khanal, A.; Kim, A.; Kim, W.; Kripko, A.; Kubarovsky, V.; Kuhn, S. E.; Lanza, L.; Leali, M.; Lee, S.; Lenisa, P.; Livingston, K.; Macgregor, I. J. D.; Marchand, D.; Marsicano, L.; Mascagna, V.; Mckinnon, B.; Mclauchlin, C.; Migliorati, S.; Mirazita, M.; Mokeev, V.; Montgomery, R. A.; Munoz Camacho, C.; Nadel-Turonski, P.; Naidoo, P.; Neupane, K.; O'Connell, T. R.; Osipenko, M.; Ouillon, M.; Pandey, P.; Paolone, M.; Pappalardo, L. L.; Paremuzyan, R.; Pasyuk, E.; Phelps, W.; Pogorelko, O.; Poudel, J.; Price, J. W.; Prok, Y.; Raue, B. A.; Reed, T.; Ripani, M.; Rizzo, A.; Rossi, P.; Rowley, J.; Sabatie, F.; Schmidt, A.; Segarra, E. P.; Sharabian, Y. G.; Shirokov, E. V.; Shrestha, U.; Sokhan, D.; Soto, O.; Sparveris, N.; Strakovsky, I. I.; Strauch, S.; Tyler, N.; Tyson, R.; Ungaro, M.; Vallarino, S.; Venturelli, L.; Voskanyan, H.; Vossen, A.; Voutier, E.; Watts, D. P.; Wei, K.; Wei, X.; Wishart, R.; Yale, B.; Zachariou, N.; Zhang, J.; Zhao, Z. W.

doi:10.1103/PhysRevLett.127.262501

We present the first measurement of the timelike Compton scattering process, γp→p′γ∗(γ∗→e+e-), obtained with the CLAS12 detector at Jefferson Lab. The photon beam polarization and the decay lepton angular asymmetries are reported in the range of timelike photon virtualities 2.25<9 GeV2, squared momentum transferred 0.1<-t<0.8 GeV2, and average total center-of-mass energy squared s=14.5 GeV2. The photon beam polarization asymmetry, similar to the beam-spin asymmetry in deep virtual Compton scattering, is sensitive to the imaginary part of the Compton form factors and provides a way to test the universality of the generalized parton distributions. The angular asymmetry of the decay leptons accesses the real part of the Compton form factors and thus the D-term in the parametrization of the generalized parton distributions.