First Measurement of Timelike Compton Scattering
P. Chatagnon, S. Niccolai, S. Stepanyan, M. J. Amaryan, G. Angelini, W. R. Armstrong, H. Atac, C. Ayerbe Gayoso, N. A. Baltzell, L. Barion, M. Bashkanov, M. Battaglieri, I. Bedlinskiy, F. Benmokhtar, A. Bianconi, L. Biondo, A. S. Biselli, M. Bondi, F. Bossù, S. Boiarinov, W. J. Briscoe, W. K. Brooks, D. Bulumulla, V. D. Burkert, D. S. Carman, J. C. Carvajal, M. Caudron, A. Celentano, T. Chetry, G. Ciullo, L. Clark, P. L. Cole, M. Contalbrigo, G. Costantini, V. Crede, A. D’Angelo, N. Dashyan, M. Defurne, R. De Vita, A. Deur, S. Diehl, C. Djalali, R. Dupré, H. Egiyan, M. Ehrhart, A. El Alaoui, L. El Fassi, L. Elouadrhiri, S. Fegan, R. Fersch, A. Filippi, G. Gavalian, Y. Ghandilyan, G. P. Gilfoyle, F. X. Girod, D. I. Glazier, A. A. Golubenko, R. W. Gothe, Y. Gotra, K. A. Griffioen, M. Guidal, L. Guo, H. Hakobyan, M. Hattawy, T. B. Hayward, D. Heddle, A. Hobart, M. Holtrop, C. E. Hyde, Y. Ilieva, D. G. Ireland, E. L. Isupov, H. S. Jo, K. Joo, M. L. Kabir, D. Keller, G. Khachatryan, A. Khanal, A. Kim, W. Kim, A. Kripko, V. Kubarovsky, S. E. Kuhn, L. Lanza, M. Leali, S. Lee, P. Lenisa, K. Livingston, I. J. D. MacGregor, D. Marchand, L. Marsicano, V. Mascagna, B. McKinnon, C. McLauchlin, S. Migliorati, M. Mirazita, V. Mokeev, R. A. Montgomery, C. Munoz Camacho, P. Nadel-Turonski
Abstract
We present the first measurement of the timelike Compton scattering process, $\ensuremath{\gamma}p\ensuremath{\rightarrow}{p}^{\ensuremath{'}}{\ensuremath{\gamma}}^{*}({\ensuremath{\gamma}}^{*}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}})$, 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 $2.25<{Q}^{\ensuremath{'}2}<9\text{ }\text{ }{\mathrm{GeV}}^{2}$, squared momentum transferred $0.1<\ensuremath{-}t<0.8\text{ }\text{ }{\mathrm{GeV}}^{2}$, and average total center-of-mass energy squared $s=14.5\text{ }\text{ }{\mathrm{GeV}}^{2}$. 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.