First Exclusive Measurement of Deeply Virtual Compton Scattering off $^4$He: Toward the 3D Tomography of Nuclei

• Published in: Physical review letters 2017, Vol.119 (20)
• Main Authors: Hattawy, M., Baltzell, N.A., Dupré, R., Hafidi, K., Stepanyan, S., Bültmann, S., de Vita, R., El Alaoui, A., El Fassi, L., Egiyan, H., Girod, F.X., Guidal, M., Jenkins, D., Liuti, S., Perrin, Y., Torayev, B., Voutier, E., Adhikari, K.P., Adhikari, S., Adikaram, D., Akbar, Z., Amaryan, M.J., Anefalos Pereira, S., Armstrong, Whitney R., Avakian, H., Ball, J., Bashkanov, M., Battaglieri, M., Batourine, V., Bedlinskiy, I., Biselli, A.S., Boiarinov, S., Briscoe, W.J., Brooks, W.K., Burkert, V.D., Cao, Frank Thanh, Carman, D.S., Celentano, A., Charles, G., Chetry, T., Ciullo, G., Clark, L., Colaneri, L., Cole, P.L., Contalbrigo, M., Cortes, O., Crede, V., d'Angelo, A., Dashyan, N., de Sanctis, E., Deur, A., Djalali, C., Elouadrhiri, L., Eugenio, P., Fedotov, G., Fegan, S., Fersch, R., Filippi, A., Fleming, J.A., Forest, T.A., Fradi, A., Garçon, M., Gevorgyan, N., Ghandilyan, Y., Gilfoyle, G.P., Giovanetti, K.L., Gleason, C., Gohn, W., Golovatch, E., Gothe, R.W., Griffioen, K.A., Guo, L., Hakobyan, H., Hanretty, C., Harrison, N., Heddle, D., Hicks, K., Holtrop, M., Hughes, S.M., Ireland, D.G., Ishkhanov, B.S., Isupov, E.L., Jiang, H., Joo, K., Joosten, S., Keller, D., Khachatryan, G., Khachatryan, M., Khandaker, M., Kim, A., Kim, W., Klein, A., Klein, F.J., Kubarovsky, V., Kuhn, S.E., Kuleshov, S.V., Lanza, L., Lenisa, P., Livingston, K., Lu, H.Y.
• Format: Article
• Language: English
• Subjects: High Energy Physics - Experiment; Nuclear Experiment; Physics
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.119.202004

We report on the first measurement of the beam-spin asymmetry in the exclusive process of coherent deeply virtual Compton scattering off a nucleus. The experiment uses the 6 GeV electron beam from the Continuous Electron Beam Accelerator Facility (CEBAF) accelerator at Jefferson Lab incident on a pressurized He4 gaseous target placed in front of the CEBAF Large Acceptance Spectrometer (CLAS). The scattered electron is detected by CLAS and the photon by a dedicated electromagnetic calorimeter at forward angles. To ensure the exclusivity of the process, a specially designed radial time projection chamber is used to detect the recoiling He4 nuclei. We measure beam-spin asymmetries larger than those observed on the free proton in the same kinematic domain. From these, we are able to extract, in a model-independent way, the real and imaginary parts of the only He4 Compton form factor, HA. This first measurement of coherent deeply virtual Compton scattering on the He4 nucleus, with a fully exclusive final state via nuclear recoil tagging, leads the way toward 3D imaging of the partonic structure of nuclei.