First Measurement of Hard Exclusive <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>π</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:msup><mml:msup><mml:mrow><mml:mi mathvariant="normal">Δ</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math> Electroproduction Beam-Spin Asymmetries off the Proton
S. Diehl, N. Trotta, K. Joo, P. Achenbach, Z. Akbar, W. R. Armstrong, H. Ataç, H. Avakian, L. Baashen, N. A. Baltzell, L. Barion, M. Bashkanov, M. Battaglieri, I. Bedlinskiy, F. Benmokhtar, A. Bianconi, A. S. Biselli, F. Bossù, Kai-Thomas Brinkmann, W. J. Briscoe, D. Bulumulla, V. D. Burkert, R. Capobianco, D. S. Carman, J.C Carvajal, A. Celentano, G. Charles, P. Chatagnon, V. Chesnokov, G. Ciullo, P. L. Cole, M. Contalbrigo, G. Costantini, V. Credé, A. D’Angelo, N. Dashyan, R. De Vita, A. Deur, C. Djalali, R. Dupré, M. Ehrhart, A. El Alaoui, L. El Fassi, L. Elouadrhiri, S. Fegan, A. Filippi, G. Gavalian, D. I. Glazier, A. A. Golubenko, G. Gosta, R. W. Gothe, Y. Gotra, K. A. Griffioen, K. Hafidi, H. Hakobyan, M. Hattawy, T. B. Hayward, D. Heddle, A. Hobart, M. Holtrop, I. Illari, D. G. Ireland, E. L. Isupov, H. S. Jo, R. Johnston, D. Keller, M. Khachatryan, A. Khanal, A. Kim, W. Kim, V. Klimenko, Á. Kripkó, V. Kubarovsky, S. Kuhn, V. Lagerquist, L. Lanza, M. Leali, S. Lee, P. Lenisa, X. Li, I. J. D. MacGregor, D. Marchand, V. Mascagna, G. Matousek, B. McKinnon, C. McLauchlin, Z.-E. Meziani, S. Migliorati, R. G. Milner, T. Mineeva, M. Mirazita, V. Mokeev, Patrick Moran, C. Muñoz Camacho, P. Naidoo, K. Neupane, S. Niccolai, G. Niculescu, M. Osipenko, P. Pandey
Abstract
The polarized cross-section ratio ${\ensuremath{\sigma}}_{L{T}^{\ensuremath{'}}}/{\ensuremath{\sigma}}_{0}$ from hard exclusive ${\ensuremath{\pi}}^{\ensuremath{-}}{\mathrm{\ensuremath{\Delta}}}^{++}$ electroproduction off an unpolarized hydrogen target has been extracted based on beam-spin asymmetry measurements using a $10.2\text{ }\text{ }\mathrm{GeV}/10.6\text{ }\text{ }\mathrm{GeV}$ incident electron beam and the CLAS12 spectrometer at Jefferson Lab. The study, which provides the first observation of this channel in the deep-inelastic regime, focuses on very forward-pion kinematics in the valence regime, and photon virtualities ranging from $1.5\text{ }\text{ }{\mathrm{GeV}}^{2}$ up to $7\text{ }\text{ }{\mathrm{GeV}}^{2}$. The reaction provides a novel access to the $d$-quark content of the nucleon and to $p\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Delta}}}^{++}$ transition generalized parton distributions. A comparison to existing results for hard exclusive ${\ensuremath{\pi}}^{+}n$ and ${\ensuremath{\pi}}^{0}p$ electroproduction is provided, which shows a clear impact of the excitation mechanism, encoded in transition generalized parton distributions, on the asymmetry.