arXiv · 2205.15914
First measurement of massive virtual photon emission from N* baryon resonances
Abstract
First information on the time-like electromagnetic structure of baryons in the second resonance region has been obtained from measurements of dielectron (e+ e-) invariant-mass and angular distributions in the quasi-free reaction $\pi-$ p $\rightarrow$ n e+ e- at $\sqrt{s_{\pi p}}$ = 1.49 GeV with the High Acceptance Di-Electron Spectrometer (HADES) at GSI using the pion beam impinging on a CH$_2$ target. We find a total cross section $\sigma$ = 2.97 $\pm$ 0.07data $\pm$ 0.21acc $\pm$ 0.31Zeff $\mu$b. In complement to the analysis of the inclusive e+ e- channel, this data set provides a crucial test of the description of baryon time-like transitions. Approaches based on a Vector Meson Dominance amplitude containing direct photon and vector meson ($\rho$) couplings to the baryon provide a satisfactory agreement with the data. A good description is also obtained by electromagnetic time-like baryon transition form factors in a covariant spectator-quark model, pointing to the dominance of meson-cloud effects. The dielectron angular distributions exhibit the contributions of virtual photons ($\gamma^*$) with longitudinal polarization, in contrast to real photons. The virtual photon angular dependence supports the dominance of J=3/2, I=1/2 contributions observed in both the $\gamma^*$n and the $\pi \pi$n channels.
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R. Abou Yassine, J. Adamczewski-Musch, O. Arnold, E. T. Atomssa, M. Becker, C. Behnke, J. C. Berger-Chen, A. Blanco, C. Blume, M. Böhmer, L. Chlad, P. Chudoba, I. Ciepał, S. Deb, C. Deveaux, D. Dittert, J. Dreyer, E. Epple, L. Fabbietti, P. Fonte, C. Franco, J. Friese, I. Fröhlich, J. Fortsch, T. Galatyuk, J. A. Garzón, R. Gernhäuser, R. Greifenhagen, M. Grunwald, M. Gumberidze, S. Harabasz, T. Heinz, T. Hennino, C. Höhne, F. Hojeij, R. Holzmann, M. Idzik, B. Kämpfer, K-H. Kampert, B. Kardan, V. Kedych, I. Koenig, W. Koenig, M. Kohls, J. Kolas, B. W. Kolb, G. Korcyl, G. Kornakov, R. Kotte, W. Krueger, A. Kugler, T. Kunz, R. Lalik, K. Lapidus, S. Linev, F. Linz, L. Lopes, M. Lorenz, T. Mahmoud, L. Maier, A. Malige, J. Markert, S. Maurus, V. Metag, J. Michel, D. M. Mihaylov, V. Mikhaylov, A. Molenda, C. Müntz, R. Münzer, M. Nabroth, L. Naumann, K. Nowakowski, J. Orliński, J. -H. Otto, Y. Parpottas, M. Parschau, C. Pauly, V. Pechenov, O. Pechenova, K. Piasecki, J. Pietraszko, T. Povar, K. Prościński, A. Prozorov, W. Przygoda, K. Pysz, B. Ramstein, N. Rathod, P. Rodriguez-Ramos, A. Rost, A. Rustamov, P. Salabura, T. Scheib, N. Schild, K. Schmidt-Sommerfeld, H. Schuldes, E. Schwab, F. Scozzi, F. Seck. 2022-05-31. First measurement of massive virtual photon emission from N* baryon resonances. https://arxiv.org/abs/2205.15914
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