Inclusive e$^+$e$^-$ production in collisions of pions with protons and nuclei in the second resonance region of baryons

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. Förtsch, 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, P. Prościnki, 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, P. Sellheim, J. Siebenson, L. Silva, U. Singh, J. Smyrski, S. Spataro, S. Spies, M. Stefaniak, H. Ströbele, J. Stroth, C. Sturm, K. Sumara, O. Svoboda, M. Szala, P. Tlusty, M. Traxler, H. Tsertos, O. Vazquez-Doce, V. Wagner, A. A. Weber, C. Wendisch, M. G. Wiebusch, J. Wirth, A Wladyszewska, H. P. Zbroszczyk, E. Zherebtsova, M. Zielinski, P. Zumbruch
Nuclear Experiment, Nuclear Experiment (nucl-ex)
2023-09-22 16:00:00
Inclusive e$^+$e$^-$ production has been studied with HADES in $\pi^-$ + p, $\pi^-$ + C and $\pi^- + \mathrm{CH}_2$ reactions, using the GSI pion beam at $\sqrt{s_{\pi p}}$ = 1.49 GeV. Invariant mass and transverse momentum distributions have been measured and reveal contributions from Dalitz decays of $\pi^0$, $\eta$ mesons and baryon resonances. The transverse momentum distributions are very sensitive to the underlying kinematics of the various processes. The baryon contribution exhibits a deviation up to a factor seven from the QED reference expected for the dielectron decay of a hypothetical point-like baryon with the production cross section constrained from the inverse $\gamma$ n$\rightarrow \pi^-$ p reaction. The enhancement is attributed to a strong four-momentum squared dependence of the time-like electromagnetic transition form factors as suggested by Vector Meson Dominance (VMD). Two versions of the VMD, that differ in the photon-baryon coupling, have been applied in simulations and compared to data. VMD1 (or two-component VMD) assumes a coupling via the $\rho$ meson and a direct coupling of the photon, while in VMD2 (or strict VMD) the coupling is only mediated via the $\rho$ meson. The VMD2 model, frequently used in transport calculations for dilepton decays, is found to overestimate the measured dielectron yields, while a good description of the data can be obtained with the VMD1 model assuming no phase difference between the two amplitudes. Similar descriptions have also been obtained using a time-like baryon transition form factor model where the pion cloud plays the major role.
PDF: Inclusive e$^+$e$^-$ production in collisions of pions with protons and nuclei in the second resonance region of baryons.pdf
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