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Atomssa, E. T; Becker, M; Berger-Chen, J. C; Blanco, A; Blume, C; Böhmer, M; Chudoba, P; Ciepał, I; Deb, S; Deveaux, C; Dittert, D; Dreyer, J; Epple, E; Fabbietti, L; Fonte, P; Franco, C; Fröhlich, I; Förtsch, J; Garzón, J. A; Gernhäuser, R; Greifenhagen, R; Grunwald, M; Gumberidze, M; Harabasz, S; Heinz, T; Hennino, T; Höhne, C; Hojeij, F; Holzmann, R; Idzik, M; Kämpfer, B; Kampert, K-H; Kardan, B; Kedych, V; Koenig, I; Kohls, M; Kolas, J; Kolb, B. W; Korcyl, G; Kornakov, G; Kotte, R; Krueger, W; Kugler, A; Kunz, T; Linev, S; Linz, F; Lopes, L; Maier, L; Malige, A; Markert, J; Maurus, S; Metag, V; Michel, J; Mikhaylov, V; Molenda, A; Münzer, R; Nabroth, M; Naumann, L; Nowakowski, K; Orliński, J; Otto, J. -H; Parpottas, Y; Parschau, M; Pechenov, V; Piasecki, K; Pietraszko, J; Povar, T; Prościnki, P; Pysz, K; Ramstein, B; Rathod, N; Rodriguez-Ramos, P; Rustamov, A; Salabura, P; Scheib, T; Schild, N; Scozzi, F; Seck, F; Silva, L; Singh, U; Smyrski, J; Spataro, S; Spies, S; Stefaniak, M; Ströbele, H; Stroth, J; Sturm, C; Szala, M; Tlusty, P; Traxler, M; Tsertos, H; Vazquez-Doce, O; Wagner, V; Wendisch, C; Wiebusch, M. G; Wirth, J; Wladyszewska, A; Zherebtsova, E; Zielinski, M; Zumbruch, P
09/2023Journal Article
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.
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