학술논문
Inclusive e$^+$e$^-$ production in collisions of pions with protons and nuclei in the second resonance region of baryons
Document Type
Working Paper
Author
Yassine, R. Abou; Adamczewski-Musch, J.; Arnold, O.; Atomssa, E. T.; Becker, M.; Behnke, C.; Berger-Chen, J. C.; Blanco, A.; Blume, C.; Böhmer, M.; Chlad, L.; Chudoba, P.; Ciepał, I.; Deb, S.; Deveaux, C.; Dittert, D.; Dreyer, J.; Epple, E.; Fabbietti, L.; Fonte, P.; Franco, C.; Friese, J.; Fröhlich, I.; Förtsch, J.; Galatyuk, T.; 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.; Koenig, W.; Kohls, M.; Kolas, J.; Kolb, B. W.; Korcyl, G.; Kornakov, G.; Kotte, R.; Krueger, W.; Kugler, A.; Kunz, T.; Lalik, R.; Lapidus, K.; Linev, S.; Linz, F.; Lopes, L.; Lorenz, M.; Mahmoud, T.; Maier, L.; Malige, A.; Markert, J.; Maurus, S.; Metag, V.; Michel, J.; Mihaylov, D. M.; Mikhaylov, V.; Molenda, A.; Müntz, C.; Münzer, R.; Nabroth, M.; Naumann, L.; Nowakowski, K.; Orliński, J.; Otto, J. -H.; Parpottas, Y.; Parschau, M.; Pauly, C.; Pechenov, V.; Pechenova, O.; Piasecki, K.; Pietraszko, J.; Povar, T.; Prościnki, P.; Prozorov, A.; Przygoda, W.; Pysz, K.; Ramstein, B.; Rathod, N.; Rodriguez-Ramos, P.; Rost, A.; Rustamov, A.; Salabura, P.; Scheib, T.; Schild, N.; Schmidt-Sommerfeld, K.; Schuldes, H.; Schwab, E.; Scozzi, F.; Seck, F.; Sellheim, P.; Siebenson, J.; Silva, L.; Singh, U.; Smyrski, J.; Spataro, S.; Spies, S.; Stefaniak, M.; Ströbele, H.; Stroth, J.; Sturm, C.; Sumara, K.; Svoboda, O.; Szala, M.; Tlusty, P.; Traxler, M.; Tsertos, H.; Vazquez-Doce, O.; Wagner, V.; Weber, A. A.; Wendisch, C.; Wiebusch, M. G.; Wirth, J.; Wladyszewska, A; Zbroszczyk, H. P.; Zherebtsova, E.; Zielinski, M.; Zumbruch, P.
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Subject
Language
Abstract
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.
Comment: (HADES collaboration)
Comment: (HADES collaboration)