학술논문
First measurement of massive virtual photon emission from N* baryon resonances
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.; Ciepal, I.; Deveaux, C.; Dittert, D.; Dreyer, J.; Epple, E.; Fabbietti, L.; Fonte, P.; Franco, C.; Friese, J.; Fröhlich, I.; Förtsch, J.; Galatyuk, T.; Garzon, 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.; Kolb, B. W.; Korcyl, G.; Kornakov, G.; Kornas, F.; Kotte, R.; Krueger, W.; Kugler, A.; Kunz, T.; Lalik, R.; Lapidus, K.; Linev, S.; 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.; Naumann, L.; Nowakowski, K.; Otto, J. -H.; Parpottas, Y.; Parschau, M.; Pauly, C.; Pechenov, V.; Pechenova, O.; Pietraszko, J.; Povar, T.; Prozorov, A.; Przygoda, W.; Pysz, K.; Ramstein, B.; Rathod, N.; Rodriguez-Ramos, P.; Rost, 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. S.; Ströbele, H.; Stroth, J.; Strzempek, P.; 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.; Zbroszczyk, H. P.; Zherebtsova, E.; Zumbruch, P.; Zetenyi, M.
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Abstract
First information on the timelike electromagnetic structure of baryons in the second resonance region has been obtained from measurements of invariant mass and angular distributions in the quasi-free reaction $\pi^- p \to nee$ at $\sqrt{s_{\pi^- p}}$ = 1.49 GeV with the High Acceptance Di-Electron Spectrometer (HADES) detector at GSI using the pion beam impinging on a CH$_2$ target. We find a total cross section $\sigma (\pi^- p \to nee) = 2.97 \pm 0.07^{data} \pm 0.21^{acc} \pm 0.31^{\rm{Z}_{\rm{eff}}} \mu$b. Combined with the Partial Wave Analysis of the concurrently measured two-pion channel, these data sets provide a crucial test of Vector Meson Dominance (VMD) inspired models. The commonly used "strict VMD" approach strongly overestimates the $e^+e^-$ yield. Instead, approaches based on a VMD amplitude vanishing at small $e^+e^-$ invariant masses supplemented coherently by a direct photon amplitude provide a better agreement. A good description of the data is also obtained using a calculation of electromagnetic timelike baryon transition form factors in a covariant spectator-quark model, demonstrating the dominance of meson cloud effects. The angular distributions of $e^+e^-$ pairs demonstrate the contributions of virtual photons 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 $\gamma^{\star} n$ and the $\pi \pi n$ channels.
Comment: 8 pages, 3 figures
Comment: 8 pages, 3 figures