First measurement of massive virtual photon emission from N baryon resonances

• Published in: arXiv.org 2024-09
• Main Authors: R Abou Yassine, Adamczewski-Musch, J, Atomssa, E T, Becker, M, Behnke, C, Blanco, A, Blume, C, Böhmer, M, Chlad, L, Chudoba, P, Ciepał, I, Deb, S, Deveaux, C, Dittert, D, Dreyer, J, Epple, E, Fonte, P, Friese, J, Fröhlich, I, tsch, J, Gernhäuser, R, Greifenhagen, R, Grunwald, M, Gumberidze, M, Harabasz, S, Hennino, T, Höhne, C, Hojeij, F, Holzmann, R, Kämpfer, B, K-H Kampert, Kardan, B, Koenig, I, Koenig, W, J Kolas, Kolb, B W, Korcyl, G, Kotte, R, Krueger, W, Kugler, A, Lapidus, K, Linz, F, Lopes, L, Lorenz, M, Mahmoud, T, Markert, J, Maurus, S, Metag, V, Michel, J, Mihaylov, D M, Mikhaylov, V, Molenda, A, Müntz, C, Nabroth, M, Naumann, L, Nowakowski, K, Orliński, J, J -H Otto, Parpottas, Y, Parschau, M, Pauly, C, Pechenov, V, Pechenova, O, Piasecki, K, Prościński, K, Prozorov, A, Przygoda, W, Ramstein, B, Rathod, N, Rodriguez-Ramos, P, Rost, A, Rustamov, A, Scheib, T, Schmidt-Sommerfeld, K, Schwab, E, Seck, F, Siebenson, J, Singh, U, Smyrski, J, Spataro, S, Spies, S, Stefaniak, M, Ströbele, H, Stroth, J, Sturm, C, Svoboda, O, M Szala, Traxler, M, Tsertos, H, Udrea, I C, Vazquez-Doce, O, Wagner, V, Wendisch, C, Wiebusch, M G, Wirth, J, Władyszewska, A, Zbroszczyk, H P, Zherebtsova, E, Zieliński, M, Zumbruch, P
• Format: Article
• Language: English
• Subjects: Amplitudes; Baryon resonance; Baryons; Dominance; Form factors; Invariants; Photon emission; Photons; Pion beams; Pions; Quark models; Vector mesons
• ISSN: 2331-8422

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.