Speaker
Description
A dedicated programme to study electromagnetic baryonic transitions in the time-like
region has started using the pion beam and the HADES (High Acceptance Di-Electron
Spectrometer) set-up at GSI [1].
First measurements have been performed in the second resonance region, at a center-of-mass
energy of 1.49 GeV, using carbon and polyethylene targets, allowing for an analysis
of the inclusive $e^+e^-$ production and of the exclusive quasi-free $\pi^-p \rightarrow ne^+e^-$ reaction.
The $e^+e^-$ yield at invariant masses larger than 300 MeV/c$^2$ strongly exceeds expectations
based on real photon couplings, signaling the effect of baryon transition form factors of the
Vector Dominance type. A quantitative description of the observed $e^+e^-$ yield is achieved by
estimating the contribution from off-shell $\rho$s deduced from a Partial Wave Analysis [2] of the
two-pion production channels, which were measured simultaneously in our experiment. The
angular distributions for the $e^+e^-$ production contain additional selective information on the
electromagnetic structure of the different transitions. An analysis based on the spin density
matrix formalism, providing in particular quantitative information on the contribution of
longitudinal virtual photons will be presented. The predictions of several models for the $\pi^-p \to ne^+e^-$ reaction [3, 4, 5, 6, 7] will also be discussed.
Finally, the prospects for future experiments focusing on the third resonance region or
on hyperon Dalitz decays will also be shortly presented.
References
[1] J. Adamczewski-Musch, et al. (HADES collaboration), Eur. Phys. J. A53 (2017) 188.
[2] A.V. Anisovich, E. Klempt, A.V. Sarantsev, U. Thoma, Eur. Phys. J. A24 (2005) 111.
[3] A. I. Titov and B. Kämpfer, Eur. Phys. J. A12 (2001) 217.
[4] M. Lutz, B. Friman, M. Soyeur, Nucl. Phys. A713 (2003) 97.
[5] M. Zetenyi and G. Wolf, Phys. Rev. C86 (2012) 065209.
[6] G. Ramalho, G. and M. T. Pena, Phys. Rev. D95 (2017) 014003.
[7] E. Speranza, M. Zetenyi and B. Friman, Phys. Lett. B764 (2017) 282.
