Speaker
Description
The High Acceptance Di-Electron Spectrometer (HADES) [1], installed at GSI/FAIR Helmholtzzentrum in Darmstadt, was designed for spectroscopy of positron-electron pairs in heavy-ion reactions in the SIS-18 energy range (1-2 GeV/nucleon). The HADES collaboration has measured inclusive e+e- production in proton-nucleus and nucleus-nucleus systems at various energies providing information on a contribution from baryonic matter which manifests itself as an excess radiation with respect to a nucleon-nucleon reference. The radiation is mediated by the rho vector meson, which properties (spectral function) are strongly modified in the cold or dense nuclear matter. These modifications are due to vector meson-baryon couplings and can also be studied in decays of baryonic resonances R → Ne+e- (Dalitz decays) providing information on the electromagnetic baryon-resonance transition form factors (eTFF) in the time-like region.
The elementary collisions, especially those with pion beams, offer a great opportunity to study eTFF in a direct way. The HADES collaboration has measured the $\Delta$(1232) Dalitz decay in pp collisions [2] delivering, for the first time, the $\Delta$ → pe+e- branching ratio. In the next step, using combined measurements of hadronic and dielectron final states in $\pi$-N collisions and Partial Wave Analysis (PWA) developed by the Bonn-Gatchina group [3], the contributions of N(1440), N(1520) and N(1535) to two pion and dielectron final states have been studied. As a result cross sections for $\Delta$$\pi$, N$\rho$, N$\sigma$ isobar contributions have been extracted. In dielectron channel the off-shell $\rho$ meson contribution to the Dalitz decays of N(1520) and N(1535) have been obtained and allowed for extraction of the mass dependence of the effective time-like eTFF [4]. Studies of angular distributions of emitted electrons have delivered information on hadronic spin density matrix elements (the helicity structure of baryon eTFF) [5]. The recent upgrade of the HADES detector [6] made possible to study also electromagnetic decays of hyperons. First measurements at HADES on both virtual and real photon decays, Y → Y e+e and Y* → Y $\gamma$ , have been performed in p+p collisions at 4.5 GeV [7].
The results of the HADES collaboration obtained with proton and pion beams will be presented. The eTFF will be compared to various versions of the Vector Dominance Model, to quark-constituent model [8] and Lagrangian microscopic calculations [5]. In addition the Dalitz decay will provide valuable information about eTFF of hyperon resonances. Prospects for HADES measurements at SIS-18 in the near future within the FAIR-Phase0 programme will also be discussed.
[1] G. Agakichiev et al. (HADES), Eur. Phys. J. A 41, 243 (2009).
[2] J. Adamczewski-Musch et al. (HADES), Phys. Rev. C95, 06520 (2017).
[3] J. Adamczewski-Musch et al. (HADES), Phys. Rev. C 102, 024001 (2020).
[4] R. Abou Yassine et al. (HADES), arXiv:2205.15914 [nucl-ex]
[5] M. Zétényi et al., Phys. Rev. C 104, 015201 (2021).
[6] J. Adamczewski-Musch et al. (HADES), Eur. Phys. J. A 57, 138 (2021).
[7] J. Rieger (HADES, PANDA@ HADES), EPJ Web of Conferences 291, 05005 (2024).
[8] G. Ramalho, M. T. Pena, Phys. Rev. D 95, 014003 (2017).
