Speaker
Description
Pion production in NN collisions is one of the sources of information on the NN
interaction and on the contribution of nucleon resonances. In particular, two-pion production in the few energy range, carries information both on $\pi\pi$ dynamics and on single and double baryon excitation.
The High Acceptance Di-Electron Spectrometer (HADES) [1] installed at GSI Helmholtz-Zentrum für Schwerionenforschung in Darmstadt, designed to investigate dielectron production in heavy-ion collisions in the range of kinetic beam energies 1-3 A GeV is also an excellent detector for charged hadron detection, due to its tracking capabilities.
Recently, differential and integrated cross sections for the reactions pp$\rightarrow$pp$\pi^0$, pp$\rightarrow$pn$\pi^{+}$ [2-3-4], pp$\rightarrow$pp$\pi^{+}\pi^{-}$, pn$\rightarrow$pn$\pi^{+}\pi^{-}$ [5], pn$\rightarrow$d$\pi^{+}\pi^{-}$ have been investigated with HADES at kinetic energies 1.25, 2.2 and 3.5 GeV. This talk will focus on the analysis of the pp$\rightarrow$pp$\pi^{+}\pi^{-}$ channel at 3,5 GeV, using results from pp$\rightarrow$pp$\pi^0$, pp$\rightarrow$pn$\pi^{+}$ [3] and pp$\rightarrow$pK$\Lambda$ [6] measured at the same energy by HADES. The contributions of the excitation on one or two baryonic resonances with masses up to 1,9 GeV and of the $\rho$ production can be quantified. The results are compared with two theoretical models [7-8].
The results of this study provide strong constraints on the pion production mechanisms, and on the various resonance contributions ($\Delta^0$(1232), N*(1440),…), as well as on the double resonance excitation and the direct $\rho$ production. These aspects are closely related to the interpretation of the dielectron spectra measured by the HADES collaboration. Baryonic resonances are indeed important sources of dileptons through two mechanisms: the Dalitz decay (e.g. R$\rightarrow$N$e^{+}e^{-}$) and the mesonic decay with subsequent dielectron production.
[1] G. Agakishiev et al., Eur. Phys. J. A41, 243-277 (2009).
[2] G. Agakishiev et al. Eur.Phys.J. A48 (2012) 74.
[3] G. Agakishiev et al. Eur.Phys.J. A50 (2014) 82.
[4] G. Agakishiev et al. , Eur.Phys.J. A51 (2015), 137.
[5] G. Agakishiev et al., Phys.Lett. B750 (2015) 184.
[6] G. Agakishiev et al. Phys.Lett. B742 (2015) 242-248.
[7] A.P.Jerusalimov et al. Study of the Reaction pn$\rightarrow$pn$\pi^{+}\pi^{-}$ at Intermediate Energies. http://arxiv.org/pdf/1102.1574.pdf
[8] X. Cao et al., Phys. Rev. C81, 065201 (2010).
