Inclusive e + e − production in collisions of pions with protons and nuclei in the second resonance region of baryons

Le résumé fourni par la source

Inclusive ${e}^{+}{e}^{\ensuremath{-}}$ production has been studied with HADES in ${\ensuremath{\pi}}^{\ensuremath{-}}+p, {\ensuremath{\pi}}^{\ensuremath{-}} +$ C, and ${\ensuremath{\pi}}^{\ensuremath{-}}+{\mathrm{CH}}_{2}$ reactions, using the GSI pion beam at $\sqrt{{s}_{\ensuremath{\pi}p}}=1.49$ GeV. Invariant mass and transverse momentum distributions have been measured and reveal contributions from Dalitz decays of ${\ensuremath{\pi}}^{0}, \ensuremath{\eta}$ mesons, and baryon resonances. The transverse momentum distributions are very sensitive to the underlying kinematics of the various processes. The baryon contribution exhibits a deviation up to a factor seven from the QED reference expected for the dielectron decay of a hypothetical pointlike baryon with the production cross section constrained from the inverse $\ensuremath{\gamma}\phantom{\rule{4pt}{0ex}}n\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}\phantom{\rule{4pt}{0ex}}p$ reaction. The enhancement is attributed to a strong four-momentum squared dependence of the timelike electromagnetic transition form factors as suggested by vector meson dominance (VMD). Two versions of the VMD that differ in the photon-baryon coupling have been applied in simulations and compared to data. VMD1 (or two-component VMD) assumes a coupling via the $\ensuremath{\rho}$ meson and a direct coupling of the photon, while in VMD2 (or strict VMD) the coupling is only mediated via the $\ensuremath{\rho}$ meson. The VMD2 model, frequently used in transport calculations for dilepton decays, is found to overestimate the measured dielectron yields, while a good description of the data can be obtained with the VMD1 model assuming no phase difference between the two amplitudes. Similar descriptions have also been obtained using a timelike baryon transition form factor model where the pion cloud plays the major role.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.