Inclusive <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>−</mml:mo> </mml:msup> </mml:mrow> </mml:math> production in collisions of pions with protons and nuclei in the second resonance region of baryons
Abstract
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.