Pion exchange interaction in the $\gamma p \to p e^+e^-$ reaction

Abstract

The $\rho^0-\omega$ interference has been studied in the dilepton invariant mass distribution spectra in the photonuclear reaction, but that is not done for the gamma-nucleon reaction. Recent past, the $e^+e^-$ invariant mass distribution spectrum in the $\gamma p$ reaction, i.e., $\gamma p \to pe^+e^-$ reaction, was measured at Jefferson Laboratory to look for the $\rho^0 -\omega$ interference in the multi-GeV region. To study the mechanism of this reaction, the differential cross section of the $e^+e^-$ invariant mass distribution is calculated in the quoted energy region. The reaction is assumed to proceed as $\gamma p \to Vp$; $V \to e^+e^-$, where $V$ denotes a vector meson, i.e., either $\rho^0$ or $\omega$ meson. The photoproduction of the vector meson is described by the Vector Meson Dominance (VMD) model which consists of diagonal and off-diagonal processes. The diagonal process is described as $\gamma \to V; ~Vp \to Vp$. The low energy $\omega$ meson photoproduction data is well described by the off-diagonal process which is illustrated as $\gamma \to \rho^0; ~\rho^0 p \to \omega p$. The reaction $\rho^0p \to \omega p$ proceeds due to one pion exchange interaction. The differential cross sections of the $\gamma p \to pe^+e^-$ reaction due to the above processes of VMD model are compared, and the significance of the pion exchange interaction is investigated in the energy region of $\gamma$ beam available at Jefferson Laboratory.