Possible mechanism for producing the threshold enhancement inJ/ψ→γpp¯

Abstract

In the $J/\ensuremath{\psi}$ radiative decay channels $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}V\overline{V}$, the result of partial wave analysis indicates that the $V\overline{V}$ systems are predominately pseudoscalar component, and most of these channels have relatively large branching ratios at an order of ${10}^{\ensuremath{-}3}$. Meanwhile, vector mesons, such as $\ensuremath{\rho}$, $\ensuremath{\omega}$, and ${K}^{*}$, have strong couplings with nucleons and/or hyperons. This suggests a dynamical mechanism describing the $\ensuremath{\eta}p\overline{p}$ form factors for higher $\ensuremath{\eta}$ mesons, such as $\ensuremath{\eta}(1405/1475)$ and $\ensuremath{\eta}(1760)$. It is thus natural to expect that rescatterings of these vector meson pairs into $p\overline{p}$ of ${0}^{\ensuremath{-}}$ partial wave could be an important source contributing to $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}p\overline{p}$ of which the branching ratio is at the order of ${10}^{\ensuremath{-}4}$. Our calculation justifies this point. In particular, we find that interferences between different rescattering amplitudes can produce a significant threshold enhancement in the invariant mass spectrum of $p\overline{p}$. Without introducing dramatic ingredients, our model provides a natural explanation for the peculiar threshold enhancement observed by BES-II in $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}p\overline{p}$. Additional experimental constraints on the $V\overline{V}\ensuremath{\rightarrow}p\overline{p}$ transitions are examined. This mechanism in $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\omega}p\overline{p}$ is also discussed.