A possible mechanism for producing the threshold enhancement in $J/ψ\toγp\bar{p}$

Abstract

In the $J/ψ$ radiative decay channels $J/ψ\to γV\bar{V}$, the result of partial wave analysis indicates that the $V\bar{V}$ systems are predominately pseudoscalar component, and most of these channels have relatively large branching ratios at an order of $10^{-3}$. Meanwhile, vector mesons, such as $ρ$, $ω$ and $K^*$, have strong couplings with nucleons and/or hyperons. This suggests a dynamical mechanism describing the $ηp\bar{p}$ form factors for higher $η$ mesons, such as $η(1405/1475)$ and $η(1760)$. It is thus natural to expect that rescatterings of these vector meson pairs into $p\bar{p}$ of $0^-$ partial wave could be an important source contributing to $J/ψ\toγp\bar{p}$ of which the branching ratio is at the order of $10^{-4}$. 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 $p\bar{p}$. Without introducing dramatic ingredients, our model provides a natural explanation for the peculiar threshold enhancement observed by BES-II in $J/ψ\to γp\bar{p}$. Additional experimental constraints on the $V\bar{V}\to p\bar{p}$ transitions are examined. This mechanism in $J/ψ\to ωp\bar{p}$ is also discussed.