Hadronic loop contributions to $J/\psi$ and ${\psi }^{\prime }$ radiative decays into $\gamma {\eta }_c$ or $\gamma {\eta }_c^{\prime }$

Abstract

Intermediate hadronic meson loop contributions to $J/\psi$, ${\psi }^{\prime }\to \gamma {\eta }_c$ ($\gamma {\eta }_c^{\prime }$) are studied apart from the dominant M1 transitions in an effective Lagrangian approach. Due to the property of the unique antisymmetric tensor coupling in $V\to VP$, the hadronic loop transitions provide explicit corrections to the M1 transition amplitudes derived from the naive "quenched" $c\bar{c}$ transitions via the coupling form factors. This mechanism interfering with the M1 transition amplitudes naturally accounts for the deviations from the Godfrey-Isgur model predictions in $J/\psi$ and ${\psi }^{\prime }\to \gamma {\eta }_c$. It also predicts a small branching ratio of ${\psi }^{\prime }\to \gamma {\eta }_c^{\prime }$, which can be examined by experimental measurements at BES and CLEO-c.

Comments: 10 pages, 2 eps figures, revtex4; version to appear on Phys. Lett. B