Radiative and leptonic decays of the pseudoscalar charmonium stateηc

Abstract

The radiative and leptonic decays of ${\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ and ${\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}}$ are studied. For a ${\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ decay, the second-order electromagnetic tree-level diagram gives the leading contribution. The decay rate of ${\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ is calculated, the prediction is in good agreement with the experimental data. For ${\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}}$, both the tree and loop diagrams are calculated. The analysis shows that the loop contribution dominates; the contribution of tree diagram with a ${Z}^{0}$ intermediate state can only modify the decay rate by less than 1%. The prediction of the branching ratios of ${\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ and ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ are very tiny within the standard model. The smallness of these predictions within the standard model makes the leptonic decays of ${\ensuremath{\eta}}_{c}$ sensitive to physics beyond the standard model. Measurement of the leptonic decay may give information about new physics.