Hindered magnetic dipole transitions $\Upsilon(nS)\to \gamma \eta_b(n'S)$ are studied in the covariant light-front approach. Compared with the allowed magnetic dipole transitions, we find that results for hindered magnetic dipole transitions are sensitive to heavy quark mass and shape parameters of the light-front wave functions. It is possible to tune the parameters so that the predictions of branching fractions of $\Upsilon(2S,3S)\to\gamma\eta_b$ are consistent with the recent experimental data, but the relevant decay constant of $\eta_b$ is much smaller than that of $\Upsilon(1S)$. We also generalize the investigation to the charmonium sector and find the the same conclusion.
Categories: hep-ph