First-principle calculation of ${\eta }_c\to 2\gamma$ decay width from lattice QCD

Abstract

We perform a lattice QCD calculation of the ${\eta }_c\to 2\gamma$ decay width using a model-independent method that requires no momentum extrapolation of the off-shell form factors. This method also provides a straightforward and simple way to examine the finite-volume effects. The calculation is accomplished using $N_f=2$ twisted mass fermion ensembles. The statistically significant excited-state effects are observed and eliminated using a multi-state fit.The impact of fine-tuning the charm quark mass is also examined and confirmed to be well-controlled. Finally, using three lattice spacings for the continuum extrapolation, we obtain the decay width ${\Gamma }_{{\eta }_c\gamma \gamma }=6.67(16{)}_{\mathrm{stat}}(6{)}_{\mathrm{syst}}$ keV, which differs significantly from the Particle Data Group's reported value of ${\Gamma }_{{\eta }_c\gamma \gamma }=5.4(4)$ keV (2.9~$\sigma$ tension). We provide insight into the comparison between our findings, previous theoretical predictions, and experimental measurements.

Comments: 8 pages + 2 page supplemental material, 5 figures,4 tables. Published version by Science Bulletin