First-principle calculation of the [formula omitted] decay width from lattice QCD

Abstract

We perform a lattice QCD calculation of the ηc→2γ 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 Nf=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 Γηcγγ=6.67(16)stat(6)syst keV, which differs significantly from the Particle Data Group’s reported value of Γηcγγ=5.4(4) keV (2.9σ tension). We provide insight into the comparison between our findings, previous theoretical predictions, and experimental measurements.