First-principle calculation of the η→2γ decay width from lattice QCD.

We perform a lattice QCD calculation of the η→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 N=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 [Formula: see text] keV, which differs significantly from the Particle Data Group's reported value of [Formula: see text] keV (2.9σ tension). We provide insight into the comparison between our findings, previous theoretical predictions, and experimental measurements.