Speaker
Description
The axion is a prime dark matter candidate that aditionally solves the strong CP problem. Its experimental detection relies on the axion-photon coupling, which receives a model-independent contribution from Quantum Chromodynamics (QCD). Previously, this contribution was only estimated via different variants of Chiral Perturbation Theory (ChPT) with inconsistent results. We present the first non-perturbative, first-principles determination of the QCD axion-photon coupling using continuum-extrapolated lattice simulations. We obtain $g_{a\gamma\gamma}^{\rm QCD}= -1.77(8)\alpha_{em}/(2\pi f_a)$, which is about 10% smaller in magnitude than the ChPT prediction quoted by the PDG. In this talk we will describe how this quantity can be extracted by studying the response of the QCD vacuum to a time-reversal violating combination of electromagnetic fields and discuss how our result modifies the constraints over the landscape of viable axion models, providing guidance for ongoing and future experimental searches.