Lattice QCD determination of the normalization of the leading-twist photon distribution amplitude and susceptibility of the quark condensate

Abstract

The normalization of the leading-twist photon distribution amplitude (DA), fγ⊥, is an important ingredient in the study of exclusive processes involving the photon emission by means of QCD sum-rules. In this paper we determine the up-, down- and strange-quark contribution to fγ⊥ by exploiting its relation to the zero-momentum two-point correlation function of the electromagnetic current Jemμ and the electric component of the tensor current Tμν. To that end we employ the gauge ensembles obtained by using Nf=2+1+1 Wilson-Clover twisted-mass quark flavors, generated by the Extended Twisted Mass (ETM) Collaboration, and after adding all sources of systematic uncertainties, we obtain a total error of 1.5% and 3.5%, respectively, for the light- (u and d) and strange-quark contribution to fγ⊥(2  GeV) in the MS¯ scheme, thus improving their accuracy by a factor of 2.3 and 2.8, respectively. For the strange-quark contribution fγ,s⊥(2  GeV), we observe a discrepancy with respect to previous lattice calculations. By combining our result with the world average lattice value of the chiral condensate, we obtain for the susceptibility of the quark condensate χdMS¯(2  GeV)≃χuMS¯(2  GeV)=2.17(12)  GeV−2. Published by the American Physical Society 2024