Pion distribution amplitude at the physical point using the leading-twist expansion of the quasi-distribution-amplitude matrix element

Abstract

We present a lattice QCD determination of the distribution amplitude (DA) of the pion and the first few Mellin moments from an analysis of the quasi-DA matrix element within the leading-twist framework. We perform our study on a HISQ ensemble with $a=0.076\text{ }\text{ }\mathrm{fm}$ lattice spacing with the Wilson-clover valence quark mass tuned to the physical point. We analyze the ratios of pion quasi-DA matrix elements at short distances using the leading-twist Mellin operator product expansion (OPE) at the next-to-leading order and the conformal OPE at the leading-logarithmic order. We find a robust result for the first nonvanishing Mellin moment $⟨{x}^{2}⟩=0.287(6)(6)$ at a factorization scale $\ensuremath{\mu}=2\text{ }\text{ }\mathrm{GeV}$. We also present different Ans\"atze-based reconstructions of the $x$-dependent DA, from which we determine the perturbative leading-twist expectations for the pion electromagnetic and gravitational form-factors at large momentum transfers.