2026/07/06 by Lei Chang, Wei-jie Fu, Chuang Huang +2
#hep-ph
We present the first functional QCD calculation of the pion distribution amplitude (DA) using the large-momentum effective theory within the functional renormalisation group (fRG) approach. With only the strong coupling and current quark masses as inputs, we compute the quasi-DA from first-principles QCD correlation functions. By pushing the pion momentum up to Pz = 4.5 GeV, the quasi-DA becomes fully saturated, rendering the extrapolation errors to the light-cone limit negligible. The resulting second-order moment ⟨ ξ2 ⟩π= 0.267 is significantly smaller than existing lattice-LaMET determinations and lies in a range consistent with other nonperturbative approaches.