We present the first lattice QCD calculation of the universal axial γW-box contribution □γWVA​ to both superallowed nuclear and
neutron beta decays. This contribution emerges as a significant component
within the theoretical uncertainties surrounding the extraction of ∣Vud​∣
from superallowed decays. Our calculation is conducted using two domain wall
fermion ensembles at the physical pion mass. To construct the nucleon 4-point
correlation functions, we employ the random sparsening field technique.
Furthermore, we incorporate long-distance contributions to the hadronic
function using the infinite-volume reconstruction method. Upon performing the
continuum extrapolation, we arrive at □γWVA​=3.65(8)lat​(1)PT​×10−3. Consequently,
this yields a slightly higher value of
∣Vud​∣=0.97386(11)exp.​(9)RC​(27)NS​,
reducing the previous 2.1σ tension with the CKM unitarity to
1.8σ. Additionally, we calculate the vector γW-box contribution
to the axial charge gA​, denoted as □γWVV​, and explore
its potential implications.Comment: 9 pages, 10 figure