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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 four-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(7)lat(1)PT×10−3. Consequently, this yields a slightly higher value of |Vud|=0.973 86(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. Published by the American Physical Society 2024
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