Electroweak axial structure functions and improved extraction of the Vud CKM matrix element

We present a comprehensive analysis of the γW interference radiative correction to the neutron β-decay matrix element. Within a dispersion relations approach, we compute the axial-vector part of the γW box amplitude □γWA in terms of the isoscalar part of the FγW3 interference structure function. Using the latest available phenomenology for FγW3 from the nucleon elastic, resonance, deep-inelastic, and Regge regions, we find the real part of the box correction to be □γWA = 3.90 (9) × 10−3. This improved correction gives a theoretical estimate of the CKM matrix element |Vud|2 = 0.94805 (26), which represents a 4σ violation of unitarity.