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

Abstract

We present a comprehensive analysis of the γ W interference radiative correction to the neutron β-decay matrix element. Within a dispersion relations approach, we compute the axial-vector part of the γ W box amplitude γ WA in terms of the isoscalar part of the F3γ W interference structure function. Using the latest available phenomenology for F3γ W from the nucleon elastic, resonance, deep-inelastic, and Regge regions, we find the real part of the box correction to be γ WA = 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.