Speaker
Description
Precision determinations of $V_{ud}$ from superallowed nuclear β decays provide an important test of the Standard Model. A leading theoretical uncertainty arises from electroweak radiative corrections, in particular the contribution from the $\gamma W$ box diagram involving the interference between the electromagnetic and axial currents. The closely related $\gamma Z$ box contribution may be written as a dispersion integral over the parity-violating structure function $F_3^{γZ}$, which motivates a determination of its first Nachtmann moment at low $Q^2$.
On behalf of the QCDSF collaboration, we present a lattice QCD calculation of the first Nachtmann moment of $F_3^{γZ}$ using the Feynman-Hellmann method. Our calculations employ $N_f=2+1$ flavours of dynamical fermions at the SU(3)-symmetric point, using three lattice spacings with pion masses $m_\pi≈415$ MeV and $220$ MeV, for various momentum transfers in the range $0.1\lesssim Q^2\lesssim 10$ $\text{GeV}^2$.