T-odd momentum correlation in radiative β decay

We consider neutron radiative β decay, n→pe ^-ν _̄eγ, and compute the T-odd momentum correlation in the decay rate characterized by the kinematical variable ξ=l _ν•(l _e×k) arising from electromagnetic final-state interactions in the standard model. Our expression for the corresponding T-odd asymmetry AξSM is exact in O(α) up to terms of recoil order, and we evaluate it numerically under various kinematic conditions. Noting the universality of the V-A law in the absence of recoil-order terms, we retain the parametric dependence on masses and coupling constants throughout, so that our results serve as a template for the computation of AξSM in allowed nuclear radiative β decays and hyperon radiative β decays as well.