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A T-odd Momentum Correlation in Radiative β-Decay

We consider neutron radiative β-decay, n\to p e^- \barν_eγ, and compute the T-odd momentum correlation in the decay rate characterized by the kinematical variable ξ= {\bf l}_ν\cdot ({\bf l}_e\times{\bf k}) arising from electromagnetic final-state interactions in the Standard Model. Our expression for the corresponding T-odd asymmetry A_ξ^{SM} 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 A_ξ^{SM} in allowed nuclear radiative β-decays and hyperon radiative β-decays as well. We consider the pattern of A_ξ^{SM} which appears in nuclear decays and show that A_ξ^{SM} can be suppressed in particular cases, facilitating searches for new sources of CP-violation in such processes.