Time reversal violation in radiative beta decay: experimental progress*

Some explanations for the excess of matter over antimatter in the universe involve sources of time reversal violation (TRV) in addition to the one known in the standard model of particle physics. We are beginning a search for TRV in a correlation between the momenta of the beta, neutrino, and the radiative gamma sometimes emitted in nuclear beta decay. Correlations involving three (out of four) momenta are sensitive to certain types of TRV physics to which observables involving spin, such as electric dipole moments and spin-polarized beta decay correlations, are less directly sensitive. TRV experiments have been done in radiative kaon decay, but not in systems involving the first generation of quarks. A class of low-energy scale physics models [Gardner and He, Phys. Rev. D 87 116012 (2013)] produces TRV effects by a cross-term with the vector current beta decay of the neutron, tritium, or some positron-decaying isotopes, and such effects grow with lepton momentum and Z. We will present progress towards measuring the TRV asymmetry in radiative beta decay of laser-trapped $^{38m}$K, adding compact gamma-ray detection to TRIUMF's neutral atom trap (TRINAT) for $\beta$-$\nu$ correlations.