Isospin breaking and time-reversal symmetry in ⁴⁷K beta decay

The neutron beta decays to the proton, its "isobaric analog" i.e. a state with same nuclear wavefunction, assuming isospin is a good symmetry. Tritium also decays to its isobaric analog, but though many other isotopes undergo beta minus decay to states of same I^π = spin^parity, the extra Coulomb energy at higher Z means that decay to the isobaric analog is energetically forbidden. So the Gamow-Teller operator dominates, while the Fermi operator linking isobaric analogs is only allowed if some low-lying final state of same I^π is mixed by an isospin-breaking interaction with the excited isobaric analog. We are studying such isospin breaking in the ⁴⁷Ca nucleus by measuring the ⁴⁷K 80% I=1/2 to I=1/2 decay branch. Interference between Gamow-Teller and isospin-suppressed Fermi amplitudes would produce a small asymmetry of the progeny recoil direction with respect to the initial nuclear spin. Since ⁴⁷Ca and ⁴⁷K have nearly closed shells, the single known final 1/2+ state may have the entire predicted analog-antianalog mixing matrix element. We will show in-progress analysis of a first experiment using TRIUMF's atom trap for beta decay, along with future plans to search for isospin-breaking time-reversal violation enhanced in such systems and thus complementary to the neutron EDM's more general sensitivity.