Speaker
Description
Building on our accurate measurement of the $\beta$ direction's asymmetry with respect to decaying polarized $^{37}$K [B. Fenker PRL 120 062502], we plan further measurements of the momenta of the recoiling progeny nucleus in coincidence. $^{37}$K's decay to its isobaric analog state has similar sensitivity to unknown physics compared to neutron decay, while a nuclear structure feature where the d$_{3/2}$ unpaired proton naturally produces a tiny nuclear magnetic moment keeps known higher-order corrections small. The angular distribution of the outgoing leptons is predicted from their helicity combined with angular momentum conservation, and we've realized one of our experiments would be the most direct measurement of the $\nu$ helicity since the Brookhaven 1958 measurement. Adding $\gamma$-ray detection with high-Z GAGG scintillators enables our search for a time reversal-breaking correlation of $\beta$, $\nu$, and $\gamma$ momenta in radiative $\beta$ decay, sensitive to a hypothetical dark strongly interacting sector. Time reversal-breaking interactions in the final nucleus in isospin-hindered $\beta$ decay compete with the Coulomb interaction instead of the strong interaction, potentially enhancing sensitivity by 1000x to make it complementary to neutron EDM and neutron resonance time reversal tests. We are beginning a program to measure isospin breaking in isospin-hindered $^{47}$K and $^{45}$K decay to determine our sensitivity. $^{47}$K decay, since parent and progeny are near closed shells so there are few final states, may exhaust the expected matrix element size in analog-antianalog isospin mixing.
| Keyword-1 | time reversal |
|---|---|
| Keyword-2 | neutrino helicity |
| Keyword-3 | beta decay |
