Speaker
Description
The reactor neutrino anomaly and $g_A$ “quenching” in neutrinoless double-β decay are two of the outstanding issues in nuclear physics. Measurement of the first-forbidden nonunique β decay of $^{137}$Xe can provide helpful input to both issues but is difficult to perform accurately. EXO-200 is a low-background neutrinoless double-β decay experiment that used close to 200 kg of $^{136}$Xe as a target. An ultraminiature AmBe neutron source was deployed in EXO-200, which allowed one to accumulate a pure sample of $^{137}$Xe β decays. The spectrum of the decay to the ground state of $^{137}$Cs was measured accurately and found to agree with the theoretical prediction, suggesting that the effects of the first-forbidden β decays lead to mitigation of the reactor anomaly and a possible explanation of the origins of the spectral bump. Extracting an accurate spectrum of the decay to the first excited state of $^{137}$Cs is more difficult and still ongoing, but it is advantageous due to the sensitivity of this mode to the effective value of the $g_A$. This talk presents the result of the first measurement and discusses the current status and outlook of the second one.
