$A_{\hat{I}}^2[E_{\hat{I}}^2]$ in ${}^{37}$K decay: new physics with opposite ${\hat{I}}^2$ helicity

Abstract

By extending our analysis and simulations of our ${}^{37}$K ${\hat{I}}^2$-decay data set to allow the ${\hat{I}}^2$ asymmetry with respect to nuclear spin to vary with ${\hat{I}}^2$ energy $E_{\hat{I}}^2$, we have gained sensitivity to new physics that depends on a helicity factor for the ${\hat{I}}^2$, $m_{\hat{I}}^2/E_{\hat{I}}^2$. In particular, we constrain Lorentz scalar and tensor quark-lepton interaction strengths at a sensitivity complementary to the similar Fierz interference term in neutron ${\hat{I}}^2$ decay. Our result for that new physics is a Fierz interference term $b$ = 0.033 $\pm$ 0.084 (stat) $\pm$ 0.039 (syst), consistent with the standard model electroweak interaction value $b=0$. We consider presently achieved complementarity to ${\hat{I}}^2$-decay and particle physics experiments, along with projectable technical improvements to our method.

Observable now labelled b throughout, pedagogy including equations fixed. Included combined Abeta, b, fT constraint on revised exclusion plot more carefully