A_β[E_β] in ³⁷K decay: new physics with opposite $β$ helicity

By extending our analysis and simulations of our ³⁷K $β$-decay data set to allow the $β$ asymmetry with respect to nuclear spin to vary with $β$ energy E_β, we have gained sensitivity to new physics that depends on a helicity factor for the $β$, m_β/E_β. In particular, we constrain Lorentz scalar and tensor quark-lepton interaction strengths at a sensitivity complementary to the similar Fierz interference term in neutron $β$ decay. Our result for that new physics is bF = -0.0002 ± 0.024 (stat) ± 0.039 (syst), consistent with the standard model electroweak interaction value bF=0. We consider presently achieved complementarity to $β$-decay and particle physics experiments, along with projectable technical improvements to our method.