First direct constraints on Fierz interference in free neutron ${\hat{I}}^2$ decay

Abstract

Precision measurements of free neutron ${\hat{I}}^2$-decay have been used to precisely constrain our understanding of the weak interaction. However the neutron Fierz interference term $b_n$, which is particularly sensitive to Beyond-Standard-Model tensor currents at the TeV scale, has thus far eluded measurement. Here we report the first direct constraints on this term, finding $b_n=0.067\pm 0.005_{\text{stat}}{}_{-0.061}^{+0.090}{}_{\text{sys}}$, consistent with the Standard Model. The uncertainty is dominated by absolute energy reconstruction and the linearity of the beta spectrometer energy response.