$β$ Recoil spectroscopy as a Beyond Standard Model laboratory

Abstract

We consider the nuclear recoil spectrum following allowed $β$ decay as a laboratory for Beyond Standard Model searches, and perform an exhaustive treatment of the Standard Model prediction. We consider final state interactions, nuclear structure corrections, kinematic corrections and radiative corrections, derive new results and show that the nuclear recoil spectrum is theoretically exceptionally clean. We construct a clean statistical framework and analyze the Fisher information for extracting exotic current constraints and testing Cabibbo-Kobayashi-Maskawa top-row unitarity, and consider a number of sterile neutrino scenarios. In addition, we find that for select isotopes separate measurement of the $β$-asymmetry and recoil spectrum can achieve an increase in the statistical power of up to a factor 40.