Neutrino Electroweak Couplings and the Neutrino Fog at Belle II

Abstract

The mono-photon process $e^{+}{e}^{-}\to \gamma +\mathrm{invisible}$ at Belle II probes both invisible new states and the electroweak couplings of neutrinos. In this work, we study the process $e^{+}{e}^{-}\to \gamma \nu \bar{\nu }$, finding that Belle II with the full expected dataset can determine the neutrino-sensitive effective weak mixing angle to a 3.6$\%$ relative statistical precision, competitive with existing neutrino probes at other energy scales. Polarized beams, as proposed for the Chiral Belle upgrade, separate the neutral- and charged-current contributions and provide independent access to each, giving a handle on the chiral structure of the neutrino gauge interactions. The same process is an irreducible background to dark boson searches, producing a neutrino fog once the luminosity exceeds 1 ab${}^{-1}$.

8 pages, 4 figures