Asymmetry Observables and the Origin of $R_{D^{(*)}}$ Anomalies

TL;DR

This paper explores how future Belle II measurements of tau asymmetries can differentiate between various new physics models explaining the $R_{D^{(*)}}$ anomalies, especially those involving right-handed neutrinos.

Contribution

It provides a detailed analysis of tau asymmetry observables to distinguish between new physics scenarios with right- and left-handed neutrinos at Belle II.

Findings

Belle II can differentiate between right- and left-handed neutrino models.

Tau asymmetries can identify specific new physics models.

Most models can be distinguished using these asymmetry measurements.

Abstract

The $R_{D^{(*)}}$ anomalies are among the longest-standing and most statistically significant hints of physics beyond the Standard Model. Many models have been proposed to explain these anomalies, including the interesting possibility that right-handed neutrinos could be involved in the $B$ decays. In this paper, we investigate future measurements at Belle II that can be used to tell apart the various new physics scenarios. Focusing on a number of $\tau$ asymmetry observables (forward-backward asymmetry and polarization asymmetries) which can be reconstructed at Belle II, we calculate the contribution of the most general dimension 6 effective Hamiltonian (including right-handed neutrinos) to all of these asymmetries. We show that Belle II can use these asymmetries to distinguish between new-physics scenarios that use right- and left-handed neutrinos, and in most cases can likely distinguish the specific model itself.