# Enhanced $B \to \mu \bar\nu$ Decay at Tree Level as Probe of Extra Yukawa Couplings

## TL;DR

This paper explores how the presence of extra Yukawa couplings in a two Higgs doublet model can significantly enhance the $B o \mu ar
u$ decay rate, providing a potential experimental probe for new physics at Belle II.

## Contribution

It demonstrates that extra Yukawa couplings can double the decay ratio, offering a new way to test for physics beyond the Standard Model using current and upcoming experiments.

## Key findings

- The decay ratio ${
m R}_B^{\mu/ au}$ can be increased by a factor of two.
- Belle II can probe these enhancements with a few inverse femtobarns of data.
- Standard Model predicts ${
m R}_B^{\mu/ au} ext{ at } 0.0045$.

## Abstract

With no New Physics seen at the LHC, a second Higgs doublet remains attractive and plausible. The ratio ${\cal R}_B^{\mu/\tau} = {\cal B}(B \to \mu \bar\nu)/{\cal B}(B \to \tau \bar\nu)$ is predicted at 0.0045 in both the Standard Model and the type II two Higgs doublet model, but it can differ if extra Yukawa couplings exist in Nature, which we deem an experimental issue. Considering recent Belle update on $B \to \mu\bar\nu$, we show that in the general two Higgs doublet model, the ratio could be up by a factor of two, which can be probed by the Belle~II experiment with just a few ab$^{-1}$.