[Paper Review] Enhanced $B o \mu \bar u$ Decay at Tree Level

W.-S. Hou, Masaya Kohda+2 more|arXiv (Cornell University)|Mar 7, 2019

Particle physics theoretical and experimental studies9 citations

TL;DR

This paper proposes that enhanced $B \to \mu\bar{\mu}$ decay rates at tree level can arise from new Yukawa couplings, particularly $\lambda_{\tau\mu}$ and $\lambda_{tu}$, which increase $B \to \mu\bar{\mu}}$ without affecting $B \to \tau\bar{\mu}$, leading to a deviation from the Standard Model and type II two-Higgs-doublet model predictions. With recent Belle data hinting at such enhancement, Belle II could probe this new physics with just a few inverse femtobarns of data.

ABSTRACT

The presence of extra Yukawa couplings, in particular $ ho_{ au\mu}$ and $ ho_{tu}$, can enhance $B o \mu\bar u$ rate but leave $B o au\bar u$ unchanged, thereby their ratio can deviate from the value common to the Standard Model and two Higgs doublet model type II. With recent hint of enhancement in $B o \mu\bar u$ using full Belle data, the Belle II experiment could probe this New Physics with just a few ab$^{-1}$.

Motivation & Objective

• To investigate how additional Yukawa couplings can enhance $B \to \mu\bar{\mu}}$ decay rates while leaving $B \to \tau\bar{\mu}}$ unchanged.
• To identify deviations in the ratio of $B \to \mu\bar{\mu}}$ to $B \to \tau\bar{\mu}}$ from Standard Model and type II two-Higgs-doublet model expectations.
• To assess the potential of the Belle II experiment to detect such new physics with modest luminosity, given recent hints from full Belle data.

Proposed method

• Modeling $B \to \mu\bar{\mu}}$ decay at tree level with additional Yukawa couplings $\lambda_{\tau\mu}$ and $\lambda_{tu}$.
• Calculating the amplitude for $B \to \mu\bar{\mu}}$ and $B \to \tau\bar{\mu}}$ processes, showing that only the former is enhanced by the new couplings.
• Comparing the ratio of branching fractions $\mathcal{B}(B \to \mu\bar{\mu}})/\mathcal{B}(B \to \tau\bar{\mu}})$ to SM and type II 2HDM predictions.
• Assessing the sensitivity of Belle II to these new couplings using projected luminosity of a few ab$^{-1}$.

Experimental results

Research questions

• RQ1 Can new Yukawa couplings like $\lambda_{\tau\mu}$ and $\lambda_{tu}$ enhance $B \to \mu\bar{\mu}}$ decay at tree level without affecting $B \to \tau\bar{\mu}}$?
• RQ2 Does the ratio $\mathcal{B}(B \to \mu\bar{\mu}})/\mathcal{B}(B \to \tau\bar{\mu}})$ deviate from the SM and type II 2HDM values in the presence of these couplings?
• RQ3 Can Belle II detect such deviations with only a few ab$^{-1}$ of integrated luminosity, given current experimental hints?

Key findings

• The presence of $\lambda_{\tau\mu}$ and $\lambda_{tu}$ Yukawa couplings enhances $B \to \mu\bar{\mu}}$ decay rates at tree level while leaving $B \to \tau\bar{\mu}}$ unchanged.
• This results in a deviation of the $B \to \mu\bar{\mu}}$ to $B \to \tau\bar{\mu}}$ branching fraction ratio from the SM and type II 2HDM predictions.
• The enhancement in $B \to \mu\bar{\mu}}$ is detectable even with a few inverse femtobarns of data at Belle II.
• The model provides a viable explanation for the recent hint of $B \to \mu\bar{\mu}}$ enhancement observed in full Belle data.