Effective theory approach to new physics in $b \to u$ and $b \to c$ leptonic and semileptonic decays

TL;DR

This paper investigates potential new physics effects in $b o u$ and $b o c$ leptonic and semileptonic B decays by analyzing experimental anomalies with a general effective Lagrangian approach.

Contribution

It performs a comprehensive analysis of all relevant data using a general effective Lagrangian to explore new physics operators and predicts effects on specific decay branching fractions.

Findings

Identifies possible new physics operators consistent with current anomalies.

Predicts altered branching ratios for $B_c o au u$ and $B o au u$ decays.

Provides constraints on new physics couplings from combined data.

Abstract

Recent measurements of exclusive $B^{-} \to \tau^{-}\,\nu$ and $B^0 \to \pi^{+}\,l^{-}\,\bar{\nu}_l$ decays via the $b \to u\,l\,\nu$ transition process differ from the standard model expectation and, if they persist in future $B$ experiments, will be a definite hint of the physics beyond the standard model. Similar hints of new physics have been observed in $b \to c$ semileptonic transition processes as well. BABAR measures the ratio of branching fractions of $B \to (D,\,D^{\ast})\,\tau\,\nu$ to the corresponding $B \to (D,\,D^{\ast})\,l\nu$, where $l$ represents either an electron or a muon, and finds $3.4\sigma$ discrepancy with the standard model expectation. In this context, we consider a most general effective Lagrangian for the $b \to u\,l\,\nu$ and $b \to c\,l\,\nu$ transition processes in the presence of new physics and perform a combined analysis of all the $b \to u$ and $b \to c$ semi-(leptonic) data to explore various new physics operators and their couplings. We consider various new physics scenarios and give predictions for the $B_c \to \tau\nu$ and $B \to \pi\tau\nu$ decay branching fractions. We also study the effect of these new physics parameters on the ratio of the branching ratios of $B \to \pi\tau\nu$ to the corresponding $B \to \pi\,l\,\nu$ decays.