Scalar contributions to $b\to c (u) \tau \nu$ transitions

TL;DR

This paper investigates scalar new physics contributions to semileptonic B decays, analyzing current experimental data, theoretical constraints, and potential ways to distinguish scalar effects from other new physics scenarios.

Contribution

It provides a comprehensive analysis of scalar contributions to both $b o c au u$ and $b o u au u$ transitions, including recent measurements and proposing methods to disentangle scalar effects.

Findings

Scalar contributions with both left- and right-handed couplings can explain $R(D^{(*)})$ and differential distributions.

Constraints from $B_c$ width create tension with some $B o D^{*} au u$ data.

Predictions for $ au$ modes in $b o u au u$ decays and their potential to reveal scalar new physics.

Abstract

We perform a comprehensive analysis of scalar contributions in $b \to c \tau \nu$ transitions including the latest measurements of $R(D^{(*)})$, the $q^2$ differential distributions in $B \to D^{(*)} \tau \nu$, the $\tau$ polarization asymmetry for $B \to D^{*} \tau \nu$, and the bound derived from the total width of the $B_c$ meson. We find that scalar contributions with the simultaneous presence of both left- and right-handed couplings to quarks can explain the available data, specifically $R(D^{(*)})$ together with the measured differential distributions. However, the constraints from the total $B_c$ width present a slight tension with the current data on $B \to D^{*}\tau \nu$ in this scenario, preferring smaller values for $R(D^*)$. We discuss possibilities to disentangle scalar new physics from other new-physics scenarios like the presence of only a left-handed vector current, via additional observables in $B \to D^{(*)}\tau \nu$ decays or additional decay modes like the baryonic $\Lambda_b \to \Lambda_c \tau \nu$ and the inclusive $B \to X_c \tau \nu$ decays. We also analyze scalar contributions in $b \to u \tau \nu$ transitions, including the latest measurements of $B \to \tau \nu$, providing predictions for $\Lambda_b \to p \tau \nu$ and $B \to \pi \tau \nu$ decays. The potential complementarity between the $b \to u$ and $b \to c$ sectors is finally investigated once assumptions about the flavour structure of the underlying theory are made.