The $B$ Anomalies and non-SMEFT New Physics

TL;DR

This paper discusses how non-SMEFT new physics could explain the B anomalies, emphasizing the importance of angular distribution measurements in ${ar B} o D^* au ar{ u}$ decays to identify such effects.

Contribution

It demonstrates that non-SMEFT operators can significantly affect certain B decay processes and proposes angular distribution measurements as a way to detect non-SMEFT new physics.

Findings

Non-SMEFT operators can contribute to B anomalies.

Constraints on $b o s \, \ell^+ \ell^-$ transitions are tight.

Angular distribution in ${\bar B} \to D^* \tau \bar{\nu}$ can reveal non-SMEFT effects.

Abstract

The modern viewpoint is that the Standard Model is the leading part of an effective field theory that obeys the symmetry $SU(3)_C \times SU(2)_L \times U(1)_Y$. Since the discovery of the Higgs boson, it is generally assumed that this symmetry is realized linearly (SMEFT), but a nonlinear realization (e.g., HEFT) is still possible. The two differ in their predictions for the size of certain low-energy dimension-6 four-fermion operators: for these, HEFT allows $O(1)$ couplings, while in SMEFT they are suppressed by a factor $v^2/\Lambda_{\rm NP}^2$, where $v$ is the Higgs vev. In this talk, I argue that (i) such non-SMEFT operators contribute to both $b \to s \ell^+ \ell^-$ and $b \to c \,\tau^- {\bar\nu}_\tau$, transitions involved in the present-day $B$ anomalies, (ii) the contributions to $b \to s \ell^+ \ell^-$ are constrained to be small, at the SMEFT level, and (iii) the contribution to $b \to c \,\tau^- {\bar\nu}_\tau$ can be sizeable. I show that the angular distribution in ${\bar B} \to D^* (\to D \pi') \, \tau^{-} (\to \pi^- \nu_\tau) {\bar\nu}_\tau$ contains enough information to extract the coefficients of all new-physics operators. The measurement of this angular distribution can tell us if non-SMEFT new physics is present.