Implications of a vector-like lepton doublet and scalar Leptoquark on $R(D^{(*)})$

TL;DR

This paper explores a new physics model involving a vector-like lepton doublet and scalar leptoquark, which can significantly reduce the discrepancy between experimental measurements and theoretical predictions of $R(D^{(*)})$ in flavor physics.

Contribution

The study demonstrates that introducing a vector-like lepton doublet and scalar leptoquark can reconcile theoretical predictions with experimental data for $R(D^{(*)})$, satisfying various experimental constraints.

Findings

Model reduces the discrepancy in $R(D^{*})$ between theory and experiment.

The proposed new physics model satisfies constraints from meson oscillations and Z-pole data.

Theoretical values of $R(D)$ and $R(D^{*})$ are shifted closer to experimental results.

Abstract

We study the phenomenological constraints and consequences in the flavor sector, of introducing a new fourth generation $\mathcal{Z}_{2}$ odd vector-like lepton doublet along with a Standard Model (SM) singlet scalar and an $SU(2)_{L}$ singlet scalar leptoquark carrying electromagnetic charge of $+2/3$, both odd under a $\mathcal{Z}_{2}$. We show that with little fine tuning among the various Yukawa couplings in the new physics (NP) Lagrangian along with the CKM parameters, the model is able to push the theoretical value of $R(D^{*})^{th}$ from $0.252 \pm 0.003$ to $0.263 \pm 0.051$ and $R(D)^{th}$ from $0.300 \pm 0.008$ to $0.313 \pm 0.158$ compared to the SM value. Especially the NP contributions are able to reduce the discrepancy between experiment and theory of $R(D^{*})$ substantially compared to SM. This is quite impressive given that the model satisfy all other very stringent constrains coming from neutral meson oscillations and precision $Z$-pole data.