Right-handed Neutrinos and $R(D^{(*)})$

TL;DR

This paper investigates how right-handed sterile neutrinos could explain the $R(D^{(*)})$ anomalies, analyzing simplified models, collider constraints, and sterile neutrino phenomenology, including displaced decays and dark radiation implications.

Contribution

It provides a comprehensive EFT analysis of models with right-handed neutrinos explaining $R(D^{(*)})$ anomalies, identifying viable models and exploring their collider and astrophysical signatures.

Findings

One simplified model is excluded by current data.

Remaining models are compatible with collider constraints at TeV scale.

Sterile neutrino phenomenology includes displaced decays and dark radiation signals.

Abstract

We explore scenarios where the $R(D^{(*)})$ anomalies arise from semitauonic decays to a right-handed sterile neutrino. We perform an EFT study of all five simplified models capable of generating at tree-level the lowest dimension electroweak operators that give rise to this decay. We analyze their compatibility with current $R(D^{(*)})$ data and other relevant hadronic branching ratios, and show that one simplified model is excluded by this analysis. The remainder are compatible with collider constraints on the mediator semileptonic branching ratios, provided the mediator mass is of order TeV. We also discuss the phenomenology of the sterile neutrino itself, which includes possibilities for displaced decays at colliders and direct searches, measurable dark radiation, and gamma ray signals.