Explaining the Cabibbo Angle Anomaly and Lepton Flavour Universality Violation in Tau Decays With a Singly-Charged Scalar Singlet

TL;DR

This paper proposes that a singly charged scalar singlet can explain the Cabibbo Angle Anomaly and lepton flavor universality violation in tau decays, predicting related effects and setting mass bounds.

Contribution

It introduces a model with a charged scalar singlet that simultaneously addresses multiple anomalies and predicts new observable effects in tau decays and collider searches.

Findings

Lower bound of ~200 GeV on scalar mass from collider searches.

Predicted loop effects in tau decays involving electron flavor.

Potential future bounds from dark matter mono-photon searches at e+e- colliders.

Abstract

The singly charged $SU(2)_L$ singlet scalar, with its necessarily flavour violating couplings to leptons, lends itself particularly well for an explanation of the Cabibbo Angle Anomaly and of hints for lepton flavour universality violation in $\tau \to \mu\bar \nu\nu$. In a setup addressing both anomalies, we predict loop-induced effects in $\tau\to e\gamma$ and in $\tau \to e\mu\mu$. A recast of ATLAS selectron and smuon searches allows us to derive a coupling-independent lower limit of $\approx 200$ GeV on the mass of the singly charged singlet scalar. At a future $e^+e^-$ collider, dark matter mono-photon searches could provide a complementary set of bounds.