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

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.