Searching for Lepton Flavour (Universality) Violation and Collider Signals from a Singly-Charged Scalar Singlet

Abstract

In recent years, evidence for lepton flavour universality violation beyond the Standard Model has been accumulated. In this context, a singly charged $SU(2{)}_L$ singlet scalar (${\varphi }^{\pm }$) is very interesting, as it can only have flavour off-diagonal couplings to neutrinos and charged leptons, therefore necessarily violating lepton flavour (universality). In fact, it gives a (necessarily constructive) tree-level effect in $\ell \to {\ell }^{\prime }\nu \nu$ processes, while contributing to charged lepton flavour violating only at the loop-level. Therefore, it can provide a common explanation of the hints for new physics in $\tau \to \mu \nu \nu /\tau (\mu )\to e\nu \nu$ and of the Cabibbo Angle Anomaly. Such an explanation predicts $\mathrm{Br}[\tau \to e\gamma ]$ to be of the order of a few times $10^{-11}$ while $\mathrm{Br}[\tau \to e\mu \mu ]$ can be of the order of $10^{-9}$ for order one couplings and therefore in the reach of forthcoming experiments. Furthermore, we derive a {novel} coupling-independent lower limit on the scalar mass of $\approx 200$GeV by recasting LHC slepton searches. In the scenario preferred by low energy precision data, the lower limit is even strengthened to $\approx 300$GeV, showing the complementary between LHC searches and flavour observables. Furthermore, we point out that this model can be tested by reinterpreting DM mono-photon searches at future $e^{+}{e}^{-}$ colliders.

arXiv comment8 pages, 4 figures, matches version published in PRD

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