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In recent years, evidence for lepton flavour universality violation beyondthe Standard Model has been accumulated. In this context, a singly charged$SU(2)_L$ singlet scalar ($\phi^\pm$) is very interesting, as it can only haveflavour off-diagonal couplings to neutrinos and charged leptons, thereforenecessarily 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 theloop-level. Therefore, it can provide a common explanation of the hints for newphysics in  $\tau\to\mu\nu\nu/\tau(\mu)\to e\nu\nu$ and of the Cabibbo Angle Anomaly.Such an explanation predicts ${\rm Br }[\tau\to e\gamma]$ to be of the order ofa few times $10^{-11}$ while ${ \rm Br}[\tau\to e\mu\mu]$ can be of the orderof $10^{-9}$ for order one couplings and therefore in the reach of forthcomingexperiments. Furthermore, we derive a {novel} coupling-independent lower limiton the scalar mass of $\approx 200\,$GeV by recasting LHC slepton searches. Inthe scenario preferred by low energy precision data, the lower limit is evenstrengthened to $\approx300\,$GeV, showing the complementary between LHCsearches and flavour observables. Furthermore, we point out that this model canbe tested by reinterpreting DM mono-photon searches at future $e^+e^-$colliders.

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