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

In recent years, evidence for lepton flavor universality violation beyond the Standard Model has been accumulated. In this context, a singly charged SU(2)L singlet scalar (ϕ±) is very interesting, as it can only have flavor off-diagonal couplings to neutrinos and charged leptons therefore necessarily violating lepton flavor (universality). In fact, it gives a (necessarily constructive) tree-level effect in ℓ→ℓ′νν processes, while contributing to charged lepton flavor violating only at the loop level. Therefore, it can provide a common explanation of the hints for new physics in τ→μνν/τ(μ)→eνν and of the Cabibbo Angle Anomaly. Such an explanation predicts Br[τ→eγ] to be of the order of a few times 10-11, while Br[τ→eμμ] 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 ≈200 GeV by recasting LHC slepton searches. In the scenario preferred by low energy precision data, the lower limit is even strengthened to ≈300 GeV, showing the complementary between LHC searches and flavor observables. Furthermore, we point out that this model can be tested by reinterpreting dark matter monophoton searches at future e+e- colliders.