Searching for lepton flavor universality violation and collider signals from a singly charged scalar singlet

Abstract

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 (${\ensuremath{\phi}}^{\ifmmode\pm\else\textpm\fi{}}$) 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 $\ensuremath{\ell}\ensuremath{\rightarrow}{\ensuremath{\ell}}^{\ensuremath{'}}\ensuremath{\nu}\ensuremath{\nu}$ 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 $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\nu}\ensuremath{\nu}/\ensuremath{\tau}(\ensuremath{\mu})\ensuremath{\rightarrow}e\ensuremath{\nu}\ensuremath{\nu}$ and of the Cabibbo Angle Anomaly. Such an explanation predicts $\mathrm{Br}[\ensuremath{\tau}\ensuremath{\rightarrow}e\ensuremath{\gamma}]$ to be of the order of a few times ${10}^{\ensuremath{-}11}$, while $\mathrm{Br}[\ensuremath{\tau}\ensuremath{\rightarrow}e\ensuremath{\mu}\ensuremath{\mu}]$ can be of the order of ${10}^{\ensuremath{-}9}$ 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 $\ensuremath{\approx}200\text{ }\text{ }\mathrm{GeV}$ by recasting LHC slepton searches. In the scenario preferred by low energy precision data, the lower limit is even strengthened to $\ensuremath{\approx}300\text{ }\text{ }\mathrm{GeV}$, 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 ${e}^{+}{e}^{\ensuremath{-}}$ colliders.

Topics & Concepts