Hiding in the Shadow of the Upsilon: Ditaus from a Light Pseudoscalar

Abstract

The CMS collaboration has reported a measurement of ${\rm Y}$ decays to ditaus using $61.9{\mathrm{fb}}^{-1}$ of scouting data. If interpreted as the decay of ${\rm Y}(1S,2S,3S)$, the measured ditau rate is more than ten times that seen in the dimuon final states at the $\sim 3\sigma$ level, and is likewise inconsistent with the branching ratios measured at $B$-factories. If confirmed with more data and at higher significance, such a violation of lepton flavor universality would necessitate new physics. In this Letter, we present a simple model with a light pseudoscalar coincidentally near the ${\rm Y}(1S)$ mass, which mixes with two Higgs doublets in the alignment limit. Such a particle naturally decays primarily to taus and evades all existing experimental constraints, while implying a number of predictions that can be tested in the near future.

Comments: 4 pages, no figures