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arXiv · 2605.29289

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

Matthew R Buckley·David Shih·Isaac R Wang
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Abstract

The CMS collaboration has reported a measurement of $\Upsilon$ decays to ditaus using $61.9~{\rm fb}^{-1}$ of scouting data. If interpreted as the decay of $\Upsilon(1S,2S,3S)$, 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 $\Upsilon(1S)$ 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.

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Matthew R Buckley, David Shih, Isaac R Wang. 2026-05-28. Hiding in the Shadow of the Upsilon: Ditaus from a Light Pseudoscalar. https://arxiv.org/abs/2605.29289

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