Probing charged lepton flavor violation with axion-like particles at Belle II

Kingman Cheung, Abner Soffer, Zeren Simon Wang*, Yu Heng Wu

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), X, at the B-factory experiment Belle II. We focus on production of the ALP in the tau decays τ → Xl with l = e, μ, followed by its decay via X → ll+. The ALP can be either promptly decaying or long-lived. We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating τ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle II sensitivity reaches in both the product of branching fractions and the ALP coupling constants, as functions of the ALP mass and lifetime. The results show that the DV search exceeds the sensitivity reach of the prompt search to the relevant branching fractions by up to about a factor of 40 in the long decay length regime.

Original languageEnglish
Article number218
JournalJournal of High Energy Physics
Issue number11
StatePublished - Nov 2021


  • Beyond Standard Model
  • Effective Field Theories


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