Lepton flavor violation by three units

Heeck, Julian; Sokhashvili, Mikheil; Thapa, Anil

doi:10.1103/2zkx-d3yk

Lepton flavor violation by three units

Journal Article · Mon Sep 29 00:00:00 EDT 2025 · Physical Review. D.

DOI:https://doi.org/10.1103/2zkx-d3yk· OSTI ID:3011132

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Univ. of Virginia, Charlottesville, VA (United States)
Univ. of Virginia, Charlottesville, VA (United States); Colorado State Univ., Fort Collins, CO (United States)

The conservation of lepton flavor is a prediction of the Standard Model and is still an excellent approximate symmetry despite our observation of neutrino oscillations. Lepton flavor violation by one or two units has been discussed for decades, with several dedicated experiments exploring the vast model landscape but no discoveries so far. Here, we explore operators and processes that violate at least one lepton flavor by three units and identify testable signatures. In the Standard Model effective field theory, such operators already arise at mass dimension 7 and can be tested through their contributions to Michel parameters in leptonic decays. True neutrinoless charged-lepton flavor violation arises at mass dimension 10 and can realistically only be seen in the tau decay channels 𝜏 → $$𝑒⁢𝑒⁢𝑒⁢\bar{𝜇}⁢\bar{𝜇}$$ or 𝜏 → $$𝜇⁢𝜇⁢𝜇\bar{𝑒}\bar{𝑒}$$, for example in Belle II. Testable rates for these tau decays require light new particles and subsequently predict an avalanche of remarkably clean but so-far unconstrained collider signatures.

View Accepted Manuscript (DOE)

Research Organization:: Univ. of Virginia, Charlottesville, VA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), High Energy Physics (HEP)

Grant/Contract Number:: SC0007974

OSTI ID:: 3011132

Journal Information:: Physical Review. D., Journal Name: Physical Review. D. Journal Issue: 5 Vol. 112; ISSN 2470-0010; ISSN 2470-0029

Publisher:: American Physical Society (APS)Copyright Statement

Country of Publication:: United States

Language:: English

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Lepton flavor violation by three units

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