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Lepton flavor violation: From muon decays to muon colliders

Pouya Asadi1,2,*, Hengameh Bagherian3,4,†, Katherine Fraser5,6,‡, Samuel Homiller7,8,§, and Qianshu Lu9,10,∥

  • 1Department of Physics and Santa Cruz Institute for Particle Physics, University of California Santa Cruz, Santa Cruz, California 95064, USA
  • 2Institute for Fundamental Science and Department of Physics, University of Oregon, Eugene, Oregon 97403, USA
  • 3Enrico Fermi Institute and Leinweber Institute for Theoretical Physics, University of Chicago, Chicago, Illinois 60637, USA
  • 4Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
  • 5Leinweber Institute for Theoretical Physics and Miller Institute University of California, Berkeley, California 94720, USA
  • 6Theoretical Physics Group, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
  • 7Pitt PACC, Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
  • 8Laboratory for Elementary Particle Physics, Cornell University, Ithaca, New York 14853, USA
  • 9School of Natural Sciences, Institute for Advanced Study, Princeton, New Jersey 08540, USA
  • 10Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, New York 10003, USA

  • *Contact author: pasadi@ucsc.edu
  • †Contact author: hengameh@uchicago.edu
  • ‡Contact author: kfraser@berkeley.edu
  • §Contact author: shomiller@pitt.edu
  • ∥Contact author: qianshu.lu@ias.edu
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Phys. Rev. D 113, 015003 – Published 5 January, 2026

DOI: https://doi.org/10.1103/bg4z-dmgb

Abstract

We investigate the unique potential of a high-energy muon collider to probe lepton-flavor-violating signals arising from physics beyond the Standard Model (SM). Low-energy, precision searches for charged lepton flavor violation (LFV) are projected to dramatically improve their sensitivity in the coming years and could provide the first evidence of new physics. We interpret the sensitivity of these searches in terms of a set of LFV operators in the SM effective field theory. The same operators are then probed at the TeV scale via new, high-energy processes only available at a high-energy muon collider, such as μμ→μτ or the scattering of a muon of an electroweak gauge boson into LFV final states. We find that, for most operators, a muon collider could confirm signals if they are seen at future low-energy experiments, whereas for certain flavor combinations it extends the reach to scales well beyond those accessible at lower energies. We also project the sensitivity of a muon collider to lepton-flavor-violating decays of the SM Higgs boson and demonstrate improved sensitivity to h→eτ and h→μτ by an order of magnitude compared to the High-Luminosity LHC. The importance of having multiple, complementary probes is illustrated by considering both various combinations of operators and relative sizes of flavor-violating transitions between generations under various assumptions for the flavor structure of new physics.

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Phys. Rev. D 113, 015003– Published 5 January, 2026
  • Received 29 October 2025
  • Accepted 8 December 2025
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