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Probing lepton-flavor-violating four-lepton operators at a muon collider

Sukanta Dutta

Purnath Unnikrishnan ^* and Yashasvi ^†

SGTB Khalsa College, University of Delhi, Delhi, India 110007

Department of Physics and Astrophysics, University of Delhi, Delhi, India 110007 and SGTB Khalsa College, University of Delhi, Delhi, India 110007

^*Contact author: purnathuk@gmail.com
^†Contact author: yamansangwan16@gmail.com

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Phys. Rev. D 114, 075015 – Published 9 October, 2026

DOI: https://doi.org/10.1103/k249-jg2x

Abstract

We investigate charged lepton-flavor violation (LFV) induced by dimension-six four-lepton operators within the Standard Model effective field theory (SMEFT) at a proposed high-energy muon collider. We study the processes $μ^{+} μ^{-} \to e^{\pm} τ^{\mp}$ , $μ^{+} μ^{-} \to e^{\pm} μ^{\mp}$ , and $μ^{+} μ^{-} \to μ^{\pm} τ^{\mp}$ at $\sqrt{s} = 3$ , 10, and 14 TeV, incorporating beam polarization and hadronic $τ$ reconstruction. Using an optimal-observable analysis of the angular distributions, we perform a global fit to the relevant set of four-lepton operators. Projected sensitivities reach $C / Λ^{2} \sim (0.5 - 1.2) \times 10^{- 4} {TeV}^{- 2}$ , depending on the flavor and chiral structure of the operator, exceeding current limits by up to an order of magnitude. A combined analysis of multiple center-of-mass energies and beam polarizations significantly improves the resolution of correlations among the Wilson coefficients. These results highlight the strong sensitivity of a future multi-TeV muon collider to charged LFV four-lepton interactions, establishing it as a powerful probe of the SMEFT parameter space.

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Phys. Rev. D 114, 075015– Published 9 October, 2026

Vol. 114, Iss. 7 — 1 October 2026

Received 2 August 2026
Accepted 21 September 2026

DOI: https://doi.org/10.1103/k249-jg2x

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

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Probing lepton-flavor-violating four-lepton operators at a muon collider

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