[Paper Review] Charged Lepton Flavour Violation in Heavy Particle DEcays

Wolfgang Altmannshofer, C. Caillol+3 more|arXiv (Cornell University)|May 21, 2022

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper reviews charged lepton flavor violation (CLFV) in decays of heavy Standard Model particles—Z bosons, Higgs bosons, and top quarks—within the context of New Physics beyond the Standard Model. Using the Standard Model Effective Field Theory framework, it evaluates current LHC limits and projects sensitivities at future colliders like HL-LHC, CEPC, and FCC-ee, showing that future experiments could probe CLFV branching fractions down to $10^{-9}$ for Z decays and $10^{-5}$ for Higgs decays, offering powerful tests of physics beyond the Standard Model.

ABSTRACT

Charged lepton flavor violation is an unambiguous signature for New Physics. Here we present a summary of the theoretical and experimental status of the search for charged lepton flavor violation in heavy particle decays, in particular in the decays of the Z and Higgs bosons, and of the top quark. Decays of beyond-Standard-Model particles such as a Z' or an additional scalar particle are also discussed. Finally the prospects for such searches at proposed future electron-positron colliders are reviewed.

Motivation & Objective

• To assess the theoretical and experimental status of charged lepton flavor violation (CLFV) in decays of heavy particles such as the Z, Higgs, and top quark.
• To evaluate the sensitivity of current and future experiments—particularly the LHC and proposed future e+e− colliders—to CLFV processes as probes of New Physics.
• To quantify the reach of future colliders like CEPC and FCC-ee in detecting CLFV in Z and Higgs decays, especially in the context of effective field theory models.
• To connect CLFV signals to potential explanations of the $g-2$ anomaly and other anomalies in flavor physics.

Proposed method

• The study employs the Standard Model Effective Field Theory (SMEFT) framework to model New Physics contributions to CLFV processes via dimension-6 operators such as dipole and Higgs current operators.
• It analyzes branching fraction limits for Z → ℓℓ′ decays using LEP and LHC (ATLAS/CMS) data, with updated limits from ATLAS at $\mathcal{B}(Z\to\mu e) < 2.62 \times 10^{-7}$.
• For Higgs decays, the analysis uses reconstructed invariant masses of eμ, eτ, and μτ final states, applying kinematic cuts and requiring consistency with the Higgs mass to suppress backgrounds.
• It projects sensitivities at future e+e− colliders (CEPC, FCC-ee, ILC) using simulated luminosities and detector performance, estimating signal efficiencies and background rejection.
• The method includes estimating the impact of detector resolution and particle identification on CLFV sensitivity, especially for the Z → μe mode where misidentification of muons as electrons is a key systematic.
• It compares limits from top quark decays (t → uμe, t → cμe) and resonant searches (e.g., Z′ → eμ, eτ, μτ) to constrain BSM models.

Experimental results

Research questions

• RQ1 What are the current experimental limits on Z → ℓℓ′ decays, and how do they constrain New Physics models?
• RQ2 How do future e+e− colliders like CEPC and FCC-ee improve sensitivity to CLFV in Z and Higgs decays compared to the LHC?
• RQ3 To what extent can CLFV in top quark decays probe New Physics, and what are the projected sensitivities at HL-LHC?
• RQ4 Can CLFV in Z and Higgs decays provide a viable explanation for the $g-2$ anomaly in muon magnetic moment?
• RQ5 How do detector performance and reconstruction efficiency affect the sensitivity to CLFV in Z and Higgs decays at future colliders?

Key findings

• The ATLAS collaboration improved the limit on Z → μe decay to $\mathcal{B}(Z\to\mu e) < 2.62 \times 10^{-7}$, a factor of seven better than previous LEP limits.
• The most stringent current limits on Higgs boson decays are $\mathcal{B}(H\to\mu\tau) < 0.15\%$ and $\mathcal{B}(H\to e\tau) < 0.22\%$ from CMS, while $\mathcal{B}(H\to e\mu) < 6.1 \times 10^{-5}$ from ATLAS.
• HL-LHC is expected to improve sensitivity to Z → μe by a factor of five, reaching $\sim 10^{-7}$, and to Z → τe/μτ by $\sim 10^{-6}$.
• Future e+e− colliders like FCC-ee could achieve sensitivities down to $\sim 10^{-9}$ for Z → μe and $\sim 10^{-5}$ for H → eμ, depending on detector performance.
• For H → τℓ modes, signal efficiencies are around 5% due to τ lepton branching fractions and reconstruction challenges, leading to higher background and less sensitivity than the eμ mode.
• The projected sensitivity for H → eμ at CEPC and FCC-ee is $\mathcal{B} < 1.2 \times 10^{-5}$, while for H → eτ and H → μτ it is $\mathcal{B} < 1.5 \times 10^{-4}$.