[Paper Review] Search for charged-lepton flavor violation in the production and decay of top quarks using trilepton final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Hayrapetyan, Aram, Tumasyan, Armen+2360 more|arXiv (Cornell University)|Jan 1, 2023

Particle physics theoretical and experimental studies2 citations

TL;DR

This study searches for charged-lepton flavor violation (CLFV) in top quark production and decay using trilepton final states in 13 TeV proton-proton collisions. Employing boosted decision trees on 138 fb⁻¹ of CMS data, it sets the most stringent limits to date on Wilson coefficients (0.024–0.424 TeV⁻²) and branching fractions (down to 0.012 × 10⁻⁶) for top quark decays into electron-muon pairs via up or charm quarks, consistent with the Standard Model.

ABSTRACT

A search is performed for charged-lepton flavor violating processes in top quark (t) production and decay. The data were collected by the CMS experiment from proton-proton collisions at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 138 fb-1. The selected events are required to contain one opposite-sign electron-muon pair, a third charged lepton (electron or muon), and at least one jet of which no more than one is associated with a bottom quark. Boosted decision trees are used to distinguish signal from background, exploiting differences in the kinematics of the final states particles. The data are consistent with the standard model expectation. Upper limits at 95% confidence level are placed in the context of effective field theory on the Wilson coefficients, which range between 0.024–0.424 TeV-2 depending on the flavor of the associated light quark and the Lorentz structure of the interaction. These limits are converted to upper limits on branching fractions involving up (charm) quarks, t→eμu (t→eμc), of 0.032(0.498)×10-6, 0.022(0.369)×10-6, and 0.012(0.216)×10-6 for tensorlike, vectorlike, and scalarlike interactions, respectively.

Motivation & Objective

• To search for charged-lepton flavor violation (CLFV) in top quark production and decay, a process forbidden in the Standard Model but predicted by new physics.
• To probe the existence of dimension-6 effective field theory operators involving top quarks, charged leptons, and up or charm quarks.
• To improve sensitivity to CLFV beyond previous LHC limits by utilizing trilepton final states with jet and flavor tagging constraints.
• To set 95% confidence level upper limits on Wilson coefficients and branching fractions for specific LFV top quark decay modes.
• To test models that explain B-meson flavor anomalies by examining whether such new physics could manifest in top quark decays at the LHC.

Proposed method

• Utilizes 138 fb⁻¹ of proton-proton collision data at √s = 13 TeV collected by the CMS experiment.
• Applies a trilepton final state selection requiring one opposite-sign electron-muon pair, a third charged lepton (e or μ), and at least one jet with no more than one b-jet.
• Employs boosted decision trees (BDTs) to discriminate signal from background using kinematic features of final-state particles.
• Models prompt backgrounds via Monte Carlo simulations and nonprompt backgrounds using data-driven control samples.
• Interprets results in the framework of effective field theory (EFT), parameterizing CLFV via dimension-6 operators with Wilson coefficients C(6)a.
• Converts observed limits on Wilson coefficients into upper bounds on branching fractions for t → eμu and t → eμc decays under different Lorentz structures (tensor, vector, scalar).

Experimental results

Research questions

• RQ1 What is the sensitivity of the LHC to charged-lepton flavor violation in top quark decays via trilepton final states?
• RQ2 How do the kinematic distributions of trilepton events differ between CLFV signal and dominant Standard Model backgrounds?
• RQ3 What are the most stringent 95% confidence level limits on Wilson coefficients for dimension-6 EFT operators involving top quarks and flavor-violating charged leptons?
• RQ4 To what extent can the observed data constrain branching fractions for t → eμu and t → eμc decays under various Lorentz structures?
• RQ5 Can the LHC probe new physics models that explain B-meson flavor anomalies through top quark CLFV?

Key findings

• The data are consistent with Standard Model expectations, with no significant excess observed in the trilepton final state.
• 95% confidence level upper limits on Wilson coefficients range from 0.024 to 0.424 TeV⁻², depending on the flavor of the associated light quark and Lorentz structure.
• For tensor-like interactions, the upper limit on the branching fraction for t → eμu is 0.032 × 10⁻⁶, while for scalar-like interactions it is 0.012 × 10⁻⁶.
• The most stringent limit is set for scalar-like interactions, yielding a branching fraction upper bound of 0.012 × 10⁻⁶ for t → eμc decays.
• The results represent the most sensitive search to date for CLFV in top quark decays, surpassing previous LHC and non-LHC bounds.
• The analysis demonstrates the effectiveness of machine learning (boosted decision trees) in isolating rare, flavor-violating signals in high-multiplicity final states.