Machine Learning Approaches to Top Quark Flavor-Changing Four-Fermion Interactions in Trilepton Signals at the LHC

TL;DR

This study employs machine learning to analyze top quark flavor-changing four-fermion interactions in trilepton signals at the LHC, setting new limits on interaction scales and interpreting results within a Z' model framework.

Contribution

It introduces machine learning techniques to improve detection sensitivity of top FCNC interactions and provides the first comprehensive EFT limits with realistic detector effects at high luminosity.

Findings

Limits of 5.5-5.7 TeV on interaction scale in t-tbar channel

Limits of 1.9-2.0 TeV in tW channel

Significant background reduction using ML classifiers

Abstract

We explore the top quark flavor-changing 4-Fermi interactions ($tuee$ and $tcee$) with scalar, vector, and tensor structures using machine learning models to analyze tri-lepton processes at the LHC. The study is performed using $t\bar{t}$ and $tW$ processes, where a top quark decays into $u/c+e^{+}+e^{-}$. The analysis incorporates both reducible and irreducible backgrounds while accounting for realistic detector effects. The dominant backgrounds for these trilepton signatures arise from $t\bar{t}$ production, single top quark production in association with $V$, and $VV$ production (where $V = W, Z$). These backgrounds are significantly reduced using machine learning-based classification models, which optimize event selection and improve signal sensitivity. For an integrated luminosity of 3000 fb$^{-1}$ at the LHC, we find that the expected $95\%$ confidence level (CL) limits on the scale of 4-Fermi FCNC interactions reach $\Lambda \leq 5.5$ TeV for $tuee$ and $\Lambda \leq 5.7$ TeV for $tcee$ in the $t\bar{t}$ channel, and $\Lambda \leq 1.9$ TeV ($tuee$) and $\Lambda \leq 2.0$ TeV ($tcee$) in the $tW$ channel. We also provide an interpretation of our EFT analysis in the context of a specific $Z'$ model, illustrating how the derived constraints translate into bounds on the parameter space of a heavy neutral gauge boson mediating flavor-changing interactions.