[Paper Review] Search for singly produced vector-like top partners in multilepton final states with 139 $\mathrm{fb}^{-1}$ of $pp$ collision data at $\sqrt{s} = 13$ TeV with the ATLAS detector

Georges Aad, Abbott, Braden Keim+2972 more|arXiv (Cornell University)|Jan 1, 2023

Particle physics theoretical and experimental studies1 citations

TL;DR

This study searches for singly produced vector-like top partners (T) decaying into a Z boson and a top quark using 139 fb⁻¹ of 13 TeV proton-proton collisions from the ATLAS experiment. No significant excess is observed, leading to 95% confidence level exclusion limits: for singlet T quarks, couplings κ between 0.22 and 0.64 are excluded for masses 1000–1975 GeV; for doublet T quarks, κ between 0.54 and 0.88 are excluded for masses 1000–1425 GeV.

ABSTRACT

A search for the single production of a vector-like top partner ($T$) decaying into a $Z$ boson and a top quark is presented, using the full Run 2 dataset corresponding to 139 $\mathrm{fb}^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV, collected in 2015-2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a pair of electrons or muons with opposite-sign charges which form a $Z$-boson candidate, as well as by the presence of $b$-tagged jets and forward jets. Events with exactly two or at least three leptons are categorized into two independently optimized analysis channels. No significant excess above the background expectation is observed and the results from the two channels are statistically combined to set exclusion limits at 95% confidence level on the masses and couplings of $T$ for different benchmark models and generalized branching ratio scenarios.

Motivation & Objective

• To search for singly produced vector-like top partners (T) decaying into a Z boson and a top quark in multilepton final states.
• To probe the existence of heavy vector-like quarks beyond the Standard Model using the full Run 2 dataset of 139 fb⁻¹ at √s = 13 TeV.
• To set model-independent exclusion limits on the mass and coupling strength (κ) of T quarks in both singlet and doublet representations.
• To improve constraints on vector-like quark couplings by analyzing dilepton (2ℓ) and trilepton (3ℓ) final states with optimized channels.
• To test the viability of vector-like quark models in addressing the hierarchy problem via electroweak production and decay signatures.

Proposed method

• The analysis uses 139 fb⁻¹ of pp collisions at √s = 13 TeV collected by the ATLAS detector during 2015–2018.
• Events are categorized into two independent channels: dilepton (exactly two opposite-sign leptons) and trilepton (at least three leptons), each optimized for sensitivity.
• The final state includes a Z-boson candidate from opposite-sign lepton pairs, b-tagged jets from top quark decays, and forward jets to enhance signal discrimination.
• Signal models are simulated with varying T quark masses (1.0–2.7 TeV) and coupling strengths (κ), with electroweak production via W T Z t and Z T Z t diagrams.
• Backgrounds from SM processes are estimated using data-driven techniques and Monte Carlo simulations, with control regions used to validate the modeling.
• Statistical significance is evaluated using a profile-likelihood ratio test, and results from both channels are combined using a hybrid method to set 95% confidence level limits.

Experimental results

Research questions

• RQ1 What are the exclusion limits on the mass and coupling strength (κ) of vector-like top partners (T) in the singlet representation, assuming single production via electroweak interactions?
• RQ2 How do the exclusion limits on T quark couplings differ in the doublet representation compared to the singlet, given the lower production cross-section?
• RQ3 To what extent can the multilepton final state with b-tagged and forward jets enhance sensitivity to singly produced T quarks?
• RQ4 What is the sensitivity of the dilepton and trilepton channels individually and in combination for probing T quark masses above 1 TeV?
• RQ5 How do the results constrain models that stabilize the Higgs boson mass through vector-like quarks?

Key findings

• No significant excess above the Standard Model background is observed in either the dilepton or trilepton analysis channels.
• The combined analysis sets 95% confidence level exclusion limits on the vector-like top partner (T) mass and coupling strength (κ), with the strongest constraints in the singlet representation.
• For singlet T quarks, coupling values between κ = 0.22 and κ = 0.64 are excluded for masses between 1000 GeV and 1975 GeV.
• For doublet T quarks, where the production cross-section is significantly lower, κ values between 0.54 and 0.88 are excluded for masses between 1000 GeV and 1425 GeV.
• The results represent the most stringent constraints to date on vector-like top partners in the single production mode using the full Run 2 dataset.
• The study demonstrates the effectiveness of multilepton final states with b-tagged and forward jets in probing high-mass vector-like quarks at the LHC.