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Search for heavy top quark partners with charge 5/3 in the single-lepton final state in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Using 138 fb $^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the CMS experiment, this study presents a search for vector-like quark partners with charge 5/3 in the single-lepton final state, setting the most stringent exclusion limits to date on their pair production cross-sections (up to 1630 GeV for decays to top-W and up to 1740 GeV for exotic top-charged Higgs decays) without observing any excess over Standard Model expectations.

Original author: CMS Collaboration

Published 2026-11-11

📖 4 min read🧠 Deep dive

CC BY 4.0

Original author: CMS Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the author. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is built on a set of rules that physicists call the Standard Model, a theory that successfully describes the fundamental particles and forces that make up everything we see. Yet, this theory is incomplete. It leaves a major puzzle unsolved: why does the Higgs boson, the particle responsible for giving other particles their mass, have the specific weight it does? Calculations suggest that without new, unseen physics, the Higgs should be impossibly heavy or impossibly light. To fix this, theorists have proposed the existence of "vector-like quarks," heavy partners to the known quarks that would stabilize these calculations. Among these hypothetical partners is a particle with an electric charge of five-thirds, a value that does not exist in the known world. If such a particle exists, it would be a massive, unstable entity that decays almost instantly into lighter, familiar particles. Finding it would not only solve a mathematical problem but also open a window into the deeper structure of reality.

At the Large Hadron Collider, a massive machine buried beneath the border of France and Switzerland, scientists have been smashing protons together at incredible speeds to recreate the conditions of the early universe. The CMS experiment, one of the giant detectors at this facility, recently completed a comprehensive search for these heavy, five-thirds charged partners. The team analyzed a vast collection of collision data gathered between 2016 and 2018, representing 138 units of integrated luminosity, a measure of the total number of collisions recorded. They were looking for a very specific signature: a pair of these heavy particles being created and then immediately decaying. The researchers focused on two possible ways these particles might break apart. In the first, more traditional scenario, the heavy particle splits into a top quark and a W boson. In a second, more exotic possibility, it decays into a top quark and a charged Higgs boson, which then breaks down further into other quarks.

The challenge for the researchers was to find a needle in a haystack. The collisions produce billions of events, most of which are ordinary interactions that mimic the signal the scientists were seeking. To separate the potential signal from the background noise, the team looked for events containing a single electron or muon, a type of heavy electron, along with missing energy carried away by invisible neutrinos and a spray of jets, which are sprays of particles created when quarks fly apart. They used sophisticated computer algorithms to sift through the data, looking for patterns that matched the predicted behavior of the heavy five-thirds charged particle. The analysis involved training machine learning models to distinguish between the complex debris of a potential new particle and the messy, common debris of standard particle interactions.

After examining the data, the scientists found no evidence of these heavy particles. The number of events they observed matched the predictions of the Standard Model perfectly, with no unexpected excesses that would indicate a new discovery. While they did not find the particle, the absence of a signal allowed them to set strict boundaries on where it could exist. They concluded that if these particles do exist, they must be heavier than 1,630 gigaelectronvolts in the traditional decay scenario. In the more exotic scenario involving the charged Higgs boson, they ruled out masses up to 1,740 gigaelectronvolts, assuming the charged Higgs itself has a mass of 800 gigaelectronvolts. These limits are the most stringent constraints established to date for the traditional decay mode and represent the first experimental limits ever set for the exotic decay mode.

The results effectively push the search for these heavy partners into a higher mass range. The researchers did not prove that the particles do not exist; rather, they proved that if they do, they are too heavy to have been produced in the collisions they analyzed. This narrows the field for future experiments, telling theorists that if they want to find these particles, they must look for them at even higher energies. The study stands as a testament to the rigorous process of scientific inquiry: by carefully ruling out what is not there, the scientific community refines its understanding of what might be, guiding the next generation of experiments toward the frontiers of the unknown.

Technical Summary: Search for Heavy Top Quark Partners with Charge 5/3 in the Single-Lepton Final State

Problem and Motivation
This paper addresses the search for Vector-Like Quarks (VLQs), specifically the $X_{5/3}$ , a hypothetical heavy fermionic partner of the top quark with an electric charge of $+5/3$ . VLQs are predicted by various Beyond-the-Standard-Model (BSM) theories, such as little Higgs and composite Higgs models, which aim to resolve the hierarchy problem by stabilizing quantum loop corrections to the Higgs boson mass. While previous searches have focused on the "conventional" decay mode where $X_{5/3} \to tW$ , this analysis extends the search to an "exotic" decay scenario motivated by extensions of the Standard Model (SM). In this alternative scenario, the $X_{5/3}$ decays into a top quark and a charged Higgs boson ( $H^+$ ), which subsequently decays into a top-bottom quark pair ( $tH^+ \to t(t\bar{b})$ ). The paper aims to set experimental limits on the pair production of $X_{5/3}$ in both decay modes using proton-proton collision data.

Methodology
The analysis utilizes data collected by the CMS experiment at the CERN Large Hadron Collider (LHC) during the 2016–2018 run periods at a center-of-mass energy of $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 138 fb $^{-1}$ .

Signal Topology: The search targets the single-lepton final state (one electron or muon).
- In the conventional mode ( $X_{5/3} \to tW$ ), pair production results in a signature with four $W$ bosons and two bottom quarks.
- In the exotic mode ( $X_{5/3} \to tH^+$ ), the decay chain yields four top quarks, resulting in four $W$ bosons and six bottom quarks.
- Events are selected requiring one isolated high- $p_T$ lepton, missing transverse momentum ( $\vec{p}^{\text{miss}}_T$ ) from the neutrino, and multiple hadronically decaying $W$ bosons and top quarks, including several $b$ -tagged jets.
Event Selection and Reconstruction:
- Events must contain at least three AK4 jets ( $p_T > 30$ GeV), with at least one $b$ -tagged.
- A scalar sum of transverse momenta ( $H_T$ ) exceeding 500 GeV is required to suppress multijet backgrounds.
- Angular separation ( $\Delta R$ ) between the lepton and the subleading jet is optimized to retain signal efficiency while rejecting background.
- A multivariate discriminant based on the XGBoost algorithm is employed to separate signal from background. The training utilizes simulated signal samples (masses 1200–1300 GeV) and $t\bar{t}$ background events. Input variables include kinematic quantities such as $S_T$ (scalar sum of lepton $p_T$ , $\vec{p}^{\text{miss}}_T$ , and $H_T$ ), invariant masses of jet-lepton pairs, sphericity, and jet tagging information.
Simulation and Background Modeling:
- Signal samples are generated at leading order (LO) using MADGRAPH5_aMC@NLO, assuming right-handed couplings and the narrow-width approximation.
- SM backgrounds (dominated by $t\bar{t}$ , single top, $W$ +jets, and QCD multijets) are simulated using POWHEG and MADGRAPH5_aMC@NLO, with parton showering modeled by PYTHIA 8.
- Systematic uncertainties, including luminosity, lepton identification, jet energy scale/resolution, $b$ -tagging, and theoretical cross-section variations (PDF, renormalization/factorization scales), are treated as nuisance parameters in the statistical fit.

Key Contributions

First Limits on Exotic Decay: This work presents the first experimental limits on the $X_{5/3}$ decaying into a top quark and a charged Higgs boson ( $X_{5/3} \to tH^+$ ).
Updated Conventional Limits: It provides the most stringent limits to date on the $X_{5/3}$ decaying into a top quark and a $W$ boson ( $X_{5/3} \to tW$ ), utilizing the full Run 2 dataset (2016–2018).
Comprehensive Kinematic Analysis: The analysis explores a wide mass range for the $X_{5/3}$ (600–1900 GeV) and, for the exotic mode, a range of charged Higgs masses (200–1000 GeV), utilizing a boosted decision tree (BDT) to maximize sensitivity in high-multiplicity final states.

Results
No significant excess of data events over the Standard Model expectation was observed in either decay mode. Upper limits on the pair production cross section were set at the 95% confidence level (CL).

Conventional Decay ( $X_{5/3} \to tW$ ):
- $X_{5/3}$ masses up to 1630 GeV are excluded at 95% CL.
- This represents the most restrictive limit for this decay channel to date.
Exotic Decay ( $X_{5/3} \to tH^+$ ):
- $X_{5/3}$ masses are excluded up to 1740 GeV for a charged Higgs mass of 800 GeV.
- Generally, for charged Higgs masses between 200 and 1000 GeV, $X_{5/3}$ masses in the range of 1620–1740 GeV are excluded.
- These are the first limits established for this specific decay topology.

Significance
The paper claims that these results constitute the most stringent constraints on the $X_{5/3}$ vector-like quark in the conventional decay mode and establish the first experimental boundaries for the exotic decay mode involving a charged Higgs boson. By covering the single-lepton final state with high signal acceptance and utilizing advanced multivariate techniques, the analysis complements previous searches in same-sign dilepton channels and multi-top signatures. The absence of an observed signal constrains the parameter space of BSM theories predicting heavy top partners and charged Higgs bosons, specifically ruling out $X_{5/3}$ masses up to 1.74 TeV in the explored scenarios.

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