⚛️ phenomenology

Agnostic search for a vector-like top partner at a lepton collider via the recoil mass technique

This paper presents an agnostic search strategy for single vector-like top partner production at a 3 TeV lepton collider using the recoil mass technique, demonstrating a discovery potential exceeding $3\sigma$ significance for masses at the lowest benchmarks when the coupling strength $\kappa_{T}$ surpasses approximately 0.3 under realistic conditions including beam polarization and initial-state radiation.

Original authors: Higinio Valle, Cristina Oropeza Barrera, Oscar Ochoa-Valeriano

Published 2026-09-09

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CC BY 4.0

Original authors: Higinio Valle, Cristina Oropeza Barrera, Oscar Ochoa-Valeriano

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 authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe as we understand it is built on a foundation of tiny, fundamental particles and the forces that govern how they interact. For decades, physicists have relied on a highly successful framework called the Standard Model to describe this microscopic world. However, this model leaves a significant gap: it cannot explain why the Higgs boson, the particle responsible for giving other particles their mass, is so much lighter than the laws of physics might suggest it should be. To solve this puzzle, scientists have proposed the existence of new, heavier partners for the known particles, particularly for the top quark, which is the heaviest known particle. These hypothetical partners, known as vector-like quarks, would act as a stabilizing force, balancing the equations and potentially revealing the deeper structure of reality. Finding them is a primary goal for the next generation of particle accelerators, which aim to smash particles together with enough energy to create these elusive, massive entities.

In a recent study, researchers explored how to find a specific type of these new partners, called a vector-like top partner, using a future electron-positron collider. Unlike the massive proton colliders currently in operation, which create chaotic showers of debris, an electron-positron collider offers a much cleaner environment. The scientists focused on a scenario where an electron and a positron collide to produce a pair of particles: a standard top quark and its heavy new partner. The challenge is that the heavy partner is unstable and immediately decays into other particles, making it impossible to see directly. To solve this, the team developed a method that does not rely on guessing exactly how the heavy partner will decay or what its mass might be. Instead, they used a technique called recoil mass. By carefully measuring the energy and momentum of the standard top quark that is produced alongside the heavy partner, they could calculate the mass of the missing partner by seeing what was "recoiled" against it. It is like determining the weight of a hidden object by watching how a known object bounces away after a collision.

The researchers simulated billions of potential collisions to test this strategy. They first established a baseline performance using ideal conditions where no extra energy is lost to radiation and the beams are not polarized. In this perfect scenario, they found that if the new partner exists and interacts with a certain strength, the method could detect it with high confidence. Specifically, for a partner with a mass of 1.2 tera-electronvolts, the team could see a clear signal if the interaction strength was above a certain threshold. However, the real world is rarely ideal. The team then introduced more realistic conditions, including the emission of initial-state radiation, where particles lose energy before colliding, and the use of polarized electron beams, which aligns the spins of the particles to increase collision rates. These realistic factors blurred the sharpness of the signal, reducing the statistical confidence of the discovery. Under these more difficult conditions, the ability to find the particle dropped, requiring a stronger interaction or a lighter mass to be seen with the same certainty.

Despite these challenges, the study demonstrated that the recoil mass technique remains a powerful tool. The researchers mapped out the range of possible masses and interaction strengths where the new particle could be found. They found that while the realistic conditions make the search harder, the method is still robust enough to probe a wide area of possibilities without needing to assume a specific decay pattern. This is a crucial advantage because, unlike searches at current colliders that often fail if the particle decays in an unexpected way, this approach stays sensitive regardless of how the heavy partner breaks apart. The study concludes that a future collider operating at an energy of 3 tera-electronvolts, with a total data collection equivalent to five inverse attobarns, could successfully uncover these vector-like top partners if they exist within the mass range of 1.2 to 2.4 tera-electronvolts and interact with sufficient strength. The work provides a clear roadmap for how to look for new physics in a way that is not limited by our current guesses about how these particles behave.

Technical Summary: Agnostic Search for a Vector-Like Top Partner at a Lepton Collider

Problem and Motivation
Vector-like quarks (VLQs), particularly the top partner ( $T$ ), are well-motivated extensions of the Standard Model (SM) addressing the Hierarchy Problem and appearing in Composite Higgs, Little-Higgs, and extra-dimensional frameworks. While the Large Hadron Collider (LHC) provides constraints, its sensitivity to single VLQ production is often limited by large QCD backgrounds and a reliance on specific decay hypotheses (e.g., specific branching fractions). If the actual decay pattern deviates from these assumptions, LHC sensitivity deteriorates rapidly. Multi-TeV $e^+e^-$ colliders, specifically the proposed CLIC program at $\sqrt{s} = 3$ TeV, offer a cleaner environment where sensitivity to single production ( $e^+e^- \to Tt$ ) is directly governed by the effective coupling strength $\kappa_T$ . This work addresses the need for a search strategy that remains robust against unknown decay modes and mass values, avoiding the pitfalls of channel-specific selections.

Methodology
The study employs a recoil mass technique designed to be agnostic to both the top-partner mass ( $m_T$ ) and its decay mode. The analysis focuses on the process $e^+e^- \to Tt$ , where the SM top quark ( $t$ ) is reconstructed as a hadronic large-radius ( $R=1.0$ ) jet, and the $T$ partner is inferred via its recoil against this system.

Simulation and Samples: Signal and background samples were generated at leading order using MadGraph5_aMC@NLO, with spin correlations handled by MadSpin and parton showering/fragmentation by Pythia 8. The signal model follows Buchkremer et al., where $T$ is a singlet mixing with the third generation, decaying to $bW$ (50%), $tZ$ (25%), and $tH$ (25%).
Event Selection:
- Reconstruction: The SM top is identified as a Cambridge/Aachen large-R jet ( $p_T > 100$ GeV) within $|\eta| < 2.5$ , tagged with the Johns Hopkins top-tagger and requiring at least one $b$ -tagged subjet.
- Recoil Mass Calculation: The recoil mass ( $m_{recoil}$ ) is computed event-by-event using the formula $m^2_{recoil} = s - 2E_{top}\sqrt{s} + m^2_{top}$ , where $E_{top}$ and $m_{top}$ are derived from the selected large-R jet.
- Background Suppression: Cuts include $m_{recoil} > 1.1$ TeV to suppress the SM continuum and total missing energy $E_{miss} < 1$ TeV to reduce $W^+W^-Z$ and $t\bar{t}Z$ backgrounds. An isolation veto is applied to large-R jets to reject candidates embedded in broader top-decay environments (e.g., from the $T$ decay chain).
Scenarios: Two configurations are evaluated:
1. Idealized Baseline: No initial-state radiation (ISR) and no beam polarization.
2. Realistic Scenario: Includes ISR and an electron beam polarization of $P_{e^-} = -80\%$ .
Parameters: The study scans $m_T$ from 1.2 to 2.4 TeV and coupling strengths $\kappa_T$ up to values yielding physical widths, with an integrated luminosity of $L = 5$ ab $^{-1}$ .

Key Contributions

Model-Agnostic Strategy: The paper introduces a selection strategy that does not require assumptions about the $T$ decay branching fractions. By relying on the kinematic recoil against a reconstructed SM top, the method maintains sensitivity across a broad range of hypothetical decay topologies.
Performance Benchmarking: The study quantifies the intrinsic performance of the recoil-mass method in an idealized setting and explicitly maps the degradation caused by realistic collider effects (ISR and beam polarization).
Discovery Reach Mapping: The authors provide a comprehensive mapping of the statistical significance ( $Z$ ) in the $(m_T, \kappa_T)$ parameter space, offering a compact interpretation of the discovery potential for CLIC.

Results

Baseline Performance: In the idealized scenario (no ISR, no polarization), the estimated statistical significance rises above $3\sigma$ for the lowest benchmark masses ( $m_T = 1.2$ TeV) once $\kappa_T$ exceeds approximately 0.3. At the baseline coupling of $\kappa_T = 0.2$ , the significance reaches $Z \approx 2.35$ for $m_T = 1.2$ TeV.
Impact of Realistic Conditions: The inclusion of ISR and $P_{e^-} = -80\%$ reduces the sensitivity. For $m_T = 1.2$ TeV, the significance drops from $Z = 2.35$ to $Z = 1.54$ at $\kappa_T = 0.2$ . The overall discovery reach is reduced by approximately 35% in significance compared to the baseline.
Mass and Coupling Dependence:
- The baseline configuration reaches $Z \simeq 5$ for $(m_T, \kappa_T) = (1.2 \text{ TeV}, 0.3)$ .
- For higher masses (e.g., $m_T = 2.4$ TeV), the significance remains below unity even for the tested coupling ranges, indicating limited reach for very heavy partners with the current selection.
- The recoil mass distributions show clear peaks around nominal masses, though a low-mass tail appears for higher $m_T$ due to the selection of secondary tops from the $T$ decay chain rather than the associated top.

Significance and Claims
The paper claims that a mass- and decay-agnostic recoil strategy is a robust tool for probing vector-like top partners at future lepton colliders, particularly in regions where hadron-collider strategies fail due to uncertain branching fractions. The study demonstrates that while realistic effects like ISR and beam polarization significantly reduce sensitivity, the recoil-mass technique retains $O(2\sigma)$ sensitivity at $\kappa_T = 0.2$ in the idealized limit. The authors suggest that the observed loss in sensitivity under realistic conditions points to specific mitigation strategies, such as utilizing transverse-momentum imbalance and acolinearity in multivariate discriminants, to recover significance while preserving the model-independent nature of the search. The work serves as a benchmark for future lepton-collider analyses in the single-production regime.

Technical Summary: Agnostic Search for a Vector-Like Top Partner at a Lepton Collider

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