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Quantum probes of invisible particles in top-quark pair production at the LHC

This paper demonstrates that quantum-information-motivated observables, particularly the spin correlation $\mathcal{D}$ measured as a function of visible invariant mass in dileptonic top-quark pair events at the High-Luminosity LHC, significantly enhance the sensitivity to invisible new physics and the discrimination of mediator couplings compared to traditional rate and kinematic analyses.

Original authors: Blanca Belmonte, Diego Blas, Alba Cervera-Lierta, Francesco Montagno

Published 2026-09-28

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CC BY 4.0

Original authors: Blanca Belmonte, Diego Blas, Alba Cervera-Lierta, Francesco Montagno

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 top quark is the heaviest known building block of matter, a particle so massive and unstable that it decays almost instantly, long before it can form the complex structures that make up atoms. Because it vanishes so quickly, it never gets tangled in the messy, sticky forces that usually scramble the properties of other particles. This means the top quark carries a unique piece of information from the moment of its creation: its spin, or the direction of its internal rotation. When two top quarks are born together in a high-energy collision, their spins are linked in a delicate quantum relationship, a connection that survives even as the particles decay into lighter debris. Scientists at the Large Hadron Collider have learned to read the angles at which these decay products fly to reconstruct that original spin connection, treating the pair like a tiny, two-part quantum system. This ability to track such a fragile link offers a new way to look for invisible particles that might be hiding in the debris of these collisions, particles that do not leave a direct trace in the detectors but might be whispering their presence through the way the top quarks spin.

In a recent study, researchers explored whether this quantum spin information could serve as a sensitive probe for invisible new physics, specifically looking for a scenario where a top-quark pair is produced alongside a mysterious, invisible particle. This invisible particle, often theorized in models of dark matter, would escape the detector entirely, carrying away energy and momentum without being seen. The team focused on the "dileptonic" channel, a specific type of collision where the top quarks decay into two charged particles and two invisible neutrinos, creating a final state with two visible jets, two charged particles, and a significant amount of missing energy. By simulating millions of these collision events at the energy levels expected for the future High-Luminosity LHC, the researchers tested four different ways to analyze the data. They compared the simple count of how many events occurred, the distribution of the visible mass of the debris, the angular patterns of the decay products, and a specific measure of the spin correlation between the two top quarks.

The researchers found that counting events or looking only at the energy and mass of the visible debris was not the most effective way to spot the invisible particle. These traditional methods often struggle because the background noise from standard collisions can mimic the signal of new physics. However, when the team included the quantum spin information in their analysis, the ability to distinguish the signal from the background improved significantly. The most powerful tool they identified was a specific measure of the spin correlation, calculated as a function of the visible mass of the top-quark pair. This method proved to be far more sensitive than the others, capable of detecting the invisible mediator even when the traditional methods would have missed it. The study showed that this approach works well whether the invisible particle is a simple scalar object or a more complex vector particle, and it remains effective across a wide range of masses for the invisible mediator.

Beyond just finding the invisible particle, the researchers discovered that this spin-based method could also tell the difference between different types of interactions. If the invisible particle interacts with the top quarks in a scalar way versus a pseudoscalar way, the pattern of the spin correlations changes in a distinct manner. The study demonstrated that the spin-correlation measure could distinguish between these two possibilities with much greater clarity than simply counting events or looking at the mass distribution. This suggests that by studying how the top quarks spin, scientists can not only confirm the presence of new, invisible physics but also begin to understand the fundamental nature of the forces connecting the visible world to the dark sector.

The work relies on detailed computer simulations that model the collisions and the subsequent decay of the particles, using the known laws of physics to predict what the detectors would see. While the study assumes ideal conditions where the paths of the top quarks are known perfectly, the researchers acknowledge that real-world experiments will face challenges in reconstructing these paths due to the presence of the invisible particle. Despite this, the simulations show that the underlying spin information is robust enough to survive the complexities of the reconstruction process. The findings indicate that quantum information tools, which were once the domain of pure theory, are becoming practical instruments for exploring the frontiers of particle physics. By treating the top-quark pair as a quantum system, scientists can extract more information from the same amount of data, turning the subtle correlations of spin into a powerful lens for viewing the invisible universe.

Technical Summary: Quantum Probes of Invisible Particles in Top-Quark Pair Production at the LHC

Problem Statement
Top-quark pair ( $t\bar{t}$ ) production at the Large Hadron Collider (LHC) offers a unique bipartite spin system where quantum correlations are preserved and transferred to the angular distributions of decay products. While quantum information (QI) observables have been used to test the Standard Model (SM) and constrain effective field theories, their sensitivity to invisible new physics remains largely unexplored. Specifically, this paper addresses the scenario of $pp \to t\bar{t} + X$ , where $X$ is an invisible spin-0 ( $Y_0$ ) or spin-1 ( $Y_1$ ) mediator motivated by top-philic simplified dark matter models. Unlike previous studies where invisible particles arise from the decay of a visible resonance or are purely virtual, here the mediator is produced on-shell but escapes detection. The central question is whether QI observables constructed from the $t\bar{t}$ spin density matrix can detect the presence of such an invisible mediator and discriminate its coupling structure, complementing traditional searches based on missing transverse momentum ( $E_T^{\text{miss}}$ ) and event rates.

Methodology
The analysis focuses on the dileptonic channel ( $t\bar{t} \to b\ell^+\nu \bar{b}\ell^-\bar{\nu}$ ) at the High-Luminosity LHC (HL-LHC) with an integrated luminosity of $3000 \text{ fb}^{-1}$ at $\sqrt{s} = 14 \text{ TeV}$ .

Theoretical Framework: The $t\bar{t}$ system is treated as a two-qubit system described by a $4\times4$ density matrix $\rho_{t\bar{t}}$ . The spin-correlation matrix $C$ is decomposed into polarization vectors and correlation coefficients. The study utilizes the basis-independent scalar observable $D \equiv \frac{1}{3}\text{Tr}(C)$ , which serves as an entanglement witness (where $D < -1/3$ implies entanglement).
Signal and Background Simulation: Signal processes ( $pp \to t\bar{t}Y_{0,1}$ ) and backgrounds ( $t\bar{t}$ , $tW$ , $t\bar{t}W$ , $t\bar{t}Z$ ) are simulated using MadGraph5_aMC@NLO and POWHEG BOX. The mediators are treated as stable invisible particles.
Event Selection: Events are selected requiring two oppositely charged leptons, two $b$ -tagged jets, and significant missing transverse momentum. Cuts are applied: $E_T^{\text{miss}} > 100 \text{ GeV}$ and stransverse mass $m_{\ell\ell}^{T2} > 90 \text{ GeV}$ to suppress the dominant SM $t\bar{t}$ background.
Reconstruction Strategy: A "truth-assisted" approach is employed. Since the presence of an additional invisible particle ( $X$ ) renders the standard kinematic reconstruction of the $t\bar{t}$ system underconstrained, the top and antitop four-momenta are taken directly from the generator-level hard-process record to define the helicity frame. Charged lepton momenta are taken from the showered state. This isolates the intrinsic sensitivity of spin observables from reconstruction ambiguities.
Observables Compared: The study compares four alternative analysis strategies on the same selected sample:
- Inclusive Yield ( $N_{\text{tot}}$ ): Total event count.
- Visible Invariant Mass ( $dN/dM_{t\bar{t}}^{\text{vis}}$ ): Kinematic distribution of the visible $t\bar{t}$ system.
- Event-Level Angular Distribution ( $dN/dD_{\text{evt}}$ ): Distribution of the event-level estimator $D_{\text{evt}} = -3\cos\phi$ , where $\cos\phi$ is the sum of products of lepton angles relative to helicity axes.
- Fiducial Spin Correlation ( $D(M_{t\bar{t}}^{\text{vis}})$ ): The observable $D$ measured as a function of the visible invariant mass, retaining the dependence of spin correlations on kinematics.
Statistical Analysis: Sensitivities are evaluated using profile-likelihood methods, incorporating systematic uncertainties (background normalization, shape, and finite Monte Carlo statistics) and finite-MC uncertainties.

Key Contributions and Results

Enhanced Exclusion Sensitivity: The paper demonstrates that observables retaining spin information significantly improve the expected exclusion sensitivity for invisible mediators compared to rate-only or purely kinematic analyses.
- For both spin-0 and spin-1 mediators, the fiducial spin-correlation observable $D(M_{t\bar{t}}^{\text{vis}})$ provides the strongest reach across all tested mediator masses (10–100 GeV).
- The event-level angular distribution $dN/dD_{\text{evt}}$ outperforms the visible invariant mass distribution $dN/dM_{t\bar{t}}^{\text{vis}}$ , indicating that angular correlations contain discriminating information beyond the kinematic spectrum.
- The inclusive yield provides the weakest constraints, highlighting the degeneracy between signal yield and background normalization.
Coupling Structure Discrimination: The study investigates the ability to distinguish between scalar ( $g_s \neq 0, g_p = 0$ ) and pseudoscalar ( $g_s = 0, g_p \neq 0$ ) couplings of the spin-0 mediator.
- $D(M_{t\bar{t}}^{\text{vis}})$ again provides the strongest discrimination power, particularly for heavier mediator masses where rate-based discrimination deteriorates.
- This suggests that the spin-correlation pattern encodes the Lorentz structure of the interaction in a way that total rates and event-level kinematics alone cannot capture.
Robustness to Systematics: The advantage of spin-sensitive observables persists even under conservative assumptions regarding shape uncertainties. However, the superiority of $D(M_{t\bar{t}}^{\text{vis}})$ over $dN/dD_{\text{evt}}$ is sensitive to the assumed systematic uncertainty on the spin correlation measurement; the advantage diminishes if the systematic uncertainty on $D$ is increased significantly (e.g., to $\sim 0.05$ ).

Significance and Claims
The authors claim that quantum-information-motivated observables offer a principled and efficient tool for probing invisible new physics that is complementary to, and often superior to, standard rate and kinematic analyses.

Dual Role: These observables serve a dual purpose: they act as sensitive probes for the presence of invisible particles and as tools to characterize the nature of their couplings (e.g., scalar vs. pseudoscalar).
Physical Motivation: Unlike arbitrary variable choices, $D$ is basis-independent and physically motivated by the underlying spin structure of the production amplitude. It is sensitive to the full spin-correlation matrix, making it robust against distortions that might leave total rates unchanged.
Limitations and Future Work: The paper explicitly acknowledges that the current results are based on a "truth-assisted" reconstruction, which bypasses experimental reconstruction challenges inherent to the $t\bar{t} + \text{invisible}$ topology. The authors state that a realistic assessment requires a dedicated reconstruction study to determine experimental systematic uncertainties and their correlations. They also note that while $D$ is used here as a discriminant, it is fundamentally an entanglement witness; future work could explore unfolding these measurements to the inclusive density matrix to study how invisible mediators genuinely degrade the entanglement of the $t\bar{t}$ pair, potentially offering a new handle to distinguish spin-0 from spin-1 mediators based on the loss of quantum coherence.

In conclusion, the paper establishes that spin-correlation observables, particularly the differential fiducial correlation $D(M_{t\bar{t}}^{\text{vis}})$ , provide a powerful, model-independent avenue to search for and characterize invisible mediators in top-quark pair production at the HL-LHC.

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