⚛️ phenomenology

Probing new physics in the top sector using quantum information

This paper demonstrates that various quantum information measures, including magic, trace distance, and fidelity distance, offer distinct and complementary sensitivities to new physics in the top quark sector within the Standard Model Effective Field Theory, highlighting the importance of employing multiple such metrics to effectively probe beyond-Standard-Model phenomena.

Original authors: Rafael Aoude, Hannah Banks, Chris D. White, Martin J. White

Published 2026-07-31

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Rafael Aoude, Hannah Banks, Chris D. White, Martin J. White

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

Imagine the universe as a giant, cosmic game of LEGO. For decades, scientists have been building a magnificent model called the Standard Model, which explains how the tiniest building blocks of nature—like electrons and quarks—stick together and interact. It's a brilliant model, but scientists suspect there are hidden pieces in the box that they haven't found yet. These missing pieces represent "New Physics," mysterious forces or particles that could explain things like dark matter or why the universe exists at all. To find them, physicists smash particles together at mind-boggling speeds in giant machines called colliders, like the Large Hadron Collider (LHC).

But looking for these hidden pieces is like trying to find a specific, slightly different brick in a pile of billions of identical ones. That's where a new tool comes in: Quantum Information. Think of this as a special set of "magic glasses" that let scientists look at the particles not just as physical objects, but as complex states of information. Two key ideas help here: entanglement, which is like having two dice that always roll the same number no matter how far apart they are, and magic, a technical term for how "weird" or computationally powerful a quantum state is compared to a boring, classical one. If the particles behave in a way that the Standard Model doesn't predict, these magic glasses might reveal a subtle shift in the pattern, hinting that a new, hidden brick is being used.

This paper is like a detective's guidebook for using these quantum glasses to hunt for new physics in the world of top quarks. Top quarks are the heaviest known particles, and when they are created in pairs at the LHC, they form a tiny, two-qubit quantum system. The authors, Rafael Aoude, Hannah Banks, Chris White, and Martin White, wanted to see if different ways of measuring "quantum weirdness" could spot the fingerprints of new physics better than the old methods. They didn't just look at one type of measurement; they compared several, including concurrence (a measure of entanglement), trace distance (how far apart two quantum states are), fidelity distance (how similar they are), and magic (how non-classical the state is).

The researchers simulated what would happen if the Standard Model were tweaked by adding "Effective Field Theory" operators—essentially, mathematical placeholders for new physics that could be lurking at high energy scales. They ran their simulations across different speeds and angles of the colliding top quarks. What they found is that there is no single "best" magnifying glass. Depending on the specific type of new physics and the exact region of the collision (like how fast the particles are moving or the angle they scatter at), different measures shine. Sometimes concurrence is the sharpest tool, other times magic or trace distance takes the lead. In fact, for some scenarios, the raw data (called Fano coefficients) was just as good as these fancy quantum measures, but for others, the quantum measures were superior.

The paper suggests that the most effective strategy isn't to pick one winner and stick with it, but to use a whole toolkit. By looking at the data through multiple quantum lenses simultaneously, scientists can catch subtle differences that a single measure might miss. The authors also noted that while "magic" generally tends to increase when new physics is added (making the quantum state more complex), it can sometimes decrease depending on the specific new force involved. This variability is actually a good thing, as it means the pattern of changes could help identify exactly what kind of new physics is hiding in the data. Ultimately, the study concludes that combining these diverse quantum information measures offers a powerful, complementary way to search for the unknown, turning the hunt for new physics into a more nuanced and effective game of cosmic hide-and-seek.

Technical Summary: Probing New Physics in the Top Sector Using Quantum Information

Problem Statement
The paper addresses the challenge of detecting Beyond the Standard Model (BSM) physics in top quark pair production at the Large Hadron Collider (LHC). While recent studies have utilized quantum information (QI) concepts, specifically entanglement (measured by concurrence) and "magic" (non-stabiliserness), to analyze top quark spins, there is a need to systematically compare these measures against other QI observables. The authors investigate whether specific QI measures offer superior sensitivity to new physics effects modeled by the Standard Model Effective Field Theory (SMEFT) compared to traditional observables like the raw Fano coefficients. The central question is whether a single QI measure can universally outperform others across all kinematic regions and new physics scenarios, or if a diverse set of measures is required.

Methodology
The authors employ the SMEFT framework to parametrize BSM physics, focusing on dimension-six operators that contribute to top quark pair production. They adopt a basis of 17 relevant operators (including gluon couplings $O_G, O_{\phi G}, O_{tG}$ and various four-fermion operators) and analyze their impact on the top quark spin density matrix.

The study evaluates four distinct quantum information measures:

Magic ( $M_2$ ): Quantified via the Second Stabiliser Rényi Entropy (SSRE), measuring the "non-stabiliserness" of the quantum state.
Concurrence ( $C$ ): A measure of spin entanglement.
Trace Distance ( $D_T$ ): A metric quantifying the distance between the SM density matrix and the SMEFT density matrix.
Fidelity Distance ( $D_F$ ): Derived from fidelity, measuring the "closeness" of two quantum states.

The analysis is performed at both the parton level (individual $gg$ and $q\bar{q}$ channels) and the proton level (incorporating Parton Distribution Functions). The authors calculate these measures differentially in terms of the top quark velocity ( $\beta$ ) and scattering angle ( $z = \cos \theta$ ), as well as in angularly averaged bins. To assess sensitivity, they compute a $\chi^2$ statistic for deviations from the Standard Model, incorporating estimated experimental uncertainties on the Fano coefficients. This allows for a ranking of measures based on their ability to distinguish SMEFT corrections from the SM background in specific kinematic bins defined by invariant mass ( $m_{t\bar{t}}$ ) and scattering angle.

Key Contributions and Results

Magic Behavior in SMEFT: The authors demonstrate that SMEFT operators generally modify the magic of top quark pairs. While naive expectations suggested magic would always increase (as operators complicate the state), the results show that magic can either increase or decrease depending on the specific operator and kinematic region. For instance, in the $gg$ channel near threshold, the state remains a stabiliser state (zero magic) even with SMEFT corrections, whereas at high velocities, magic increases significantly.
Comparison of Measures: The study reveals that no single QI measure is universally the most sensitive probe of new physics.
- Magic ( $M_2$ ) emerges as the most sensitive observable for the gluon coupling $c_G$ across most of the kinematic range.
- Concurrence ( $C$ ) performs relatively well at high invariant masses, particularly in the central scattering region ( $|z| < 0.4$ ).
- Trace Distance ( $D_T$ ) and Fidelity Distance ( $D_F$ ), which are zero in the pure SM by definition, show linear sensitivity to Wilson coefficients. However, their performance varies; $D_T$ sensitivity degrades at high invariant masses due to increasing uncertainties in the SM prediction.
Comparison with Raw Fano Coefficients: The authors find that QI measures are not inherently less sensitive than the raw Fano coefficients. In many kinematic bins, QI measures outperform the direct fitting of Fano coefficients, while in others, the raw coefficients are superior. The relative sensitivity is highly dependent on the specific Wilson coefficient and the kinematic bin being probed.
Quadratic Corrections: The analysis confirms that for the considered Wilson coefficients, quadratic SMEFT corrections (order $\Lambda^{-4}$ ) are generally small compared to linear corrections (order $\Lambda^{-2}$ ), suggesting the EFT expansion is well-behaved within the valid energy regime.

Significance and Claims
The paper claims that the exploration of multiple quantum information measures is essential for the hunt for BSM physics. The authors argue that because different measures exhibit peak sensitivity in different regions of phase space and for different types of new physics operators, relying on a single metric (such as concurrence or magic alone) is insufficient.

The study provides quantitative evidence that QI measures can be as sensitive, and in some cases more sensitive, than traditional observables like Fano coefficients. By highlighting the complementary nature of magic, concurrence, trace distance, and fidelity distance, the authors aim to strengthen the dialogue between quantum information theory and collider physics. They conclude that a strategy employing a range of QI measures is necessary to effectively constrain new physics, as the optimal observable depends on both the specific collider process and the kinematic region under investigation.

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