Unlocking New Physics with Quantum Top Quarks

Author: Denis Avetisyan

A new study explores how quantum correlations in top quark pairs produced at the Large Hadron Collider could reveal subtle signals of physics beyond the Standard Model.

The distributions of concurrence and quantum discord, alongside a Bell variable, characterize quantum entanglement within the <span class="katex-eq" data-katex-display="false">\bar{t}t</span> process at 13 TeV LHC, specifically examining the di-leptonic decay of top quarks in the <span class="katex-eq" data-katex-display="false">k\bar{k}-r\bar{r}-n\bar{n}</span> helicity basis to reveal underlying quantum correlations. — The distributions of concurrence and quantum discord, alongside a Bell variable, characterize quantum entanglement within the $\bar{t}t$ process at 13 TeV LHC, specifically examining the di-leptonic decay of top quarks in the $k\bar{k}-r\bar{r}-n\bar{n}$ helicity basis to reveal underlying quantum correlations.

This review investigates the sensitivity of quantum information measures to beyond-the-Standard-Model effects within the Standard Model Effective Field Theory (SMEFT) framework, focusing on potential signatures of CP violation and the interplay with conventional analyses.

Despite the successes of the Standard Model, fundamental questions about the nature of particle interactions remain open, motivating searches for new physics at high energies. This paper, ‘Quantumness of top quark pairs produced at LHC within SMEFT framework’, explores the sensitivity of quantum information measures-including entanglement, discord, and Bell parameter violations-to beyond-the-Standard-Model effects parameterized within the Standard Model Effective Field Theory (SMEFT) in top quark pair production at the LHC. Analyses of the $t\bar{t}$ spin density matrix reveal distinct responses to anomalous chromo- and weak-dipole moments, offering a complementary probe of CP-violating interactions and subtle correlations beyond conventional particle physics observables. Could these quantum information signatures provide a novel pathway towards discovering and characterizing new physics at the TeV scale?

Probing the Limits of Known Physics

Despite its extraordinary predictive power, the Standard Model of particle physics remains incomplete. Phenomena such as the existence of dark matter and dark energy, the observed mass of neutrinos, and the matter-antimatter asymmetry of the universe all fall outside its scope. These unresolved puzzles strongly suggest the presence of undiscovered particles and interactions, hinting at a more fundamental theory awaiting discovery. While the Standard Model accurately describes the known fundamental forces – the strong, weak, and electromagnetic interactions – and classifies all known elementary particles, its inability to account for these critical observations motivates ongoing searches for physics beyond its current framework. These investigations aren’t about disproving the Standard Model, but rather identifying where it breaks down, providing crucial clues to the underlying structure of reality and the laws governing the universe.

The top quark, the most massive elementary particle known, presents a unique opportunity to explore physics beyond the Standard Model. Its substantial mass amplifies the effects of potential new particles or interactions, making top quark production and decay particularly sensitive to deviations from predicted behaviors. Researchers meticulously analyze the characteristics of top quarks – their production rate, spin correlations, and decay products – seeking subtle discrepancies between experimental observations and the precise predictions of the Standard Model. These measurements aren’t simply confirming existing theory; they function as a high-precision search for new physics, with even minute anomalies potentially signaling the existence of undiscovered particles or forces that could revolutionize our understanding of the universe. Because of this sensitivity, the top quark serves as a crucial testing ground for models proposing extensions to the Standard Model, such as supersymmetry or extra dimensions.

Despite the high precision of current experiments, fully unlocking the potential of top quark data requires overcoming significant analytical hurdles. Existing methods struggle to account for the intricate quantum correlations between the particles produced in top quark collisions, effectively blurring the signals of potential new physics. These correlations, arising from the fundamental laws of quantum mechanics, introduce subtle dependencies that are not fully captured by standard analysis techniques. Consequently, current sensitivity is limited to detecting deviations from Standard Model predictions of only around 10%, meaning that more subtle – yet potentially crucial – effects remain hidden. Researchers are actively developing advanced statistical methods and theoretical frameworks to disentangle these correlations, aiming to sharpen the precision of measurements and open a window onto physics beyond the established model.

Contour plots of concurrence, the GQD measure, and a Bell variable reveal correlations between <span class="katex-eq" data-katex-display="false">CP</span>-odd and <span class="katex-eq" data-katex-display="false">CP</span>-even weak dipole couplings in <span class="katex-eq" data-katex-display="false">pp \to t\bar{t}</span> production at <span class="katex-eq" data-katex-display="false">\sqrt{s} = 13</span> TeV, analyzed in both a threshold (top) and intermediate mass (bottom) region for the <span class="katex-eq" data-katex-display="false">t\bar{t}</span> system. — Contour plots of concurrence, the GQD measure, and a Bell variable reveal correlations between $CP$ -odd and $CP$ -even weak dipole couplings in $pp \to t\bar{t}$ production at $\sqrt{s} = 13$ TeV, analyzed in both a threshold (top) and intermediate mass (bottom) region for the $t\bar{t}$ system.

Quantum Correlations as Probes of Fundamental Interactions

Quantum information theory offers a distinct approach to characterizing correlations beyond traditional methods used in particle physics. While classical correlation measures focus on statistical dependencies, quantum information theory leverages concepts like entanglement and quantum discord to identify and quantify non-classical correlations – those arising from the superposition and inherent uncertainty of quantum mechanics. Entanglement, a specific type of quantum correlation, describes a strong correlation between quantum systems where their fates are intertwined, even when spatially separated. Quantum discord, a broader measure, captures correlations even in the absence of entanglement. These tools allow for a more complete description of the relationships between particles, potentially revealing subtle effects not captured by classical analyses and offering new insights into the underlying physics of particle interactions. $\rho_{AB}$ represents a joint quantum state of two particles A and B, and is the basis for calculating these measures.

Concurrence and the Bell parameter are quantifiable metrics used to determine the degree to which a quantum system deviates from classical behavior. Concurrence, ranging from 0 to 1, specifically measures the entanglement present in a two-qubit system; a value of 1 indicates maximal entanglement, while 0 signifies a separable, classically-correlated state. The Bell parameter, derived from Bell inequalities, assesses the non-locality of a system; violations of these inequalities, indicated by values exceeding the classical limit, demonstrate the presence of non-classical correlations exceeding those permitted by local realism. Both metrics provide a numerical assessment of quantum non-classicality, enabling the characterization and comparison of quantum states based on their departure from classical descriptions.

Traditional analyses of top quark pairs primarily focus on spin correlations, which describe the alignment of the top and antitop quark spins. However, employing quantum information theory allows for the characterization of correlations beyond these spin effects, providing a more complete description of the quantum state of the top quark pair. This approach utilizes measures like Concurrence and the Bell Parameter to quantify non-classical correlations, including entanglement and discord, that are not captured by conventional spin analysis. Consequently, a richer understanding of the top quark pair’s quantum properties becomes accessible, potentially revealing subtle dependencies on underlying physics beyond the Standard Model.

The distributions of concurrence, geometric quantum discord, and the Bell variable are shown as functions of CP-even <span class="katex-eq" data-katex-display="false">\hat{\mu}_t</span> and CP-odd <span class="katex-eq" data-katex-display="false">\hat{d}_t</span> anomalous couplings for <span class="katex-eq" data-katex-display="false">pp \to t\bar{t}</span> events with leptonic decay, analyzed in four bins of <span class="katex-eq" data-katex-display="false">m_{t\bar{t}}</span> at a 13 TeV LHC. — The distributions of concurrence, geometric quantum discord, and the Bell variable are shown as functions of CP-even $\hat{\mu}_t$ and CP-odd $\hat{d}_t$ anomalous couplings for $pp \to t\bar{t}$ events with leptonic decay, analyzed in four bins of $m_{t\bar{t}}$ at a 13 TeV LHC.

The Standard Model Effective Field Theory: A Framework for Discovery

The Standard Model Effective Field Theory (SMEFT) provides a framework for analyzing potential new physics by systematically introducing higher-dimensional operators to the Standard Model Lagrangian. This approach avoids the need to specify a complete ultraviolet (UV) completion – the exact theory at very high energies – and instead focuses on the low-energy effects of such physics. By adding operators with increasing dimensionality, such as dimension-five or dimension-six, the SMEFT parameterizes all possible deviations from the Standard Model that are consistent with the underlying symmetries. The coefficients of these operators represent the strength of the new physics effects and can be constrained by experimental measurements, providing a model-independent way to search for physics beyond the Standard Model. This methodology allows physicists to analyze experimental data for subtle deviations, linking observed anomalies to specific operator coefficients and thus inferring properties of the underlying new physics.

Dimension six operators within the Standard Model Effective Field Theory (SMEFT) represent the lowest-order quantum effects of physics beyond the Standard Model. These operators, constructed from Standard Model fields and their derivatives, introduce interactions that are suppressed by a high energy scale Λ. The presence of these operators modifies Standard Model predictions, manifesting as deviations in observable quantities. The magnitude of these deviations is inversely proportional to $\Lambda^2$ , meaning that higher energy scales correspond to smaller effects, and vice versa. Consequently, precision measurements at lower energy scales, such as those achievable at the LHC or future colliders, provide sensitivity to new physics at much higher, currently inaccessible, energy scales. The SMEFT approach allows for a model-independent parameterization of these effects, avoiding the need to specify the details of the underlying new physics model.

Anomalous interactions, arising from dimension-six operators within the SMEFT, deviate from Standard Model predictions through modified couplings to gauge bosons and fermions. Specifically, Weak Dipole Moments (WDMs) represent interactions between fermions and gauge bosons that violate Lorentz invariance, while Chromo Dipole Moments (CDMs) exhibit similar deviations within the strong interaction sector. These anomalous magnetic and electric dipole moments, though typically suppressed in the Standard Model, can become observable effects at sufficiently high energies or through precision measurements at lower energies. Experimental searches for these moments, conducted through analyses of particle decays, scattering processes, and high-luminosity collider data, provide stringent constraints on the coefficients of the corresponding dimension-six operators and offer a pathway to indirectly detect new physics beyond the Standard Model.

The distribution of differences in the Generalized Quark-Dipole (GQD) <span class="katex-eq" data-katex-display="false">\Delta\mathcal{D}[\rho_{t\bar{t}}] </span> reveals sensitivity to anomalous weak dipole moments in <span class="katex-eq" data-katex-display="false">pp \to t\bar{t} \to b\bar{b}\ell^{+}\ell^{-}\nu_{\ell}\bar{\nu}_{\ell}</span> collisions at <span class="katex-eq" data-katex-display="false">\sqrt{s} = 13</span> TeV. — The distribution of differences in the Generalized Quark-Dipole (GQD) $\Delta\mathcal{D}[\rho_{t\bar{t}}]$ reveals sensitivity to anomalous weak dipole moments in $pp \to t\bar{t} \to b\bar{b}\ell^{+}\ell^{-}\nu_{\ell}\bar{\nu}_{\ell}$ collisions at $\sqrt{s} = 13$ TeV.

Amplifying Sensitivity with Optimized Analyses

The search for physics beyond the Standard Model benefits significantly from a multifaceted approach to analyzing top quark pair production. Investigations aren’t limited to a single kinematic regime; instead, both the Threshold Region, where top quarks are produced with low momentum, and the Boosted Region, characterized by highly energetic and collimated jets, offer unique avenues for discovery. The Threshold Region allows for precise measurements of top quark mass and couplings, acting as a sensitive probe of new interactions that subtly modify these fundamental parameters. Conversely, the Boosted Region, where individual jets are reconstructed from the decay products of highly relativistic top quarks, provides enhanced sensitivity to new, heavier particles that might couple to the top quark. By combining the complementary strengths of these two analyses, researchers maximize their ability to detect and characterize potential deviations from the Standard Model, effectively broadening the scope of the search for new physics.

A precise understanding of particle interactions demands a detailed examination of spin correlations, and utilizing the Helicity Basis provides a powerful framework for systematically achieving this. This approach focuses on the projection of particle spins along the direction of motion, simplifying the analysis of angular distributions and allowing researchers to isolate the effects of subtle new physics contributions. By decomposing interactions into distinct helicity states – where particles are either aligned or anti-aligned with their momentum – the analysis becomes more sensitive to deviations from the predictions of the Standard Model. This method not only enhances the precision of measurements, particularly in top quark pair production, but also facilitates a clearer interpretation of the underlying quantum mechanical processes governing these interactions, ultimately improving the ability to detect and characterize physics beyond current understanding.

The pursuit of increasingly precise measurements in particle physics demands innovation beyond conventional techniques, and quantum information tools offer a promising avenue for achieving this. Researchers are actively developing methods to harness quantum entanglement and superposition – phenomena where particles exist in multiple states simultaneously – to enhance the sensitivity of particle detectors and data analysis. These tools aren’t simply about building more powerful instruments; they focus on cleverly exploiting quantum correlations to tease out exceedingly faint signals obscured by noise. For instance, quantum-enhanced sensors could dramatically improve the detection of rare particle decays or subtle deviations from predicted interaction strengths. This approach moves beyond simply accumulating more data; it’s about extracting more information from the data already at hand, potentially revealing new physics currently beyond the reach of existing experiments and offering a deeper understanding of fundamental interactions.

Simulations of the <span class="katex-eq" data-katex-display="false">pp \to t\bar{t} \to l^{+}l^{-}b\bar{b}\nu_{l}\bar{\nu}_{l}</span> process at <span class="katex-eq" data-katex-display="false">\sqrt{s}=13</span> TeV reveal correlations between concurrence, geometric discord, and the Bell variable, and anomalous weak dipole moments at the parton level. — Simulations of the $pp \to t\bar{t} \to l^{+}l^{-}b\bar{b}\nu_{l}\bar{\nu}_{l}$ process at $\sqrt{s}=13$ TeV reveal correlations between concurrence, geometric discord, and the Bell variable, and anomalous weak dipole moments at the parton level.

Towards a Quantum-Enhanced View of Fundamental Interactions

The search for CP violation – a symmetry that, when broken, explains the matter-antimatter asymmetry in the universe – receives a promising new avenue through the investigation of CPOdd asymmetries and their connection to dimension six operators. While the Standard Model of particle physics accounts for some CP violation, it is insufficient to explain the observed abundance of matter. Exploring these higher-dimensional operators, which represent deviations from the Standard Model’s predictions, provides a framework for detecting new sources of CP violation. Specifically, measurements of CPOdd asymmetries – differences in the behavior of particles and their antiparticles – can reveal the presence of these operators and constrain their parameters. This approach offers a direct pathway to probing physics beyond the Standard Model, potentially uncovering new particles and interactions that contribute to CP violation and ultimately shed light on the origins of our matter-dominated universe.

The convergence of particle physics and quantum information science is fostering a potentially revolutionary shift in how physicists approach the study of fundamental interactions. Traditionally, particle physics has focused on identifying particles and their interactions through direct observation of collisions and decays. However, this emerging interdisciplinary field proposes that quantum correlations – entanglement and superposition – aren’t merely a byproduct of these interactions, but may be integral to defining them. Researchers are beginning to explore scenarios where these quantum phenomena manifest as subtle deviations from Standard Model predictions, offering a novel means of probing physics beyond current understanding. This approach suggests that a deeper, more complete picture of reality may require understanding not just what particles are, but how they are quantumly connected, potentially revealing hidden symmetries and forces that govern the universe at its most basic level.

The pursuit of a deeper understanding of the universe’s fundamental principles drives this research, extending beyond the mere cataloging of particles and forces. It seeks to resolve long-standing enigmas – the origin of matter-antimatter asymmetry, the nature of dark matter and dark energy, and the unification of quantum mechanics with general relativity. Through precision measurements and novel theoretical frameworks, scientists aim to identify the underlying laws governing reality, potentially revealing a more complete and elegant description of the cosmos than currently offered by the Standard Model. This endeavor is not simply about filling gaps in existing knowledge; it is about fundamentally reshaping humanity’s perception of its place within the universe and illuminating the deepest secrets of existence, pushing the boundaries of what is known and venturing into the realm of the truly unknown.

The pursuit of understanding top quark pair production, as detailed in this study, echoes a fundamental principle of elegant design: simplicity revealing complexity. This research demonstrates how subtle quantum correlations-concurrence, quantum discord, and the Bell parameter-can serve as sensitive probes for physics beyond the Standard Model. It suggests that a seemingly straightforward system, like top quark interactions, possesses an intricate structure susceptible to the influence of new physics. As Albert Einstein observed, “Everything should be made as simple as possible, but not simpler.” The investigation into these quantum information measures isn’t merely about adding layers of complexity; rather, it’s about identifying the essential elements that betray the underlying structure, revealing the potential for CP violation and new interactions with remarkable precision.

Where Do We Go From Here?

The pursuit of precision in top quark pair production, framed through the lens of quantum information, reveals a subtle, yet insistent, challenge. The current work demonstrates the potential sensitivity of quantum correlations to new physics, but potential remains a fragile state. The true test lies not merely in detecting a signal, but in disentangling it from the inherent complexities of QCD and the limitations of our current theoretical approximations. The SMEFT framework, while elegant, is ultimately a truncation – a map that is not the territory. Future investigations must rigorously address the impact of higher-order operators and the interplay between them.

A persistent question arises: are these quantum information measures merely sensitive probes of conventional CP violation, or do they offer a pathway to uncover entirely new sources of T-odd effects? The answer likely resides in a deeper understanding of the spin structure of the top quark and the correlations between its decay products. Furthermore, the computational demands of fully simulating these quantum phenomena within a realistic collider environment present a significant hurdle. A symbiotic relationship between theoretical development and algorithmic innovation will be essential.

Ultimately, this line of inquiry forces a re-evaluation of the relationship between quantum mechanics and particle physics. It suggests that the most profound signatures of new physics may not reside in the discovery of new particles, but in the subtle distortions of quantum entanglement and correlation – a whisper in the noise, discernible only through a refined theoretical and experimental apparatus. The elegance of the system demands nothing less.

Original article: https://arxiv.org/pdf/2605.12033.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/