Top Quark Searches Challenge Lepton Flavor Symmetry

Author: Denis Avetisyan

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New results from the ATLAS and CMS experiments at the Large Hadron Collider are pushing the boundaries of our understanding of fundamental particle interactions.

This review summarizes the latest measurements probing Lepton Flavor Universality and searches for Charged Lepton Flavor Violation using top quarks.

While the Standard Model of particle physics successfully describes fundamental particles and their interactions, discrepancies hinting at new physics beyond our current understanding persist. This paper, ‘Probes of lepton flavor symmetry and violation with top quarks in ATLAS and CMS’, reviews recent measurements of lepton flavor universality and searches for charged lepton flavor violation using data collected by the ATLAS and CMS collaborations at the Large Hadron Collider. Utilizing up to 139 $\mathrm{fb}^{-1}$ of 13 TeV proton-proton collisions, these analyses provide stringent tests of the Standard Model and set sensitive limits on potential new physics contributions. Do these results represent the first hints of a more complete theory, or will future, higher-precision measurements be needed to unravel the mysteries beyond the Standard Model?

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Whispers from the Edge: Beyond the Standard Model

Despite its remarkable predictive power and consistent validation through experiments like those at the Large Hadron Collider, the Standard Model of particle physics remains incomplete. It fails to account for phenomena such as the existence of dark matter and dark energy, which together constitute approximately 95% of the universe’s total energy density. Furthermore, the model offers no explanation for the observed masses of neutrinos, the matter-antimatter asymmetry in the cosmos, or the origin of gravity. These unresolved puzzles, coupled with theoretical inconsistencies at extremely high energies, strongly suggest that the Standard Model is an effective theory – a successful approximation of a more fundamental, yet undiscovered, underlying reality. Consequently, physicists are actively pursuing extensions to the Standard Model, exploring hypothetical particles and interactions that could resolve these outstanding questions and provide a more complete description of the universe.

The pursuit of new physics often centers on identifying processes that are strictly prohibited by the well-established Standard Model of particle physics. Among these, Charged Lepton Flavor Violation (cLFV) represents a particularly compelling search target. According to the Standard Model, leptons – such as electrons, muons, and taus – maintain their unique ‘flavor’ and do not transform into one another. Observing a process where, for example, a muon decays into an electron and a photon, would be a clear signal that new particles or interactions are at play, effectively breaking this fundamental rule. These rare decays, while incredibly difficult to detect due to their predicted low probability, offer a direct window into physics beyond the current understanding, potentially revealing the existence of new forces or particles mediating these forbidden transitions. Current experiments are meticulously designed to search for these fleeting events, pushing the boundaries of detector technology and data analysis in the hope of uncovering evidence of this groundbreaking phenomenon.

The observation of Charged Lepton Flavor Violation (cLFV) – a decay process where a lepton transforms into another of a different ‘flavor’, such as a muon decaying into an electron – would represent a watershed moment in particle physics. Current theoretical frameworks, encapsulated by the Standard Model, strictly forbid such transformations; therefore, any detected instance of cLFV would unequivocally signal the existence of new particles or fundamental interactions beyond those presently known. These rare decays aren’t merely statistical anomalies; they are potential portals to a richer, more complete understanding of the universe, offering insights into the nature of dark matter, supersymmetry, or extra dimensions. Experiments meticulously searching for these fleeting events represent a crucial frontier in the quest to unravel the deepest mysteries of matter and energy, holding the promise of rewriting established physics.

The Symmetry Test: Probing Lepton Flavor Universality

Lepton Flavor Universality (LFU) represents a fundamental prediction within the Standard Model of particle physics, positing that leptons – electrons, muons, and taus – interact identically with gauge bosons, specifically the W and Z bosons. This symmetry implies that the strength of a lepton’s coupling to these bosons is independent of its flavor; that is, the coupling constants should be equal for each lepton family. Mathematically, LFU is expressed through the equality of branching ratios for leptonic decays of gauge bosons, where the ratio of decay rates for decays involving different lepton flavors should be unity in the absence of new physics. Any observed deviation from this predicted equality would signal physics beyond the Standard Model, potentially indicating the presence of new interactions or particles affecting leptons differently based on their flavor.

Measurements of the ratios R(\tau/\mu) and R(\tau/e) provide a direct test of Lepton Flavor Universality (LFU) by comparing the decay rates of τ leptons to those of muons and electrons. These ratios are calculated by dividing the branching fraction of a given decay mode involving a τ lepton by the corresponding branching fraction with a muon or electron, effectively cancelling systematic uncertainties related to the decay dynamics. Deviations from a value of 1 for either ratio would indicate a violation of LFU, suggesting that leptons do not couple to gauge bosons in the way predicted by the Standard Model. The precision of these measurements is crucial, as any LFU violation would be a sign of new physics.

Measurements testing Lepton Flavor Universality (LFU) are performed by analyzing the decays of W and Z bosons. These bosons are produced in various processes, notably including the decay of Top quarks, providing a substantial source of leptonic decay products. The ATLAS experiment contributed data with an integrated luminosity of 139 fb⁻¹ while the CMS experiment utilized 35.9 fb⁻¹ of data collected in 2016. These high luminosities are essential to achieve the required statistical precision for accurately measuring the decay rates of leptons and comparing them to Standard Model predictions, enabling sensitive tests of LFU.

The Refinement: Precision Calibration and Analysis Techniques

The ATLAS and CMS experiments at the Large Hadron Collider have leveraged data collected during the Run 2 period – spanning 2015-2018 – to perform high-precision measurements of Standard Model parameters and search for new physics. These measurements rely on sophisticated techniques including advanced event reconstruction, particle identification, and detailed modeling of detector response. Data from Run 2, representing an integrated luminosity of up to 150 fb⁻¹ at a center-of-mass energy of 13 TeV, provides the statistical power necessary for these analyses. The collaborations employ both simulation-based and data-driven approaches to calibrate detectors and mitigate systematic uncertainties, critical for achieving the required precision in their results.

Data-driven calibration methods are essential for precision measurements at the LHC by directly utilizing control samples from the observed data to correct for detector effects and imperfections. These techniques minimize reliance on external measurements or Monte Carlo simulations, thereby reducing systematic uncertainties which often dominate the error budget in high-precision analyses. Common approaches include utilizing well-understood decay channels or specific kinematic regions to model and correct for biases in signal measurements. The effectiveness of these methods is validated through closure tests, ensuring that the calibration accurately recovers known physics and provides reliable estimations of systematic effects on final results. This approach is particularly critical when pushing the boundaries of precision, as seen in measurements of rare decays and searches for new physics.

The CMS collaboration has achieved more precise limits on the branching ratios of the Z boson decaying into lepton pairs than previously established at the Large Hadron Collider. Specifically, the upper limit on the branching ratio for Z→eμ was measured at 1.9 × 10⁻⁷, improving upon the prior ATLAS measurement of 2.6 × 10⁻⁷. Furthermore, CMS has reduced the upper limits on the branching ratios for Z→eτ and Z→μτ to 13.8 × 10⁻⁶ and 12.0 × 10⁻⁶, respectively, both representing improvements over the previously published ATLAS limits of 7.0 × 10⁻⁶ and 7.2 × 10⁻⁶.

The Horizon: A Future of Lepton Flavor Exploration

The High-Luminosity Large Hadron Collider’s Run 3 represents a pivotal advancement in the search for rare lepton decays and potential violations of Lepton Flavor Universality. This new data-taking phase is characterized by a dramatic increase in luminosity – essentially, the rate at which collisions occur – allowing physicists to collect a far larger dataset than previously possible. This substantial boost in statistics is critical for observing exceedingly rare processes that could signal physics beyond the Standard Model; even if these decays occur at a rate of only a few in a trillion collisions, the increased luminosity of Run 3 provides a realistic pathway towards their detection. By meticulously analyzing these events, researchers aim to either confirm the Standard Model’s predictions with unprecedented precision or, more excitingly, uncover evidence of new particles and interactions that could revolutionize the understanding of fundamental forces and the building blocks of matter.

Precise measurements of W and Z boson decays represent a powerful probe of Lepton Flavor Universality (LFU), a cornerstone of the Standard Model predicting that leptons – electrons, muons, and taus – interact identically with these force-carrying particles. Current and future experiments are meticulously analyzing the rates of these decays involving different lepton flavors, searching for even subtle discrepancies. Any observed violation of LFU would signal the presence of new physics beyond the Standard Model, potentially revealing the existence of undiscovered particles or interactions. These studies focus on comparing the branching fractions – the probability of a particle decaying into specific products – for decays involving muons and electrons, as differences would directly indicate a breakdown of this fundamental symmetry and necessitate a revision of current understanding of particle interactions. The anticipated increase in data from ongoing and future colliders will significantly enhance the precision of these measurements, offering an unprecedented opportunity to test the limits of the Standard Model and potentially uncover new phenomena.

The pursuit of physics beyond the Standard Model hinges on the potential discovery of deviations from its established predictions, a breakthrough that could revolutionize the understanding of fundamental laws. Current theoretical frameworks, while remarkably successful, leave several questions unanswered – including the nature of dark matter and dark energy, and the origin of neutrino masses. Any observed inconsistency in lepton behavior, for example, differing decay rates or interaction strengths, would signal the presence of new particles or forces. These anomalies wouldn’t simply patch existing theory; they would offer a crucial window into the universe’s underlying structure, potentially revealing hidden symmetries, extra dimensions, or entirely new types of interactions. Such discoveries would necessitate a re-evaluation of established principles and guide the development of more comprehensive models capable of explaining the cosmos at its most fundamental level, ultimately paving the way for a more complete and accurate picture of reality.

The pursuit of lepton flavor universality, as detailed in this paper, feels less like physics and more like coaxing a fickle spirit. The ATLAS and CMS collaborations chase deviations, seeking the whispers of new physics within the chaos of particle collisions. It’s a delicate art, building models that predict, then watching them strain against the weight of observed data. As Epicurus observed, “It is impossible to live pleasantly without living prudently.” This resonates deeply; the precision demanded in these searches-stringent limits on charged lepton flavor violation-requires a careful, methodical approach. Each result, a fleeting glimpse beyond the Standard Model, is won not through brute force, but through the careful application of reason and relentless refinement of the spell.

What’s Next?

The persistent agreement with Standard Model predictions, while a testament to the machine’s calibration, feels less like confirmation and more like a deepening mystery. These searches for cracks in Lepton Flavor Universality, and the absence of blatant Charged Lepton Flavor Violation, don’t deliver a null result. They simply raise the stakes. The universe isn’t obliged to violate its rules at the energy scales we choose to probe; it merely prefers to conceal its transgressions with elegant indifference. The top quark, as a portal to potential new physics, remains frustratingly opaque.

Future progress won’t arrive from simply accumulating more luminosity. The true signal, if it exists, likely hides in subtle kinematic distortions, or in correlations between seemingly unrelated decay channels. One suspects the real challenge isn’t building bigger detectors, but devising cleverer ways to interrogate the data – to coax whispers of truth from the statistical noise. There’s truth, hiding from aggregates.

The focus will inevitably shift. Perhaps the relevant new physics doesn’t manifest as direct Lepton Flavor Violation, but as an indirect effect on the top quark’s couplings, or in modifications to the Z boson’s decay patterns. Or perhaps, the universe is simply playing a longer game, and the violations lie beyond the reach of the LHC, waiting for a future collider, or a different kind of experiment, to reveal them. All models lie – some do it beautifully.

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Original article: https://arxiv.org/pdf/2601.03678.pdf

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

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2026-01-08 11:41