Properties of the top quark
The top quark, the heaviest elementary particle known to date, was discovered by the CDF and D0 collaborations in 1995. In more than 25 years since the discovery the properties of the top quark have been studied in detail.
Introduction
The top quark is one of the six flavors of quarks in the Standard Model of particle physics, which describes the fundamental building blocks of matter and interactions between them. With its remarkably high mass and extremely short lifetime, the top quark is unique among elementary particles and serves as a vital probe into the deepest functioning of nature’s fundamental forces. Since its discovery in 1995, the top quark has become a central focus of both theoretical and experimental particle physics, providing crucial insights into the Standard Model, the mechanism of mass generation by the Higgs boson, and potential glimpses of new physics beyond our current theories.
The existence of the top quark was predicted long before its discovery, completing the third generation of the Standard Model’s quark families. Its remarkably high mass meant that its direct observation required unprecedented energies. It was finally discovered by the CDF and DØ collaborations at Fermilab’s Tevatron collider in 1995, after an extended search[1,2]. This milestone confirmed the Standard Model’s structure and set the stage for a new era of high-energy particle physics, especially in the study of electroweak symmetry breaking.
Fundamental Properties
The top quark is an “up-type” quark, sharing quantum numbers with the up and charm quarks but possessing entirely different physical characteristics due to its mass:
- Spin: ½ (fermion)
- Electric charge: +2/3 e
- Color charge
- Mass: 172.52±0.33 GeV/c²
- Lifetime: about 5×10⁻²⁵ seconds
Its mass is the largest of any known elementary particle, nearly the same mass as a tungsten atom, and 40 times heavier than the bottom quark.
Its lifetime is so short that the top typically decays before combining with other quarks to form hadrons. This ultra-brief lifetime means the top quark is observed directly as a “bare” quark, unlike all other quark types, which are seen only inside composite particles.
The very short lifetime of the top quark is related to its high mass value. It is the only quark that has a mass higher than the W boson (with a mass of approximately 80.4 GeV/c²), the force carrier of the Weak nuclear force. This means that it is energetically favorable for the top quark to decay to a W boson and a bottom quark. Thus, the "weak" interaction effectively becomes relatively strong for the top quark, leading to a short decay time.
Like other quarks, the top quark carries a color charge, which means that it can couple to the Strong nuclear force. However, due to the short lifetime of the top quark, the effects of the Strong force are very different for the top quark than for other quarks. The Strong force has the peculiar property that its effect becomes stronger at larger distances between color force carriers. This leads to asymptotic freedom of quarks at very short distances, and color confinement at larger distances. Because of this, other quarks form tightly bound states and cannot be observed as free particles. The top quark is unique in this respect: its lifetime is too short to experience the effects of color confinement, as the top quark decays before it can travel a distance at which the Strong force becomes strong, even if traveling fast, at relativistic velocities. The absence of screening effects by the Strong force allows unique experimental access to the top quark's properties, and indirectly to fundamental parameters and symmetries that are reflected in the top quark's interactions with other particles.
The only top quark decay mode that has been observed experimentally to date is to a W boson and a bottom quark, although decays to other bottom-type quarks (down and strange) are also allowed according to the Standard Model. The relative probabilities for the top quark to decay to different types of quarks are defined by the CKM matrix. The predicted fraction of decays to a W boson and a bottom quark is 99.8%, leaving 0.2% for the decays to down and charm quarks. The latter decay modes have however not yet been observed.
Production and Decay
Top quarks are copiously produced in high-energy particle collisions at the Large Hadron Collider (LHC) through the strong interaction (via gluon-gluon or quark-antiquark fusion)[5][6]. Most are created in pairs (top and antitop), though single top quark production also occurs via the electroweak force.
After formation, a top quark rapidly decays, typically producing a W boson and a bottom quark. This process is so quick (around 5×10⁻²⁵ s) that the top does not participate in hadronization. The W boson also has a very short life time (around 3x10⁻²⁵ s) and either decays hadronically to a quark and anti-quark (66.7%), or leptonically to a lepton and a neutrino (33.3%) The W boson can decay. This leads to the presence of characteristic decay products —jets from bottom quarks and leptons/neutrinos or jets from W decays-, enabling experimental reconstruction of top events, despite the fleeting existence of the top quark [6]. In the case of a top quark pair, this leads to three possible final states that can be reconstructed experimentally:
- the fully leptonic channel (approximately 11%)
- the semi-leptonic (or lepton+jets) channel (approximately 45%)
- the fully hadronic (or all-jets) channel (approximately 45%)
While the fully leptonic channel has the lowest abundance, the presence of the leptons from the W decays from both top quarks give these events a striking signature that easy to trigger on, and to distinguish from other physics background processes.
In the case of single top quark production, only the leptonic decay channel has been employed in experimental measurements to date.
The production of top quarks has been measured with great precision at the Tevatron and LHC experiments and agrees very well with theoretical calculations.
Plots of LHC Top Working Group of ttbar production: tt_production
single top: single_top_production single_top_ratio
tt+X ttX_production
t+X tX_production
tttt: tttt_production
Importance in the Standard Model
The top quark’s immense mass is directly tied to a very strong coupling with the Higgs boson (the “Yukawa coupling”), nearly equal to one—the strongest such interaction among known particles[3][5]. This coupling means the top quark has a unique relation to the mechanism that generates masses for fundamental particles, and it can strongly affect the effective masses of other particles through quantum loop effects. Through quantum loop effects, the top quark affects precision measurements of Standard Model parameters (like the W boson mass) and even contributes to the stability of the Higgs boson and the broader nature of the Higgs field’s vacuum. As a result, measuring top quark properties to high precision has become a critical ingredient in pinpointing the foundation of the Standard Model and potentially a powerful search tool for new physics, such as supersymmetry or exotic Higgs sectors[4][6].
Experimental Studies and Results
With the advent of the LHC, top quark research entered a new era of precision. Collaborations such as ATLAS and CMS have produced huge numbers of top quarks—allowing measurement of heir mass, spin correlations, production rates, and interaction strengths with ever-increasing accuracy.
Recent major achievements include:
- Production cross-section & mass precision: The most up-to-date measurements place the top quark mass at 172.5±0.3 GeV/c², with production cross-sections measured in both top pair and single top events spanning proton-proton and heavy-ion collisions[7][6][8].
- Rare processes: Top quark pairs have now been observed produced together with Higgs, Z, or W bosons—allowing study of the corresponding couplings and searching for deviations from the Standard Model[5][9].
- Rare decays: various searches for anomalous decays modes have been performed, such as :::
- Quantum phenomena: Detailed studies have shown evidence of top quark spin correlations and even quantum entanglement effects, as manifest in the distributions of decay products[6].
- Toponium: A landmark recent result has been the possible observation of toponium—a short-lived bound state of a top quark and its antiquark, something once thought impossible given the top’s short lifetime[10][11][12][13]. The CMS and ATLAS collaborations observed an excess of top quark-antiquark pairs that could indicate the formation of this bound state, supported by statistical significance far above random fluctuations.
These ongoing results not only continually test the Standard Model but also provide windows to potential new particles and interactions.
Unique Aspects and Physics Insights
What sets the top quark apart is its “naked” identity—unlike all other quarks, its extremely rapid decay means it is observed directly, not shielded by hadronic binding. This enables precise study of parameters such as spin, charge, and mass, uncontaminated by strong interaction effects that cloud measurements in other quark types[3][6].
The top quark also plays a significant role in testing Quantum Chromodynamics (QCD) at high energies and the interplay of strong and electroweak forces. Studies of its production, decay, and quantum properties illuminate deeper symmetries and may reveal new phenomena such as CP violation, plus potential new particles at energy scales probed by current and future colliders[4][6].
One area of active research is the behavior of top quarks in heavy-ion collisions—such as lead-lead collisions at the LHC—where effects like color screening and collective excitation can be explored[14].
The rarest process involving top quarks studied at the LHC so far is the simultaneous production of two top quarks and two top antiquarks, referred to as ‘four top production’. This is expected to be around 70,000 times rarer than top-pair production, yet ATLAS and CMS have recently seen first hints of such events using their full Run 2 dataset. The analyses give observed numbers of events about twice the expected rate, but still compatible with the Standard Model prediction within uncertainties. Many theories proposing new particles or interactions beyond the Standard Model could give rise to such an enhancement. This just maybe a first tantalising hint of excitement to come, but Run 3 data will be needed to know for sure.
Top quark mass
The mass of the top quark is a free parameter of the Standard Model, and its value is not predicted directly by the theory. The exact value of its mass, however, has big consequences for how the top quark manifests itself in direct experimental observations and in the way it affects other particles and processes.
Modern top quark mass determinations employ three primary experimental strategies, each addressing different theoretical and practical challenges. Direct measurements reconstruct the mass from the invariant mass of top quark decay products, utilizing techniques that fit the kinematic properties of the events to extract mass values, that are corrected using detailed Monte Carlo simulations. These corrections are needed to account for subtle experimental effects that include detector efficiencies and energy reconstruction biases that originate from detector calibration as well as reconstruction algorithms implemented in the analysis software. These measurements achieve exceptional precision, with the most recent CMS result yielding 171.77 ± 0.38 GeV, representing a relative precision of approximately 0.22%.
Indirect measurements extract the top mass by comparing measured production cross sections with theoretical predictions for different mass values. This approach provides a theoretically cleaner connection to well-defined field theory mass parameters, though typically with reduced precision compared to direct methods.
Subtle theoretical effects on the observed masses from Quantum Chromo Dynamics effects are complex and hard to calculate. Experts have differing views on how to best treat these effects that are important for the theoretical interpretation of top quark mass measurements at a level of precision below about 0.2 - 0.3%. Using different techniques and methods to measure the top mass quark in complementary approaches has been the goal of the LHC experiments. Overall results are consistent with each other, boosting confidence in the obtained results.
A new category of measurements focuses on boosted top quarks produced with high transverse momentum, where decay products of each top quark are collimated into single jets of particles. Recent ATLAS measurements using this technique achieved 172.95 ± 0.53 GeV, demonstrating the method's potential as experimental precision improves with increased luminosity at the LHC. Earlier measurements by the CMS experiment evolved 10-fold in measurement precision within the time span of several years, profiting from improving analysis methods and increasing data samples. This clearly illustrates the future potential of boosted top measurements with the large increase in luminosity expected from the High-Luminosity LHC.
The most precise determination comes from the first combined ATLAS+CMS top quark mass analysis, which took into account 15 individual measurements from LHC Run-1 data. This landmark collaboration achieved 172.52 ± 0.14 (stat.) ± 0.30 (syst.) GeV, with a total uncertainty of 0.33 GeV, representing a 31% improvement over the most precise input measurement.
This makes the top quark the heaviest point-like particle known, even surpassing the Higgs boson, W and Z bosons. Compared to other fermions (quarks, charged leptons, neutrinos), the difference in mass is even more striking. The second-heaviest quark is the bottom quark, with a mass of 4.18+-0.04 GeV/c2.
Current measurements are dominated by systematic uncertainties, particularly jet energy scale calibrations and theoretical modeling uncertainties. Statistical uncertainties have been reduced below 300 MeV with current datasets[19].
Future improvements are expected from several directions. The High-Luminosity LHC will provide substantially larger datasets, potentially enabling systematic uncertainties to reach 200-600 MeV. Boosted top quark measurements show particular promise, with precision improvements by factors of 3-10 already demonstrated. Theoretical advances in understanding the relationship between the Monte Carlo simulation and field theory masses will be crucial for ultimate precision.
The following plot (credit: tikz.org) compares mass and lifetime of elementary (and a few composite) particles known:
Top quarks with a boost
Top quarks and Quantum Entanglement
In the production of top quark pairs (\( t\bar{t} \)), the top quark and its antiparticle can be created in an entangled state. This entanglement manifests in correlations between their spins and other quantum numbers. For instance, if one of the top quarks is measured to have a particular spin, the corresponding antitop quark will have a complementary spin state due to conservation of angular momentum.
The entangled state of the top quark pairs can be probed experimentally through various decay channels and final states. The measurement of these correlations provides a unique opportunity to test the predictions of quantum mechanics and understand the underlying principles of particle interactions.
The study of top quark entanglement has several implications:
1. Testing Quantum Mechanics: Investigating the entangled states of top quarks provides a testbed for the principles of quantum mechanics, particularly in understanding how entanglement behaves in high-energy environments.
2. Exploring Nonlocality: Top quark entanglement can shed light on the nonlocal nature of quantum mechanics, where the properties of entangled particles are linked regardless of the distance separating them. This can lead to insights into the fundamental nature of reality.
3. Quantum Computing and Information: The entangled states of top quarks may have applications in quantum computing and quantum information theory. Understanding how to harness entanglement in particle interactions could contribute to advancements in quantum technologies.
4. Connection to Cosmology: Entangled states of top quarks may have implications for understanding early universe phenomena, particularly during rapid expansion phases where quantum effects could influence the evolution of matter.
The exploration of entanglement in top quark production and decay represents an exciting frontier in both theoretical and experimental physics, with potential connections to broader questions about the foundations of quantum mechanics.
Future Outlook
Top quark physics stands at the forefront of particle research. The next decade will see increased precision in mass, coupling, and decay property measurements, benefiting from detector upgrades and larger datasets at the HL-LHC and proposed future colliders.
Focus areas include:
- Improving constraints on Higgs-top couplings and searching for new interactions. - Measuring rare decay channels and searching for evidence of new particles. - Investigating the top quark’s role in the stability of the universe’s vacuum and the Higgs sector. - Using machine learning and advanced analysis techniques to improve extraction of subtle quantum and collective effects.
Conferences such as the annual International Workshops on Top Quark Physics [] bring together theorists and experimentalists to chart the path ahead. The future remains bright—as the top quark, with its heavy mass and unusual quantum behavior, continues to be a guide to new realms of understanding[6].
Twenty-five years after the discovery of the top quark, we now know a great deal about this super-heavy fundamental particle. As far as we can see, it looks like a quark, it swims like a quark, and quacks like a quark. And yet – it does not fit the pattern. It is so much heavier than all the other quarks, and seems to sit more easily among the heavy bosons of the electroweak sector of the particle zoo. Is this a coincidence? Trying to understand patterns has been a powerful technique in our exploration of Nature thus far, and has brought us the periodic table, our understanding of hadrons, and much more. But when something does not fit the pattern, this is often a hint that something new is lurking around the corner. We have not found it yet – but we will keep looking!
References
1. F.Abe et al., Phys.Rev.Lett. 74, 2626 (1995)
2. S.Abachi et al., Phys. Rev. Lett. 74, 2632 (1995)
3. PDG review of top quark
4. CERN 'evidence' of top quark [1]
5. The Last Quark | ATLAS Experiment at CERN https://atlas.cern/updates/feature/top-quark
6. [PDF] Physics of the Top Quark at the LHC: An Appraisal and Outlook of ... https://cds.cern.ch/record/2891535/files/ferreira-da-silva-2023-physics-of-the-top-quark-at-the-lhc-an-appraisal-and-outlook-of-the-road-ahead.pdf
= = =
[1] Top quark - Wikipedia https://en.wikipedia.org/wiki/Top_quark
[2] Properties of the top quark - Scholarpedia http://www.scholarpedia.org/article/Properties_of_the_top_quark
[3] [PDF] 61. Top Quark - Particle Data Group https://pdg.lbl.gov/2023/reviews/rpp2023-rev-top-quark.pdf
[4] [PDF] 61. Top Quark - Particle Data Group https://pdg.lbl.gov/2024/reviews/rpp2024-rev-top-quark.pdf
[5] The Last Quark | ATLAS Experiment at CERN https://atlas.cern/updates/feature/top-quark
[6] [PDF] Physics of the Top Quark at the LHC: An Appraisal and Outlook of ... https://cds.cern.ch/record/2891535/files/ferreira-da-silva-2023-physics-of-the-top-quark-at-the-lhc-an-appraisal-and-outlook-of-the-road-ahead.pdf
[7] CMS Top Quark Physics Summary Figures - Twiki - CERN https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsTOPSummaryFigures
[8] [2403.01313] Review of top quark mass measurements in CMS - arXiv https://arxiv.org/abs/2403.01313
[9] Decoding top quarks with precision - CERN https://home.cern/news/news/physics/decoding-top-quarks-precision
[10] Do top quarks combine for a fleeting moment to form a new particle ... https://desy.de/desy_latest_news/2025/toponium/index_eng.html
[11] Elusive romance of top-quark pairs observed at the LHC - CERN https://home.cern/news/press-release/physics/elusive-romance-top-quark-pairs-observed-lhc
[12] Elusive romance of top-quark pairs observed at the LHC - CERN https://home.web.cern.ch/news/press-release/physics/elusive-romance-top-quark-pairs-observed-lhc
[13] Bound to be discovered? ATLAS explores top-quark interactions ... https://atlas.cern/Updates/Briefing/Quasi-Bound-Tops
[14] Top quarks spotted at mega-detector could reveal clues to ... - Nature https://www.nature.com/articles/d41586-025-01075-2
[15] 18th International Workshop on Top Quark Physics (TOP2025) https://indico.cern.ch/event/1501204/
[16] TopPublicResults < AtlasPublic < TWiki - CERN https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResults
[17] Shiny top quarks: what happens when top quarks emit light? https://cms.cern/news/shiny-top-quarks-what-happens-when-top-quarks-emit-light
[18] [PDF] Properties of the Top Quark - Fermilab | Technical Publications https://lss.fnal.gov/archive/habil/fermilab-habil-2009-01.pdf
[19] [PDF] Top Quark Physics at the LHC https://hep.physics.utoronto.ca/PekkaSinervo/talks/tsi2009/TSITopLectures.pdf
[20] [PDF] Recent results on top quark mass and properties and rare ... https://cds.cern.ch/record/2826232/files/document.pdf
Recommended Reading
See Also
1. Symmetry Magazine (2007) Secrets of a Heavyweight
2. M. Cristinziani, M. Mulders (2016) Top-quark physics at the Large Hadron Collider
3. F. Deliot et al in Annual Review of Nuclear and Particle Science Properties of the Top Quark
4. Special Issue Universe (2023) Top Quark at the New Physics Frontier
5. P. Silva Review, Outlook and Road ahead
