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.
Six quark flavors, see color charge and [1]
History and Discovery
The existence of the top quark was predicted long before its discovery, filling out 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[3]. 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. It has [3]:
- Spin: ½ (fermion)
- Electric charge: +2/3 e
- Mass: ≈172.76±0.3 GeV/c², the largest of any known elementary particle, nearly the same mass as a tungsten atom, and 40 times heavier than the bottom quark.
- Lifetime: About 5×10⁻²⁵ seconds — 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.
A top quark almost always decays via the weak force into a W boson and a bottom quark. Its properties, especially its mass and lifetime, allow unique experimental access to fundamental parameters and symmetries.
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 presence of characteristic decay products —jets from bottom quarks and leptons/neutrinos from W decays- enables experimental reconstruction of top events, despite the fleeting existence of the top quark [6].
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 ingreadient 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].
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.
With an observed of 172.5 +- 0.3 GeV [] it is the heaviest point-like particle known, even surpassing the Higgs boson (125.x +-y GeV), 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 XX+-YY GeV. As the mass is higher than the W boson mass + b quark mass combined, the top quark decay is able to decay to an on-shell W boson and a b-quark. It does so 99.x % of the time, with a very short lifetime of XX. 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
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].
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 [2]
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
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
