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. 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 Standard Model of particle physics describes the interactions between the elementary building blocks of matter: quarks and leptons.
Six quark flavors, color charge and [1]
Top quark discovery
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 generically produced in high-energy particle collisions at accelerators such as 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 an outsized influence on how masses are generated for fundamental particles. 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 test of the Standard Model and a search tool for new physics, such as supersymmetry or exotic Higgs sectors[4][6].
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, 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 liftetime of elementary (and a few composite) particles known:
Top quark decay
Top quark interactions
Top quarks with a boost
Top quarks and Quantum Entanglement
Outlook
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
