# Quark

A quark is a fundamental particle of matter that carries fractional electric charge, participates in the strong interaction, and combines with other quarks to form composite particles called hadrons, including protons and neutrons. Six flavors of quark are known, arranged in three generations, and every hadron yet observed is built from them.<sup>[1](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf)</sup><sup> • </sup><sup>[2](https://home.cern/science/physics/standard-model/)</sup>

| Key fact | Detail |
|---|---|
| Flavors and generations | Six flavors in three generations: up and down (first), charm and strange (second), top and bottom or beauty (third)<sup>[2](https://home.cern/science/physics/standard-model/)</sup> |
| Electric charge | Up-type quarks carry +2/3 e, down-type quarks carry −1/3 e; every known charged system has charge in integer multiples of 1/3 e<sup>[1](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf)</sup><sup> • </sup><sup>[3](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-free-quark-searches.pdf)</sup> |
| Other quantum numbers | Spin 1/2 fermions with baryon number B = 1/3 (antiquarks −1/3); flavors include isospin, strangeness, charm, bottomness, and topness<sup>[1](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf)</sup> |
| Masses (PDG 2024) | \( m_u = 2.16 \pm 0.07 \) MeV, \( m_d = 4.70 \pm 0.07 \) MeV, \( m_s = 93.5 \pm 0.8 \) MeV, \( m_c = 1.2730 \pm 0.0046 \) GeV, \( m_b = 4.183 \pm 0.007 \) GeV, \( m_t = 172.57 \pm 0.29 \) GeV (from event kinematics)<sup>[4](https://pdg.lbl.gov/2024/download/db2024.pdf)</sup> |
| Confinement | Quarks are not observed as free particles; they are confined inside hadrons or deconfined in quark-gluon plasma, and experiments have produced no evidence for free quarks<sup>[3](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-free-quark-searches.pdf)</sup> |
| Quark-gluon plasma | Recreated in heavy-ion collisions at the LHC at temperatures over 100,000 times hotter than the center of the Sun; the universe was in this state for its first few microseconds<sup>[5](https://home.cern/quark-matter-2017-understanding-early-universe/)</sup><sup> • </sup><sup>[6](https://home.cern/science/physics/heavy-ions-and-quark-gluon-plasma/)</sup> |
| Mass of ordinary matter | Most of the proton's mass comes from the gluon field binding the quarks, not from the quark masses themselves<sup>[7](https://spinquest.fnal.gov/)</sup> |

## Flavors and quantum numbers

Quarks are strongly interacting fermions with spin 1/2 and, by convention, positive parity; antiquarks have negative parity and opposite flavor quantum numbers.<sup>[1](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf)</sup> Each quark carries baryon number 1/3, so three quarks make a baryon such as the proton or neutron, while a quark and an antiquark make a meson.<sup>[1](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf)</sup><sup> • </sup><sup>[8](https://home.cern/lhcb-collaboration-discovers-new-proton-particle/)</sup> The additive quantum numbers are related to electric charge through the generalized Gell-Mann–Nishijima formula, and by convention each flavor quantum number has the same sign as the quark's charge.<sup>[1](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf)</sup>

The two lightest quarks, up and down, make up all stable matter: a proton has the quark content uud and a neutron udd.<sup>[2](https://home.cern/science/physics/standard-model/)</sup><sup> • </sup><sup>[9](https://conferences.fnal.gov/lp2003/forthepublic/qcd/)</sup> The down quark is slightly heavier than the up quark, which explains why the neutron is slightly heavier than the proton.<sup>[9](https://conferences.fnal.gov/lp2003/forthepublic/qcd/)</sup> Heavier quarks are unstable: charm was discovered in 1974, bottom in 1977, and top in 1995, and any heavier particle quickly decays toward the more stable first generation.<sup>[9](https://conferences.fnal.gov/lp2003/forthepublic/qcd/)</sup><sup> • </sup><sup>[2](https://home.cern/science/physics/standard-model/)</sup>

## Color and the strong interaction

Quarks carry a property called color charge, coming in three colors, and they combine only in ways that form colorless objects.<sup>[2](https://home.cern/science/physics/standard-model/)</sup> The theory of the strong interaction, Quantum Chromodynamics (QCD), describes colored quarks exchanging gluons, the force carriers of the strong force.<sup>[2](https://home.cern/science/physics/standard-model/)</sup><sup> • </sup><sup>[10](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-qcd.pdf)</sup> Neither quarks nor gluons are observed as free particles; hadrons are color-singlet combinations of quarks, antiquarks, and gluons.<sup>[1](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf)</sup>

**Confinement and hadronisation.** The strong force binds quarks so strongly at low energies that they cannot be pulled apart, a phenomenon known as confinement; their transition into hadrons is called hadronisation.<sup>[11](https://home.cern/partons-hadrons/)</sup> At high energies the strong force binds quarks weakly, while at lower energies they interact more strongly and form hadrons.<sup>[11](https://home.cern/partons-hadrons/)</sup> The top quark is the one exception to confinement in practice: it decays so quickly that it never hadronizes, and is observed decaying as a still-free quark.<sup>[3](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-free-quark-searches.pdf)</sup>

## Quark masses

Quark masses are fundamental parameters of the [Standard Model](https://www.edgepedia.org/standard-model), but unlike the electron's mass they cannot be measured directly, because quarks are confined inside hadrons.<sup>[12](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-masses.pdf)</sup> Any quantitative statement about a quark mass must refer to the theoretical framework used to define it, a property called scheme dependence; the most commonly used scheme for QCD perturbation theory is the \( \overline{\mathrm{MS}} \) scheme, in which the masses "run" with the energy scale.<sup>[12](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-masses.pdf)</sup> The u-, d-, and s-quark masses quoted by the Particle Data Group are \( \overline{\mathrm{MS}} \) masses at the scale \( \mu = 2 \) GeV, while the c- and b-quark masses are renormalized at their own mass and the t-quark mass is extracted from event kinematics.<sup>[4](https://pdg.lbl.gov/2024/download/db2024.pdf)</sup>

The earliest mass determinations came from quark models and are called constituent quark masses, of order 350 MeV for the u and d quarks; these model the effects of dynamical chiral symmetry breaking and are not directly related to the mass parameters of the QCD Lagrangian.<sup>[12](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-masses.pdf)</sup> The most reliable determinations of the light-quark masses come from lattice QCD simulations, in which QCD is solved numerically from its Lagrangian.<sup>[13](https://pdg.lbl.gov/2025/web/viewer.html?file=https%3A%2F%2Fpdg.lbl.gov%2F2025%2Freviews%2Frpp2025-rev-quark-masses.pdf)</sup> It is conventional to call quarks heavy if their mass exceeds the chiral symmetry-breaking scale \( \Lambda_\chi \), around 1 GeV, so the c, b, and t quarks are heavy and the u and d are light, with the s considered light in SU(3) chiral perturbation theory.<sup>[12](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-masses.pdf)</sup>

**Direct mass access.** In 2022 the ALICE collaboration at the LHC made the first direct observation of the dead-cone effect, a fundamental feature of QCD in which a radiating quark shows a cone of suppressed gluon emission at angles smaller than the ratio of its mass to its energy. The observation provided direct experimental access to the mass of a single charm quark before it is confined inside hadrons.<sup>[14](https://home.cern/alice-makes-first-direct-observation-fundamental-effect-particle-physics/)</sup>

## Quarks in the early universe and in collisions

In the first moments after the [Big Bang](https://www.edgepedia.org/big-bang) the universe was extremely hot and dense; as it cooled, quarks and electrons arose as the building blocks of matter, and a few millionths of a second later the quarks aggregated into protons and neutrons.<sup>[15](https://home.cern/science/physics/early-universe/)</sup> For those first few microseconds the universe was filled with quark-gluon plasma, a state in which quarks and gluons were bound only weakly and moved freely.<sup>[6](https://home.cern/science/physics/heavy-ions-and-quark-gluon-plasma/)</sup>

Accelerators recreate this state by colliding heavy ions such as lead nuclei at energies of a few trillion [electronvolts](https://www.edgepedia.org/electronvolt) per nucleon; the collisions reach temperatures more than 100,000 times hotter than the center of the Sun, melting protons and neutrons and freeing quarks from their gluon bonds.<sup>[5](https://home.cern/quark-matter-2017-understanding-early-universe/)</sup><sup> • </sup><sup>[6](https://home.cern/science/physics/heavy-ions-and-quark-gluon-plasma/)</sup> Each collision creates only a small droplet of plasma, and studies of how it expands and cools help explain how it gave rise to today's particles.<sup>[5](https://home.cern/quark-matter-2017-understanding-early-universe/)</sup> Recent results show the effect is not limited to large systems: all four main LHC experiments have reported signs of quark-gluon plasma in oxygen and neon collisions, and ALICE has observed a common pattern across proton-proton, proton-lead, and lead-lead collisions bearing on plasma formation in small systems.<sup>[16](https://home.cern/oxygen-collisions-at-the-lhc-show-new-indications-of-extreme-state-of-matter/)</sup><sup> • </sup><sup>[17](https://home.cern/news/news/physics/alice-sees-new-sign-primordial-plasma-proton-collisions)</sup>

## Quarks inside the proton

The three valence quarks of the proton are only part of the story. The gluon field binding them is strong enough to create fleeting pairs of quarks and antiquarks, the so-called quark sea, whose presence in the nucleon cannot be ignored.<sup>[7](https://spinquest.fnal.gov/)</sup> Most of the proton's mass comes from this gluon field rather than from the quarks themselves.<sup>[7](https://spinquest.fnal.gov/)</sup> [Deep inelastic scattering](https://www.edgepedia.org/deep-inelastic-scattering) experiments at SLAC in the late 1960s found electrons scattering off charged points inside the proton, and follow-up experiments determined the charges and spin of these partons, identifying them with the quarks proposed by Gell-Mann and Zweig; the later discovery of charm at SPEAR convinced the remaining doubters that quarks were real.<sup>[18](https://colloquium.slac.stanford.edu/events/2023-10-05-early-days-slac-quark-discoveries)</sup>

Quark flavor also mixes: the charged-current weak interaction couples quarks of different generations through the CKM quark-mixing matrix, whose elements are fundamental parameters of the Standard Model measured with high precision.<sup>[19](https://pdg.lbl.gov/2023/reviews/rpp2022-rev-ckm-matrix.pdf)</sup> Searches for composites of quarks continue in other directions as well; the [LHCb experiment](https://www.edgepedia.org/lhcb-experiment) has discovered baryons containing two charm quarks, including one with two charm quarks and a down quark, about four times heavier than the proton, and the final member of the doubly charmed family with one strange and two charm quarks.<sup>[8](https://home.cern/lhcb-collaboration-discovers-new-proton-particle/)</sup><sup> • </sup><sup>[20](https://home.cern/lhcb-discovers-the-final-missing-member-of-a-doubly-charmed-particle-family/)</sup>

## References

1. [C. Amsler, V. Crede, T. DeGrand (2023). Quark Model, Review of Particle Physics, Particle Data Group.](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-model.pdf)
2. [The Standard Model, CERN](https://home.cern/science/physics/standard-model/)
3. [Free Quark Searches, Review of Particle Physics, Particle Data Group (2024)](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-free-quark-searches.pdf)
4. [Quark Summary Table, Review of Particle Physics, Particle Data Group (2024)](https://pdg.lbl.gov/2024/download/db2024.pdf)
5. [Quark Matter 2017: understanding the early universe, CERN](https://home.cern/quark-matter-2017-understanding-early-universe/)
6. [Heavy ions and quark-gluon plasma, CERN](https://home.cern/science/physics/heavy-ions-and-quark-gluon-plasma/)
7. [SpinQuest/E1039 Experiment, Fermilab](https://spinquest.fnal.gov/)
8. [LHCb Collaboration discovers new proton-like particle, CERN](https://home.cern/lhcb-collaboration-discovers-new-proton-particle/)
9. [QCD and the structure of matter, Fermilab](https://conferences.fnal.gov/lp2003/forthepublic/qcd/)
10. [J. Huston, K. Rabbertz, G. Zanderighi (2023). Quantum Chromodynamics, Review of Particle Physics, Particle Data Group.](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-qcd.pdf)
11. [From partons to hadrons, CERN](https://home.cern/partons-hadrons/)
12. [R.M. Barnett, L.P. Lellouch, A.V. Manohar (2023). Quark Masses, Review of Particle Physics, Particle Data Group.](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-quark-masses.pdf)
13. [Quark Masses, Review of Particle Physics 2025 edition, Particle Data Group](https://pdg.lbl.gov/2025/web/viewer.html?file=https%3A%2F%2Fpdg.lbl.gov%2F2025%2Freviews%2Frpp2025-rev-quark-masses.pdf)
14. [ALICE makes first direct observation of a fundamental effect in particle physics, CERN](https://home.cern/alice-makes-first-direct-observation-fundamental-effect-particle-physics/)
15. [The early universe, CERN](https://home.cern/science/physics/early-universe/)
16. [Oxygen collisions at the LHC show new indications of extreme state of matter, CERN](https://home.cern/oxygen-collisions-at-the-lhc-show-new-indications-of-extreme-state-of-matter/)
17. [ALICE sees new sign of primordial plasma in proton collisions, CERN](https://home.cern/news/news/physics/alice-sees-new-sign-primordial-plasma-proton-collisions)
18. [Early days at SLAC - the quark discoveries, SLAC Colloquium (Marty Breidenbach)](https://colloquium.slac.stanford.edu/events/2023-10-05-early-days-slac-quark-discoveries)
19. [A. Ceccucci, Z. Ligeti, Y. Sakai (2022). CKM Quark-Mixing Matrix, Review of Particle Physics, Particle Data Group.](https://pdg.lbl.gov/2023/reviews/rpp2022-rev-ckm-matrix.pdf)
20. [LHCb discovers the final missing member of a doubly charmed particle family, CERN](https://home.cern/lhcb-discovers-the-final-missing-member-of-a-doubly-charmed-particle-family/)

---
*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Particle physics › Standard Model particle content › Quarks and leptons › Fermion generations and family structure*

*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026; Oct 21, 2026; Oct 25, 2026 · Last review: Oct 15, 2026*

*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*

License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license