Quarks are the elementary particles inside protons and neutrons — the only particles that feel all four fundamental forces. And they are never, ever found alone. The proton floating on this page is real physics: grab a quark and try to pull it free.
The strong force is unlike anything else in nature. Electromagnetism fades with distance; the color force between quarks does not. Stretch two quarks apart and the gluon field between them tenses like an elastic band, storing more and more energy.
Past a certain point, it's energetically cheaper for the field to snap and create a brand-new quark–antiquark pair out of pure energy than to keep stretching. Your "freed" quark instantly pairs up into a meson. The proton heals. Nature wins. This is color confinement — and it's why, in 60 years of looking, no experiment has ever seen an isolated quark.
The flip side is asymptotic freedom: squeeze quarks close together and the force gets weaker — they rattle around almost freely. Strong when far, gentle when near. Backwards, and true.
Everything you have ever touched is up quarks, down quarks, and electrons. The other four flavors are heavier, unstable, and appear only in cosmic-ray collisions, particle accelerators — and the first microseconds after the Big Bang. Mass bars below are on a logarithmic scale; on a linear one, the top quark's bar would be about 80,000× longer than the up quark's.
Two of these live in every proton. Lighter than the electron's big sibling has any right to be.
The proton's third wheel and the neutron's majority. Beta decay is a down turning into an up.
Named for the "strangely" long-lived cosmic-ray particles it inhabited — seen decades before quarks were proposed.
Its 1974 discovery — the J/ψ "November Revolution" — convinced physics that quarks were real. Now the star of exotic-hadron hunting.
The flavor physicist's favorite: its decays are where matter–antimatter differences show themselves. LHCb was built for it.
Heavier than a gold atom. Decays in ~5×10⁻²⁵ s — before it can even hadronize — which makes it a uniquely clean quantum probe.
MASSES: PDG REVIEW OF PARTICLE PHYSICS (CURRENT-QUARK / MS̄ VALUES, ROUNDED); TOP MASS FROM THE ATLAS+CMS COMBINATION ≈ 172.5 GeV. THE * ON STRANGE: ITS HADRONS WERE SEEN IN COSMIC RAYS IN 1947, LONG BEFORE THE QUARK MODEL NAMED THEM.
Quarks carry a second kind of charge with three values, whimsically named red, green, and blue (no relation to light — it's just a useful analogy, because of how they combine). The rule of quantum chromodynamics is absolute: only color-neutral combinations can exist in isolation. Build one yourself.
Three quarks — one red, one green, one blue — sum to "white": that's a baryon, like the proton or neutron. A quark and an antiquark carrying a color and its anti-color also cancel: that's a meson. Anything colored is forbidden from existing on its own.
The force itself is carried by gluons — eight of them — and here's the twist that makes the strong force strong: gluons carry color charge themselves, so gluons attract gluons. The field feeds back on itself, the force lines bundle into tight flux tubes, and confinement falls out. (Photons, by contrast, are electrically neutral — which is why electromagnetism politely fades with distance.)
Since the 2000s, experiments have also confirmed color-neutral tetraquarks (qq̄qq̄) and pentaquarks (qqqqq̄) — exotic hadrons the original model only dreamt of.
Here is the most under-told fact in physics: add up the rest masses of the proton's three valence quarks and you get roughly 9 MeV. The proton weighs 938 MeV. The missing ~99% isn't matter at all — it's the energy of the gluon field and the furious motion of confined quarks, converted to mass by E = mc². The Higgs gives quarks their tiny intrinsic masses; QCD builds nearly everything you weigh.
Jefferson Lab experiments probing the proton with near-threshold J/ψ production, and lattice-QCD calculations of the proton's "gravitational form factors," are now mapping exactly where this mass — and even pressure and shear — sits inside the proton.
Only the weak force can change a quark's flavor, by emitting or absorbing a W boson. That single fact powers radioactive beta decay: a down quark inside a neutron flips into an up quark, and the neutron becomes a proton. The probabilities of every possible flavor jump are encoded in one 3×3 table — the Cabibbo–Kobayashi–Maskawa matrix. Hover the cells: bright means likely, dim means rare.
|Vij| MAGNITUDES, APPROXIMATE (PDG). DIAGONAL ≈ 1: QUARKS PREFER THEIR OWN GENERATION.
Kobayashi and Maskawa noticed in 1973 that with three generations of quarks, this matrix naturally contains an irreducible complex phase — and that phase makes matter and antimatter behave differently. That's CP violation, a required ingredient for why the universe contains anything at all.
The catch: the Standard Model's CP violation is far too small to explain the cosmic matter excess. So physicists hunt for extra CP violation in quark decays — and in 2025, LHCb observed it in baryon decays (the Λb) for the first time ever, opening a whole new arena for the search.
Beta decay's flavor flip is also quietly medical: positron emission — a proton becoming a neutron at the quark level — is the physics underneath every PET scan.
Independently, both physicists realize the chaotic "particle zoo" of hadrons snaps into order if built from three constituents: up, down, strange. Many physicists treat them as bookkeeping fictions.
Deep inelastic scattering — firing electrons through protons — reveals hard, point-like scatterers inside. Feynman calls them "partons." They are quarks. Nobel Prize, 1990.
Two teams (SLAC and Brookhaven) simultaneously find the J/ψ — a charm–anticharm meson. The fourth quark, predicted by the GIM mechanism, is real. The quark model wins overnight.
Lederman's team finds the Υ, a bottom–antibottom state. The third generation — predicted in 1973 by Kobayashi and Maskawa to explain CP violation — has begun.
CDF and DØ at Fermilab observe the top quark — shockingly heavy at ~173 GeV, nearly a gold atom in a single point particle.
LHCb confirms pentaquarks, then a doubly charmed tetraquark (Tcc⁺), then a strange pentaquark and a tetraquark pair in one announcement. Hadrons with 4 and 5 valence quarks go from conjecture to catalog.
Entangled top quarks, CP violation in baryons, toponium hints, doubly charmed baryons completing their family, and lattice QCD reaching record precision — see below.
"Three quarks for Muster Mark!
Sure he hasn't got much of a bark
And sure any he has it's all beside the mark."
Gell-Mann had the sound first — "kwork" — and found the spelling while rereading Joyce. The number three fit perfectly. Zweig had wanted to call them "aces." History chose the stranger word.
Recent results, with primary sources. This is where the field actually is right now.
ATLAS (then CMS) observed quantum entanglement between top–antitop pairs at >5σ — the highest-energy entanglement ever measured, and the first in quarks. Possible only because tops decay before hadronization can scramble their spins.
Nature 633, 542 (2024) · ATLAS/CMS, CERNLHCb observed matter–antimatter asymmetry in Λb baryon decays — the first time CP violation has been seen in the baryon family that includes protons and neutrons. A new arena for explaining why anything exists.
CERN / LHCb (2025) · Nature 643, 1223CMS and ATLAS found an unexpected excess right at the top–antitop production threshold, consistent with a fleeting quasi-bound "toponium" state — from a quark thought to decay too fast to ever bind.
CMS, arXiv:2503.22382 · ATLAS confirmation 2025LHCb's upgraded detector found the Ξcc⁺ (ccd) in March — four times the proton's mass — and announced the Ωcc⁺ (ccs) in June, completing the doubly charmed baryon trio. Two heavy quarks orbited by one light one: a hydrogen atom of the strong force.
arXiv:2603.28456 · LHCb outreach (Jun 2026)A lattice-QCD calculation in Nature delivered the quark–gluon coupling αs with unprecedented, model-free precision. Every Higgs prediction and new-physics search just got sharper.
Dalla Brida et al., Nature (2026)CMS searched for any sign that quarks have substructure — and found none, pushing the limit to roughly 10⁻²⁰ meters (compositeness scale ~37 TeV). If quarks are made of anything smaller, it's hiding extraordinarily well.
CERN / CMS (Apr 2026)The QGP — deconfined quark matter at ~2×10¹² K — flows as a near-perfect fluid. CMS now sees jets plowing through it leaving boat-like diffusion wakes; ALICE found QGP-like collective flow even in plain proton–proton collisions; first oxygen–oxygen runs show parton energy loss in small systems.
CERN heavy ions · CMS PLB (2026) · ALICE (Mar 2026)Combining gravitational waves, X-ray radii, and QCD theory, analyses find it likely (estimates of 75–90%) that the heaviest neutron stars hide cores of deconfined quark matter — the only place in today's universe quarks may roam "free."
Annala et al., Nat. Phys. (2020) · Univ. Helsinki (2024)Brookhaven's EIC will fire polarized electrons at polarized protons and nuclei to make the first true 3-D maps of quarks and gluons — and finally settle the proton spin puzzle: quark spins supply only ~30% of the proton's spin, and the rest (gluon spin? orbital motion?) is still unaccounted for.
BNL EIC ScienceRare b→s transitions are so suppressed in the Standard Model that unknown heavy particles could visibly distort them. The famous lepton-universality anomalies realigned with the SM in 2022 — but the precision program continues, and it remains one of the best long-shot bets for a crack in the theory.
CERN / LHCb (2022)Before 10⁻⁶ seconds after the Big Bang, it was too hot for protons to exist: the cosmos was a single quark–gluon plasma. Colliders recreate femto-droplets of it today — and found not a gas, but the most perfect liquid ever measured, with near-zero viscosity for its entropy. At the other extreme — cold but absurdly dense — quark matter may form color-superconducting Cooper pairs inside neutron stars.
Confinement means you'll never wire a quark into a chip. The technology arrives sideways: through the machines, detectors, and nuclear processes the field created.
Every PET scan rides on beta-plus decay — a proton becoming a neutron, which at the bottom is a quark changing flavor via the weak force. Quark physics, in clinical rotation daily.
Particle accelerators built for collision physics now treat tumors with proton and ion beams; detector crystals and chips from experiments became medical imagers. CERN's FLUKA simulation code plans hadron-therapy doses.
Lattice QCD's record-precision strong coupling sharpens Higgs predictions and new-physics searches — and quark-matter equations of state now feed directly into gravitational-wave and neutron-star astrophysics.
Lattice computations demonstrate it numerically; a full analytic derivation from first principles remains one of the deepest unsolved problems in quantum field theory — adjacent to the Yang–Mills Millennium Prize problem.
Quark spins cover roughly a third. Gluon spin and orbital motion must supply the rest, in proportions still unmeasured. The EIC is being built largely to answer this.
Compact tetraquarks? Loosely bound hadron "molecules"? Threshold effects? Tcc⁺ and the doubly charmed baryons are the cleanest test cases yet.
One quark outweighs a gold atom while its sibling weighs less than an electron-pair. The 100,000× spread of quark masses — set by their Higgs couplings — has no accepted explanation.
The CKM phase is real but cosmologically insufficient. Baryon CP violation, rare beauty decays, and the quark–lepton connection (CKM vs PMNS) are the open hunting grounds.
Point-like down to ~10⁻²⁰ m as of 2026. Three near-identical generations whisper "pattern" — and patterns, historically, have substructure. Or not. Nobody knows.