Quark

A quark is a fundamental particle that feels the strong force. Quarks are the stuff inside every Proton and Neutron, and therefore inside every Atom you have ever touched. There are six kinds, called flavours.

Two things make quarks strange compared with an Electron:

• Their Electric charge comes in thirds — +⅔ e or −⅓ e — while every particle you can catch on its own carries a whole number of e.
• You cannot catch one on its own. Quarks are confined: they only ever appear locked inside composite particles called hadrons.

The six flavours

flavour	symbol	charge	mass	generation
up	u	+⅔ e	2.16 ± 0.07 MeV	1st
down	d	−⅓ e	4.70 ± 0.07 MeV	1st
strange	s	−⅓ e	93.5 ± 0.8 MeV	2nd
charm	c	+⅔ e	1.2730 ± 0.0046 GeV	2nd
bottom	b	−⅓ e	4.183 ± 0.007 GeV	3rd
top	t	+⅔ e	172.57 ± 0.29 GeV	3rd

All masses are PDG 2024. Read the fine print: quark mass is not a single unambiguous number. The u, d and s values are MS-bar masses at a scale of 2 GeV; c and b are quoted at their own mass; the top mass comes from the kinematics of the events it produces.

The span is the headline. The top quark is about 79 900 times heavier than the up quark. Converted into atomic mass units it comes to 185.3 u, so one top quark outweighs a whole atom of tungsten (183.84 u) and is just shy of an atom of rhenium (186.21 u). A single fundamental particle, with no known size or parts, heavier than 74 protons and 110 neutrons bound together.

Only the first generation is stable and ordinary. Everything around you is up and down quarks plus electrons. The other four flavours are made in accelerators and cosmic rays, and decay in a flash.

Confinement: why you can never hold one

Pull two quarks apart and the strong force does not weaken with distance the way gravity or electromagnetism do. The energy stored between them keeps climbing. Long before a quark comes free, that energy is enough to create a fresh quark–antiquark pair out of the vacuum — so you end up with two hadrons instead of one loose quark.

The bookkeeping behind this is colour charge. In quantum chromodynamics (QCD):

• There are 3 colours, and 8 gluons carrying the force between them.
• Every observable particle must be colour-neutral — a "colour singlet".
• The two simplest ways to be neutral are a meson (quark + antiquark) and a baryon (three quarks). Protons and neutrons are baryons.

The top quark is the one exception to the "you never see a bare quark" rule, in a sideways sense. It is so heavy that its decay width is 1.42 GeV, giving it a life of roughly 5 × 10⁻²⁵ seconds — it decays before it has time to grab a partner and form a hadron at all.

Quarks and leptons

Quarks pair off in three generations, exactly as Leptons do. That mirror is one of the Standard Model's real patterns, and nobody knows why it exists or why it stops at three.

generation	quarks	leptons
1st	u, d	electron, electron neutrino
2nd	c, s	muon, muon neutrino
3rd	t, b	tau, tau neutrino

How they were found

1. 1964 — Murray Gell-Mann (and, independently, George Zweig) proposes the quark model to explain why the growing zoo of hadrons falls into neat geometric families like the decuplet above. Gell-Mann takes the 1969 Nobel Prize for the classification work.
2. Late 1960s — deep inelastic scattering at SLAC fires electrons hard into protons and sees them bounce off small hard objects inside. That turned quarks from bookkeeping into things. Friedman, Kendall and Taylor share the 1990 Nobel Prize for it.
3. 1995 — the top quark, last of the six, is finally produced at Fermilab.

Where to read next

• Proton — uud, and why the quark masses only add up to 1 % of it
• Neutron — udd, and the down-to-up flip that makes beta decay
• Lepton — the other family of matter particles, the ones that ignore the strong force