Quark: The Elementary Building Block Inside Every Proton and Neutron
Everything you can touch is built from atoms, atoms contain a dense nucleus of protons and neutrons, and those protons and neutrons are themselves built from smaller objects still: quarks. A quark is one of the truly elementary particles of nature - it has no known internal structure and, as far as every experiment so far can tell, cannot be divided further. What makes quarks unusual is that they are never found alone. They are permanently locked inside larger particles by the strongest force known, and almost everything we know about them has been pieced together indirectly, from the debris of high-energy collisions and the mathematics that explains it. This page is a reference on what a quark is, the six flavours quarks come in, the fractional electric charge and colour charge they carry, why they cannot be isolated, and how they were discovered.
What is a quark?
A quark is an elementary particle - one with no measurable size and no known internal structure - and one of the fundamental constituents of matter. Quarks are the only elementary matter particles that feel the strong nuclear force; they also take part in the electromagnetic and weak interactions, and, like everything carrying energy, in gravity. Together with the leptons (the family that includes the electron and the neutrino), quarks make up the matter content of the Standard Model of particle physics, the theory that describes the known elementary particles and the forces between them.
Quarks are fermions with spin 1/2, which means they obey the Pauli exclusion principle and behave as classic matter particles rather than as force carriers. Their defining feature is that they combine into composite particles called hadrons. Three quarks bound together make a baryon - the proton and the neutron are the familiar examples - while a quark paired with an antiquark makes a meson, such as the pion or the kaon. Quarks are not themselves hadrons; they are the ingredients from which hadrons are built. To place them in the wider family tree of matter, see subatomic particles and lepton.
The six flavours of quark
Quarks come in six varieties, whimsically called flavours: up, down, charm, strange, top and bottom. They are organised into three generations, each generation a heavier copy of the one before. The first generation - up and down - makes up all ordinary, stable matter. The heavier flavours appear only fleetingly, in high-energy collisions and in cosmic-ray interactions, and decay almost immediately into lighter quarks.
The generations pair the flavours as (up, down), (charm, strange) and (top, bottom). Mass rises steeply across them. In the conventional scheme used to quote them, the up quark comes in at roughly 2 MeV/c² and the down quark at roughly 5 MeV/c², while the top quark is measured at about 173 GeV/c² - comparable to the mass of an entire tungsten atom, and by far the most massive elementary particle known. Between the extremes sit the strange (~93 MeV/c²), charm (~1.27 GeV/c²) and bottom (~4.18 GeV/c²) quarks. Because heavier quarks decay into lighter ones, effectively all of the ordinary matter in the universe is built from the two humblest flavours, up and down.
Fractional charge and how quarks build matter
Quarks are the only known particles that carry fractional electric charge. The up-type quarks (up, charm, top) each carry +2/3 of the elementary charge, while the down-type quarks (down, strange, bottom) each carry −1/3. Everything else observed in nature carries whole-number multiples of the electron's charge, which is precisely why fractional charge struck many physicists in the 1960s as a bold and unlikely idea.
The arithmetic works out exactly. A proton is two up quarks and one down quark: (+2/3) + (+2/3) + (−1/3) = +1, the proton's charge. A neutron is one up quark and two down quarks: (+2/3) + (−1/3) + (−1/3) = 0, precisely neutral. Yet these quark rest masses account for only around one per cent of a proton's mass. The remainder is the energy of the strong interaction binding the quarks and gluons together, appearing as mass through E = mc². The quarks' own small rest masses come from their coupling to the Higgs field, the mechanism behind the Higgs boson found at CERN's Large Hadron Collider in 2012 - but that mechanism supplies only that small share. The mass of everyday matter is overwhelmingly strong-interaction binding energy, not a gift of the Higgs.
Colour charge, gluons and the strong force
Beyond electric charge, quarks carry a second kind of charge with no everyday analogue, called colour. It comes in three values, labelled red, green and blue - names chosen only because, like the primary colours of light, they combine to something neutral, or "white". Colour has nothing to do with visible colour; it is simply the charge of the strong interaction, described by the theory of quantum chromodynamics (QCD). Observable hadrons are always colour-neutral: three quarks carrying one of each colour, or a quark and an antiquark carrying a colour and its anticolour.
The strong force between quarks is carried by gluons, massless force-carrying particles of the boson family - there are eight of them in QCD. Unlike the photon of electromagnetism, which is electrically neutral, gluons carry colour charge themselves, so they interact not only with quarks but with one another. This self-interaction is what makes the strong force behave so unlike the others, and it is ultimately why quarks cannot escape.
Confinement: why a quark is never seen alone
No experiment has ever detected a free, isolated quark, and QCD says none ever will under ordinary conditions. The phenomenon is called colour confinement. The reason lies in how the strong force behaves with distance: unlike gravity or electromagnetism, which weaken as objects separate, the force between two quarks stays roughly constant as they are pulled apart, as though they were joined by an unbreakable elastic string.
Try to separate two quarks and you pour ever more energy into that string. Long before either quark can break free, the stored energy becomes large enough to create a new quark–antiquark pair, which caps the ends of the broken string. The result is always more hadrons, never a lone quark. The opposite behaviour holds at very short distances: squeezed close together, quarks interact only weakly and behave almost as free particles, a property called asymptotic freedom. Its discovery by David Gross, Frank Wilczek and H. David Politzer earned the 2004 Nobel Prize in Physics and turned QCD into a predictive theory.
A short history: from Finnegans Wake to Fermilab
Quarks began as a bookkeeping idea. In 1964, Murray Gell-Mann and George Zweig independently proposed that the growing zoo of hadrons could be explained if those particles were built from a few more fundamental pieces; Zweig called his version "aces", and Gell-Mann's name for them is the one that stuck. He took "quark" from a line in James Joyce's novel Finnegans Wake - "Three quarks for Muster Mark" - a fitting choice for particles that come three to a proton.
At first many physicists doubted that fractionally charged objects were physically real rather than a useful piece of algebra. Evidence arrived from deep inelastic scattering experiments at the Stanford Linear Accelerator Center (SLAC) in the late 1960s, in which electrons fired into protons occasionally bounced back hard, as if from small point-like objects inside - the quarks. Jerome Friedman, Henry Kendall and Richard Taylor shared the 1990 Nobel Prize for that work. The charm quark followed in the "November Revolution" of 1974, when groups led by Burton Richter at SLAC and Samuel Ting at Brookhaven announced the J/ψ particle within days of each other. The bottom quark appeared in 1977 at Fermilab, in the discovery of the upsilon meson, and the long-sought top quark completed the set in 1995, found by the CDF and DZero collaborations at Fermilab's Tevatron collider.
Quarks, beta decay and where neutrinos come from
Quark flavour is not immutable: the weak nuclear force can turn one flavour into another, and that is the engine of radioactivity. In beta-minus decay, a down quark inside a neutron turns into an up quark, converting the neutron into a proton and emitting an electron and an electron antineutrino. A subtle rearrangement at the quark level therefore drives whole categories of radioactive decay - and closely related weak processes are what fill the universe with neutrinos, from the antineutrinos streaming out of nuclear reactors to the neutrinos produced when protons fuse into helium in the core of the Sun.
That is where fundamental quark physics touches applied research: the Neutrino Energy Group studies graphene-based neutrinovoltaic materials, an early-stage laboratory line of inquiry into whether small amounts of energy can be harvested from ambient radiation and thermal motion, rather than a finished or commercially available technology.
Open questions about quarks
Even with all six flavours found, quarks keep physicists busy. The Standard Model does not explain why there are exactly three generations, nor why quark masses span roughly five orders of magnitude - a pattern the theory accommodates but does not predict. The strengths with which quarks couple to the Higgs field, and the values in the mixing matrix that governs how one flavour turns into another, are inputs measured from experiment rather than consequences of the theory.
There are deeper puzzles too. In the first few microseconds after the Big Bang, matter existed as a quark–gluon plasma, a state now recreated in heavy-ion collisions at RHIC and the Large Hadron Collider and still far from fully understood. And the longstanding question of why the universe contains so much more matter than antimatter remains open: the difference in behaviour between quarks and antiquarks that the Standard Model does contain - the CP violation explained by Makoto Kobayashi and Toshihide Maskawa, recognised with a share of the 2008 Nobel Prize - is far too small to account for the imbalance. Quarks are therefore not a closed chapter but a live frontier in the effort to understand what the universe is made of.
Frequently asked questions
What is a quark in simple terms?
A quark is one of the smallest known building blocks of matter - an elementary particle with no known internal structure. Quarks combine in groups to form protons and neutrons, which in turn make up the nucleus of every atom. There are six kinds, and they are never found on their own.
What are the six types of quarks?
The six quark flavours are up, down, charm, strange, top and bottom, grouped into three generations. Up and down quarks make up all ordinary stable matter, forming protons and neutrons. The heavier four - strange, charm, bottom and top - appear only briefly in high-energy collisions before decaying into lighter quarks.
What are up and down quarks?
Up and down quarks are the two lightest and most common quarks, and they make up all everyday matter. An up quark carries an electric charge of +2/3 and a down quark −1/3. A proton is two up quarks and one down quark; a neutron is one up quark and two down quarks.
Why can't quarks be seen on their own?
Because of colour confinement. The strong force between quarks does not weaken with distance, so pulling two quarks apart takes ever more energy. Before either can break free, that energy creates new quark–antiquark pairs, so you always end up with more composite particles, never an isolated quark.
Who discovered quarks and when?
Murray Gell-Mann and George Zweig proposed quarks independently in 1964. Scattering experiments at SLAC in the late 1960s revealed point-like objects inside protons. The six flavours were completed over three decades, ending with the top quark, discovered by the CDF and DZero collaborations at Fermilab's Tevatron in 1995.
Does the Higgs boson give quarks their mass?
The Higgs field gives quarks their small rest masses, and the Higgs boson was found at CERN's Large Hadron Collider in 2012. But roughly 99 per cent of a proton's mass is not Higgs mass at all: it is the binding energy of the strong interaction among the quarks and gluons inside.
What is the difference between a quark and a lepton?
Both are elementary matter particles, but quarks feel the strong nuclear force and carry colour charge, while leptons - such as the electron and the neutrino - do not. Quarks carry fractional electric charge and are always bound into composite particles; leptons carry whole-number charge and can exist on their own.
