A hadron is a composite particle made of smaller building blocks called quarks, held together by the strong force, the most powerful force in nature. Protons and neutrons, the particles that make up every atomic nucleus in your body, are both hadrons. So when physicists talk about hadrons, they’re talking about a broad family of particles that includes the very stuff you’re made of.
How Quarks Build a Hadron
Quarks are fundamental particles, meaning they have no smaller components. They never exist on their own in nature. Instead, they’re always locked together inside hadrons by the strong force, which acts like an impossibly powerful glue. The more you try to pull quarks apart, the stronger the force pulls them back together.
Each quark carries a property physicists call “color charge” (which has nothing to do with actual color). For a hadron to exist, the color charges of its quarks must cancel out to zero, producing a “color neutral” particle. There are exactly two ways this can happen, and those two ways define the two main types of hadrons.
Baryons: Three-Quark Hadrons
Baryons are hadrons made of three quarks. The proton is a baryon (two “up” quarks and one “down” quark), and the neutron is a baryon (two down quarks and one up quark). These two are by far the most familiar baryons because they form every atomic nucleus. But dozens of other baryons exist, most of them extremely short-lived and only produced in high-energy collisions.
The proton is essentially the only stable hadron. It has never been observed to decay, though some theoretical models suggest it might with a half-life around 10³² years, a span so vast it dwarfs the age of the universe. The neutron is a different story. Inside a nucleus, neutrons are stable. But a free neutron, one that has been knocked loose from an atom, decays with a half-life of about 10.3 minutes, breaking apart into a proton, an electron, and a tiny particle called an antineutrino.
Mesons: Two-Quark Hadrons
Mesons are the other main class of hadron, made of one quark paired with one antiquark (the antimatter version of a quark). Because they contain an antimatter component, mesons are inherently unstable. They’re produced in particle collisions and cosmic ray interactions but decay almost instantly, typically in fractions of a microsecond or less.
The pion is the most well-known meson. It was predicted in the 1930s as the particle responsible for carrying the strong force between protons and neutrons inside a nucleus, and it was discovered about a decade later. Other mesons, like kaons, played a key role in the discovery of new types of quarks and helped physicists piece together the rules governing the subatomic world.
Exotic Hadrons: Four and Five Quarks
For decades, physicists only observed hadrons fitting neatly into the two-quark or three-quark categories. That changed with discoveries at particle colliders, particularly CERN’s Large Hadron Collider. Experiments there have confirmed the existence of exotic hadrons composed of four quarks (tetraquarks) and five quarks (pentaquarks).
Most tetraquarks and pentaquarks discovered so far contain a charm quark and its antimatter counterpart, with the remaining quarks being lighter varieties. More recently, the CMS experiment at CERN reported measurements of a family of three “all-charm” tetraquarks, particles made entirely of two charm quarks and two charm antiquarks. These exotic particles are helping physicists understand how the strong force works in configurations beyond the familiar two- and three-quark arrangements.
Exotic hadrons are fleeting. They exist for vanishingly small fractions of a second before decaying into lighter, more conventional particles. But their brief existence tells researchers a great deal about the rules that govern how quarks can combine.
Why the Large Hadron Collider Is Named After Them
The Large Hadron Collider, the world’s most powerful particle accelerator, gets its name because it accelerates and smashes hadrons together, primarily protons. In a typical run, two beams of protons travel in opposite directions around a 27-kilometer ring beneath the Swiss-French border, reaching energies of several teraelectronvolts before slamming into each other. The debris from those collisions is where new particles, including exotic hadrons, are discovered.
The LHC also collides lead ions, which are much heavier and produce far more complex collision debris. To make sense of those lead-lead results, physicists first run proton-proton collisions at the same energy as a baseline for comparison. Protons are used as the default because they’re the simplest stable hadron: just three quarks, producing relatively clean collisions that are easier to analyze.
Where Hadrons Fit in the Particle World
Not every subatomic particle is a hadron. Electrons, for instance, are not made of quarks and do not interact through the strong force. They belong to a different family called leptons. Photons (particles of light) and neutrinos also fall outside the hadron family. The key distinction is straightforward: if a particle is built from quarks and held together by the strong force, it’s a hadron. If it isn’t, it belongs to another category entirely.
This classification matters because the strong force operates on completely different rules than the electromagnetic force you encounter in daily life. It’s roughly 100 times stronger than electromagnetism at subatomic distances, which is why atomic nuclei hold together despite the protons inside them electrically repelling each other. Every atom in the universe owes its existence to the fact that hadrons are bound tightly enough to resist that repulsion.