Hadrons are categorized into two families: baryons, made of three quarks; and mesons, made of one quark and one antiquark. Protons and neutrons are examples of baryons; pions are an example of a meson. Hadrons containing more than three valence quarks (exotic hadrons) have been discovered in recent years. A tetraquark state (an exotic meson), named the Z(4430)−, was discovered in 2007 by the Belle Collaboration and confirmed as a resonance in 2014 by the LHCb collaboration. Two pentaquark states (exotic baryons), named P+
c(4380) and P+
c(4450), were discovered in 2015 by the LHCb collaboration. There are several more exotic hadron candidates, and other colour-singlet quark combinations that may also exist.
Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions; free neutrons decay with a half-life of about 611 seconds. Experimentally, hadron physics is studied by colliding protons or nuclei of heavy elements such as lead or gold, and detecting the debris in the produced particle showers.
According to the quark model, the properties of hadrons are primarily determined by their so-called valence quarks. For example, a proton is composed of two up quarks (each with electric charge +2⁄3, for a total of +4⁄3 together) and one down quark (with electric charge −1⁄3). Adding these together yields the proton charge of +1. Although quarks also carry color charge, hadrons must have zero total color charge because of a phenomenon called color confinement. That is, hadrons must be "colorless" or "white". These are the simplest of the two ways: three quarks of different colors, or a quark of one color and an antiquark carrying the corresponding anticolor. Hadrons with the first arrangement are called baryons, and those with the second arrangement are mesons.
Hadrons, however, are not composed of just three or two quarks, because of the strength of the strong force. More accurately, strong force gluons have enough energy (E) to have resonances composed of massive (m) quarks (E > mc2) . Thus, virtual quarks and antiquarks, in a 1:1 ratio, form the majority of massive particles inside a hadron. The two or three quarks that compose a hadron are the excess of quarks vs. antiquarks, and so too in the case of anti-hadrons (anti-particles). Because the virtual quarks are not stable wave packets (quanta), but an irregular and transient phenomenon, it is not meaningful to ask which quark is real and which virtual; only the excess is apparent from the outside in the form of a hadron. Massless virtual gluons compose the numerical majority of particles inside hadrons.
Like all subatomic particles, hadrons are assigned quantum numbers corresponding to the representations of the Poincaré group: JPC(m), where J is the spin quantum number, P the intrinsic parity (or P-parity), C the charge conjugation (or C-parity), and m the particle's mass. Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due to mass–energy equivalence, most of the mass comes from the large amount of energy associated with the strong interaction. Hadrons may also carry flavor quantum numbers such as isospin (G parity), and strangeness. All quarks carry an additive, conserved quantum number called a baryon number (B), which is +1⁄3 for quarks and −1⁄3 for antiquarks. This means that baryons (groups of three quarks) have B = 1 whereas mesons have B = 0.
Hadrons have excited states known as resonances. Each ground state hadron may have several excited states; several hundreds of resonances have been observed in experiments. Resonances decay extremely quickly (within about 10−24 seconds) via the strong nuclear force.