Any of the subatomic particles that are built from quarks and thus interact via the strong force. The hadrons fall into two groups: mesons and baryons. Except for protons and neutrons, which are bound in nuclei, all hadrons have short lives and are produced in high-energy collision of subatomic particles. All hadrons are subject to gravitation; charged hadrons are subject to electromagnetic forces. Some hadrons break up by way of the weak force (as in radioactive decay); others decay via the strong and electromagnetic forces.
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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 (or P) parity, and C, the charge conjugation, or C parity, and the particle four-momentum, m, (i.e., its 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 nuclear force. Hadrons may also carry flavor quantum numbers such as isospin (or G parity), and strangeness. All quarks carry an additive, conserved quantum number called baryon number (B), which is +1/3 for quarks and -1/3 for anti-quarks. This means that baryons --which are groups of three quarks-- have B=1 while mesons have B=0.
Hadrons have excited states known as resonances. Each ground-state hadron may have several excited states; hundreds of resonances have been observed in particle physics experiments. Resonances decay extremely quickly (within about 10−24 seconds) via the strong nuclear force.
In other phases of QCD matter the hadrons may disappear. For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, the theory of quantum chromodynamics (QCD) predicts that quarks and gluons will interact weakly and will no longer be confined within hadrons. This property, which is known as asymptotic freedom, has been experimentally confirmed at the energy scales between a GeV and a TeV.