Pentaquarks are exotic baryons made of four quarks and one antiquark. While there is some evidence for their existence, data are controversial and the existence of pentaquarks is neither firmly established nor generally accepted.
Generally speaking, baryons must have three more quarks than antiquarks. A baryon made of five quarks and two antiquarks would be a heptaquark, one made of six quarks and three antiquarks would be a nonaquark, and so on. It is not known if these particles can even exist.
These are fermions such as a neutron or a proton, made up of three quarks.
Diquarks are mesons made of one quark and one antiquark, such as the rho meson and the kaon.
Tetraquarks are mesons made of two quarks and two antiquarks. While there is some evidence for their existence, data are controversial and the existence of tetraquarks is neither firmly established nor generally accepted.
Generally speaking, mesons must have an equal number of quarks and antiquarks. A meson made of three quarks and three antiquarks would be a hexaquark, one made of four quarks and four antiquarks would be a octaquark, and so on. It is not known if these particles can even exist.
It should be noted that while mesons are composite bosons, they are not made of bosons. Rather they are bosons made of quarks, which are fermions.
Most hadrons can be classified by the quark model which posits that all the quantum numbers are derived from those of the valence quarks (the quarks which form the hadron). For instance, since each quark has B=1/3, each baryon, composed of three quarks, has B=1.
Excited baryon or meson states are known as resonances. Each ground state hadron may have many excited states, and hundreds have been observed in particle experiments. Resonances decay extremely quickly (within about 10−24 s) via strong interactions.
Mesons which lie outside the quark model classification are called exotic mesons. These include glueballs, hybrid mesons and tetraquarks. The only baryons which lie outside the quark model at present are the pentaquarks, but evidence for their existence is unclear as of 2006.
All hadrons are single particle excitations of the basic theory of strong interactions, called quantum chromodynamics. Due to a property called confinement that this theory enjoys at energies below the QCD scale, these excitations are not quarks and gluons, which are the basic fields, but the hadrons which are composite, and carry no color charge.
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, QCD predicts that quarks and gluons will interact weakly and in particular no longer be confined. This property, which is known as asymptotic freedom, has been experimentally confirmed at the energy scales between a GeV and a TeV.
