Exotic hadrons are

subatomic particles In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a baryon, like ...

composed of

quarks A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...

and

gluons A gluon ( ) is a type of massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a spin of 1. Through the s ...

, but which – unlike "well-known"

hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

s such as

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s and

meson In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...

s – consist of more than three

valence quark In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks that give rise to the quantum numbers of the hadrons. The quark model underlies Flavour (particle physics), ...

s. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral. Consistent with ordinary hadrons, exotic hadrons are classified as being either

fermion In particle physics, a fermion is a subatomic particle that follows Fermi–Dirac statistics. Fermions have a half-integer spin (spin 1/2, spin , Spin (physics)#Higher spins, spin , etc.) and obey the Pauli exclusion principle. These particles i ...

s, like ordinary baryons, or

boson In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0, 1, 2, ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have half odd-intege ...

s, like ordinary mesons. According to this classification scheme,

pentaquark A pentaquark is a human-made subatomic particle, consisting of four quarks and one antiquark bound together; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them. As quarks have a bar ...

s, containing five valence quarks, are exotic baryons, while tetraquarks (four valence quarks) and hexaquarks (six quarks, consisting of either a dibaryon or three quark-antiquark pairs) would be considered

exotic meson In particle physics, exotic mesons are mesons that have quantum numbers not possible in the quark model; some proposals for non-standard quark model mesons could be: ;glueballs or gluonium: Glueballs have no valence quarks at all. ;tetraquarks: ...

s. Tetraquark and pentaquark particles are believed to have been observed and are being investigated; hexaquarks have not yet been confirmed as observed. Exotic hadrons can be searched for by looking for

S-matrix In physics, the ''S''-matrix or scattering matrix is a Matrix (mathematics), matrix that relates the initial state and the final state of a physical system undergoing a scattering, scattering process. It is used in quantum mechanics, scattering ...

poles with

quantum numbers In Quantum mechanics, quantum physics and chemistry, quantum numbers are quantities that characterize the possible states of the system. To fully specify the state of the electron in a hydrogen atom, four quantum numbers are needed. The traditi ...

forbidden to ordinary hadrons. Experimental signatures for such exotic hadrons had been seen by 2003 at the latest, but they remain a topic of controversy in

particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

. Jaffe and Low suggested that the exotic hadrons manifest themselves as poles of the P matrix, and not of the S matrix. Experimental

P-matrix In mathematics, a -matrix is a complex square matrix with every principal minor is positive. A closely related class is that of P_0-matrices, which are the closure of the class of -matrices, with every principal minor \geq 0. Spectra of -matric ...

poles are determined reliably in both the meson–meson channels and nucleon–nucleon channels.

History

When the quark model was first postulated by

Murray Gell-Mann Murray Gell-Mann (; September 15, 1929 – May 24, 2019) was an American theoretical physicist who played a preeminent role in the development of the theory of elementary particles. Gell-Mann introduced the concept of quarks as the funda ...

and others in the 1960s, it was to organize the states known then to be in existence in a meaningful way. As

quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...

(QCD) developed over the next decade, it became apparent that there was no reason why only three-quark and quark-antiquark combinations could exist. Indeed, Gell-Mann's original 1964 paper alludes to the possibility of exotic hadrons and classifies hadrons into baryons and mesons depending upon whether they have an odd (baryon) or even (meson) number of valence quarks. In addition, it seemed that gluons, the mediator particles of the strong interaction, could also form bound states by themselves ( glueballs) and with quarks ( hybrid hadrons). Several decades have passed without conclusive evidence of an exotic hadron that could be associated with the S-matrix pole. In April 2014, the

LHCb The LHCb (Large Hadron Collider beauty) experiment is a particle physics detector collecting data at the Large Hadron Collider at CERN. LHCb specializes in the measurements of the parameters of CP violation in the interactions of b- and c-hadro ...

collaboration confirmed the existence of the Z(4430)⁻, discovered by the

Belle experiment The Belle experiment was a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers, at the High Energy Accelerator Research Organisation ( KEK) in Tsukuba, Ibara ...

, and demonstrated that it must have a minimal quark content of cd. In July 2015, LHCb announced the discovery of two particles, named and , which must have minimal quark content cuud, making them

Candidates

Notes

{{Authority control Hadrons Hypothetical composite particles

History

Candidates

See also

Notes