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 ...

, XYZ particles (also referred to as XYZ states) are recently discovered heavy

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 whose properties do not appear to fit the standard picture of

charmonium In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. The name "quarkonium" is analogo ...

and bottomonium states. They are therefore types of

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. The term arises from the names given to some of the first such particles discovered:

X(3872) The X(3872) is an exotic meson candidate with a mass of 3871.68 MeV/c2 which does not fit into the quark model. It was first discovered in 2003 by the Belle experiment in Japan and later confirmed by several other experimental collaborations. Seve ...

, Y(4260) and Z_c(3900), although the symbols X and Y have since been deprecated by the

Particle Data Group The Particle Data Group (PDG) is an international collaboration of particle physicists that compiles and reanalyzes published results related to the properties of particles and fundamental interactions. It also publishes reviews of theoretical ...

Theoretical significance

Since 2003 a frontier for the Standard Model (SM) has emerged at low energies through XYZ particle discoveries. The well-established theory of

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) is tested by many exotic charmonium discoveries since the X(3872) was first identified at 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 ...

in 2003. The basic model of hadron physics is the assembling 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 ...

into groups of 3 (

baryons In particle physics, a baryon is a type of composite subatomic particle that contains an odd number of valence quarks, conventionally three. Protons and neutrons are examples of baryons; because baryons are composed of quarks, they belong to ...

) or a quark and anti-quark pair (

mesons 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 ...

). A meson with a

charm quark The charm quark, charmed quark, or c quark is an elementary particle found in composite subatomic particles called hadrons such as the J/psi meson and the charmed baryons created in particle accelerator collisions. Several bosons, including th ...

and an anti-charm quark is called ''charmonium'', and the same parallels with the

bottom quark The bottom quark, beauty quark, or b quark, is an elementary particle of the third generation. It is a heavy quark with a charge of − ''e''. All quarks are described in a similar way by electroweak interaction and quantum chromodynamic ...

and bottomonium. More than two dozen previously unpredicted charmonium- and bottomonium-like states have been discovered, and the understanding of heavy

quarkonium In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavor (physics), flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. The name "quarko ...

physics is undetermined. Previously postulated exotic Standard Model states might apply to these new unique particles. One proposed state is the hybrid state of a quark, anti-quark, and a

gluon A gluon ( ) is a type of Massless particle, 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 (physi ...

, sometimes mentioned with charm quarks as an excited charmonium. A multi-quark state of 4 or more quarks (

tetraquark In particle physics, a tetraquark is an exotic meson composed of four valence quarks. A tetraquark state has long been suspected to be allowed by quantum chromodynamics, the modern theory of strong interactions. A tetraquark state is an example ...

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 ...

, etc.) is also proposed as well as a molecule-like state of multiple mesons. While each of these three types of states have had some success of explaining the newly discovered particle, a complete explanation has not been found.

Types of particle

The first charmonium state with an unpredicted mass was

. The Belle collaboration was searching for the B -> K π+ π- J/ψ decay when they discovered a peak in the π+ π- J/ψ invariant energy at 3872 with JCP

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 ...

of 1++. X(3872) was quickly confirmed by

BaBar Babar (), also variously spelled as Baber, Babur, and Babor is a male given name of Persian language, Persian origin, and a popular male given name in Pakistan. It is generally taken in reference to the Persian language, Persian ''babr'' (Persian ...

, CDF, and D0. The mass of X(3872) is close to the mass of DD* and makes it a candidate as a meson molecule or a possible tetraquark. In 2005 the

collaboration found Y(4260) from Initial state radiation as well in π+ π- J/ψ production. Again a charmonium-like particle with a large coupling to final states without open charm mesons. Continued search shows a lack of an observation in the inclusive hadronic cross section. The

BES III The Beijing Spectrometer III (BES III) is a particle physics experiment at the Beijing Electron–Positron Collider II (BEPC II) at the Institute of High Energy Physics (IHEP). It is designed to study the physics of charm, charmonium, and light ha ...

collaboration in 2012 started taking data at 4260

MeV In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an electric potential difference of one volt in vacuum. When us ...

and could observe direct production instead of B decay or Initial State Radiation to continue the study with a higher luminosity. The Z_c(3900) state was discovered at BESIII in 2013.

References

{{reflist Mesons

Theoretical significance

Types of particle

See also

References