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In 2003, a particle temporarily called X(3872), by the
The Belle experiment
* {{Authority control Mesons Hypothetical composite particles Nuclear physics Quantum chromodynamics
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 ...
, a tetraquark is an 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: ...
composed of four valence quark
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 nucleus, atomic nuclei ...
s. A tetraquark state has long been suspected to be allowed by 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 ...
, the modern theory of strong interaction
In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interaction, fundamental interactions. It confines Quark, quarks into proton, protons, n ...
s. A tetraquark state is an example of an exotic hadron that lies outside the conventional quark model
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 "flavor SU(3)", or the Eig ...
classification. A number of different types of tetraquark have been observed.
History and discoveries
Several tetraquark candidates have been reported by particle physics experiments in the 21st century. The quark contents of these states are almost all qQ, where q represents a light ( up, down or strange) quark, Q represents a heavy ( charm or bottom) quark, and antiquarks are denoted with an overline. The existence and stability of tetraquark states with the qq (or QQ) have been discussed by theoretical physicists for a long time, however these are yet to be reported by experiments.
;Timeline
In 2003, a particle temporarily called X(3872), 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 ...
in Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
, was proposed to be a tetraquark candidate, as originally theorized. The name X is a temporary name, indicating that there are still some questions about its properties to be tested. The number following is the mass of the particle in .
In 2004, the DsJ(2632) state seen in Fermilab
Fermi National Accelerator Laboratory (Fermilab), located in Batavia, Illinois, near Chicago, is a United States Department of Energy United States Department of Energy National Labs, national laboratory specializing in high-energy particle phys ...
's SELEX was suggested as a possible tetraquark candidate.
In 2007, Belle announced the observation of the Z(4430)
Z(4430) is a mesonic resonance
Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a ...
state, a tetraquark candidate. There are also indications that the Y(4660), also discovered by Belle in 2007, could be a tetraquark state.
In 2009, Fermilab
Fermi National Accelerator Laboratory (Fermilab), located in Batavia, Illinois, near Chicago, is a United States Department of Energy United States Department of Energy National Labs, national laboratory specializing in high-energy particle phys ...
announced that they have discovered a particle temporarily called Y(4140), which may also be a tetraquark.
In 2010, two physicists from DESY and a physicist from Quaid-i-Azam University re-analyzed former experimental data and announced that, in connection with the (5S) meson (a form of bottomonium), a well-defined tetraquark resonance
Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
exists.
In June 2013, the BES III experiment in China and the Belle experiment in Japan independently reported on Zc(3900), the first confirmed four-quark state.
In 2014, the Large Hadron Collider
The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It was built by the CERN, European Organization for Nuclear Research (CERN) between 1998 and 2008, in collaboration with over 10,000 scientists, ...
experiment 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 ...
confirmed the existence of the Z(4430)
Z(4430) is a mesonic resonance
Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a ...
state with a significance of over 13.9 σ.
In February 2016, the DØ experiment reported evidence of a narrow tetraquark candidate, named X(5568), decaying to .
In December 2017, DØ also reported observing the X(5568) using a different final state.
However, it was not observed in searches by the LHCb, CMS, CDF, or ATLAS
An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets.
Atlases have traditio ...
experiments.
In June 2016, LHCb announced the discovery of three additional tetraquark candidates, called X(4274), X(4500) and X(4700).
In 2020, LHCb announced the discovery of a
tetraquark: X(6900).
In 2022, ATLAS also observed X(6900), and in 2023, CMS reported an observation of three such states, X(6600), X(6900), and X(7300).
In 2021, LHCb announced the discovery of four additional tetraquarks, including cu.
In 2022, LHCb announced the discovery of cu and cd.
See also
*Color confinement
In quantum chromodynamics (QCD), color confinement, often simply called confinement, is the phenomenon that color-charged particles (such as quarks and gluons) cannot be isolated, and therefore cannot be directly observed in normal conditions b ...
* Double-charm tetraquark
* 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 ...
* 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 ...
* Tetraneutron
References
External links
The Belle experiment
* {{Authority control Mesons Hypothetical composite particles Nuclear physics Quantum chromodynamics