A tetraquark, in

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

, is an

exotic meson 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: Tetraquarks have t ...

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 nuclei. All commonly ...

s. A tetraquark state has long been suspected to be allowed by

quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the theory 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 ...

, the modern theory of

strong interaction The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called th ...

s. A tetraquark state is an example of an

exotic hadron Exotic hadrons are subatomic particles composed of quarks and gluons, but which – unlike "well-known" hadrons such as protons, neutrons and mesons – consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just tw ...

which 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 which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Ei ...

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 Charm may refer to: Social science * Charisma, a person or thing's pronounced ability to attract others * Superficial charm, flattery, telling people what they want to hear Science and technology * Charm quark, a type of elementary particle * Ch ...

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. TQ EB ape hyp r1 8 r2 14 Act 3D Sim

;Timeline In 2003, a particle temporarily called

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 because of its quantum numbers. It was first discovered in 2003 by the Belle experiment in Japan and later confirmed by several other ex ...

, 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, Ibarak ...

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...

, 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 D_sJ(2632) state seen in

Fermilab Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy national laboratory specializing in high-energy particle physics. Since 2007, Fermilab has been oper ...

's SELEX was suggested as a possible tetraquark candidate. In 2007, Belle announced the observation of the Z(4430) 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,

announced that they have discovered a particle temporarily called

Y(4140) The Y(4140) particle is an electrically neutral exotic hadron candidate that is about 4.4 times heavier than the proton. It was observed at Fermilab and announced on 17 March 2009. This particle is extremely rare and was detected in only 20 of billi ...

, which may also be a tetraquark. In 2010, two physicists from

DESY The Deutsches Elektronen-Synchrotron (English ''German Electron Synchrotron''), commonly referred to by the abbreviation DESY, is a national research center in Germany. It operates particle accelerators used to investigate the structure of mat ...

and a physicist from

Quaid-i-Azam University Quaid-i-Azam University Islamabad ( ur, ; commonly referred to as QAU), founded as University of Islamabad, is a ranked 1 public research university in Islamabad, Pakistan. Founded as the University of Islamabad in 1967, it was initially d ...

re-analyzed former experimental data and announced that, in connection with the (5S) meson (a form of

bottomonium 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. Light quarks Light quarks ( up ...

), a well-defined tetraquark

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscil ...

exists. In June 2013, 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 ...

experiment in China and the Belle experiment in Japan independently reported on Z_c(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 collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundr ...

experiment

LHCb The LHCb (Large Hadron Collider beauty) experiment is one of eight particle physics detector experiments collecting data at the Large Hadron Collider at CERN. LHCb is a specialized b-physics experiment, designed primarily to measure the paramet ...

confirmed the existence of the Z(4430) 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 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 observed X(6900). In 2021, LHCb announced the discovery of four additional tetraquarks, including cu. In 2022, LHCb announced the discovery of cu and cd.

References

External links

The Belle experiment
* {{Authority control Mesons Hypothetical composite particles Nuclear physics Quantum chromodynamics

History and discoveries

See also

References

External links