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A tetraquark, in
;Timeline
In 2003, a particle temporarily called
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 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 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 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
Bottom may refer to:
Anatomy and sex
* Bottom (BDSM), the partner in a BDSM who takes the passive, receiving, or obedient role, to that of the top or dominant
* Bottom (sex), a term used by gay couples and BDSM
* Buttocks or bottom, part of th ...
) 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) 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 exp ...
, 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 ...
in 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 north ...
, 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 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 opera ...
'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 discovered by the Belle experiment. It has a mass of . The resonant nature of the peak has been confirmed by the LHCb experiment with a significance of at least 13.9 σ. The particle is charged and is thought ...
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 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 opera ...
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 bill ...
, which may also be a tetraquark.
In 2010, two physicists from DESY 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 dedi ...
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 function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...
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 Zc(3900), the first confirmed four-quark state.
In 2014, the Large Hadron Collider 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)
Z(4430) is a mesonic resonance discovered by the Belle experiment. It has a mass of . The resonant nature of the peak has been confirmed by the LHCb experiment with a significance of at least 13.9 σ. The particle is charged and is thought ...
state with a significance of over 13.9 σ.
In February 2016, the DØ experiment
The DØ experiment (sometimes written D0 experiment, or DZero experiment) was a worldwide collaboration of scientists conducting research on the fundamental nature of matter. DØ was one of two major experiments (the other was the CDF experim ...
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.
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 ...
* Hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the e ...
* Double-charm tetraquark
* 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 baryo ...
*Tetraneutron A tetraneutron is a hypothetical stable cluster of four neutrons. The existence of this cluster of particles is not supported by current models of nuclear forces.
There is some empirical evidence suggesting that this particle does exist, based on ...
References
External links
The Belle experiment
* {{Authority control Mesons Hypothetical composite particles Nuclear physics Quantum chromodynamics