Tetraquark

Tetraquarks (from the Greek tetra , four) are hadrons that are composed of four quarks (two quarks and two antiquarks ). Like the pentaquarks and hexaquarks, they belong to the exotic hadrons , i. H. to those hadrons that are not composed of two quarks like mesons (quark-antiquark pair) or like baryons of three quarks. The existence of tetraquarks had long been discussed by theoretical physicists.

On June 28, 2016, the LHCb experiment at CERN announced the observation of tetraquarks, thereby confirming the tetraquark model that has been controversial for decades. Before that, there had been many alleged observations of a tetraquark, but these turned out to be incorrect or the interpretation was controversial.

Proof at the LHCb from 2016

In June 2016, LHCb announced that it had seen four resonances of a tetraquark during B meson decays . All consist of a Strange -Antistrange-Quark pair and a Charm -Anticharm-Quark pair ( ) and split into a φ meson and a J / ψ meson . They were named: X (4140), X (4274), X (4500) and X (4700); the number in brackets stands for the respective mass of the particle in the unit MeV . X (4274), X (4500) and X (4700) are excited states of X (4140). ${\displaystyle c,{\bar {c}},s,{\bar {s}}}$

According to the LHCb collaboration, the existence of the states was established with a certainty of five standard deviations and can not be explained by common hadrons. The exact nature of the bond is still the subject of discussion after the publication in June 2016: closely bonded tetraquarks or pairs of D mesons with a strange quark as a second partner ( ), which repel and become J / ψφ (a "cusp") ) arrange. ${\displaystyle D_{s}D_{s}^{*}}$

The X (4140) state had already been found by the CDF collaboration in 2009 and confirmed by the CMS and D0 collaborations (while the search by Belle and the BaBar collaborations for the X (4140) state was negative).

In 2020, tetraquark states consisting of two charm and two anticharm quarks were detected at the LHC b (X (6900) resonance at 6.9 GeV) decaying into two J / ψ mesons .

Announced evidence of the Tetraquark 2013

Two teams independently reported the discovery of the short-lived four-quark resonance Z _c (3900) , the Belle team at KEK in Tsukuba, Japan, and the Beijing Spectrometer III ( BES III ) at the Beijing Electron Positron Collider in Beijing . The name indicates the mass (3900 MeV, roughly four times the mass of the proton). It is simply positively charged and probably consists of ( , , , ) quarks. The Belle team had already reported candidates for tetraquarks in 2008 and 2011, but these were not confirmed by other accelerators. The original search for the Y (4260) tetraquark candidate, which was observed at SLAC in 2005. However, there are still theoretical differences about the interpretation of the newly discovered four-quark states as genuine tetraquarks or the bond between two mesons (meson molecules). ${\displaystyle u}$ ${\displaystyle {\bar {d}}}$ ${\displaystyle c}$ ${\displaystyle {\bar {c}}}$

Announced evidence of the Tetraquark 2014

In April 2014, researchers in the LHCb experiment confirmed the existence of Z (4430) , a tetraquark candidate that had already been observed by the Belle collaboration.

In the interpretation of the candidates discussed earlier, two theories faced each other, one that wanted to describe them using a model of two bound color-charged diquarks (the two quarks and antiquarks each form a diquark), i.e. a new type of particle (tetraquark), the other interpreted this in a more conventional way as a short-term resonance of two color-neutral mesons (each consisting of a quark and an antiquark). One problem with meson theory is the resulting weak bond between the two color-neutral mesons in the QCD, which makes it difficult to imagine why they even occur in the high-energy proton collisions at the LHC. One problem with diquarial theory is the fact that many of the candidates have a mass almost exactly that of two mesons. The meson theory was therefore the favorite for some of the resonances, such as the X (3872) found in Japan in 2003, because their mass was close to the sum of the mesons composing them according to the meson theory. For the candidates found at the LHC 2014, however, an explanation in meson theory from the observed properties (e.g. mass, spin parity) seems difficult, which suggests the discovery of a new type of strongly interacting particle (tetraquarks). An explanation within the QCD is still pending.

Further announcements of the detection of tetraquarks

In 2010 a group of German and Pakistani physicists (Ahmed Ali and Christian Hambrock from DESY , Jamil Aslam from Quaid-i-Azam University) announced the possible discovery of a tetraquark, which was made possible by re-analyzing data from the Belle experiment from 2008 . At that time, compared to the theoretical expectations (of quantum chromodynamics ), several orders of magnitude higher decay rates of what was assumed to be an excited Bottomonium state Y (5 S) into lower states with the generation of a pair of charged pions were observed. Ali, Hambrock and Aslam interpret the initial state as Tetraquark Y _b (10890) from two light - or quarks and two heavy - or quarks. The authors state that they can reproduce the masses and decay rates well with their model. But there are also alternative explanations. ${\displaystyle \,u}$ ${\displaystyle {\overline {u}}}$ ${\displaystyle \,b}$ ${\displaystyle {\overline {b}}}$

Naming

Suspected tetraquarks were mostly denoted by the symbols X, Y or Z, depending on the discoverer.

At the end of 2017, the naming scheme for hadrons was expanded by the Particle Data Group  (PDG). The naming of tetraquarks follows the naming of mesons .

As of 2018, some Tetraquark candidates will appear under a different name:

new PDG name	earlier names
χ c1 (3872)	X (3872)
χ c1 (4140)	X (4140), Y (4140)
χ c1 (4274)	X (4274), Y (4274)
χ c0 (4500)	X (4500)
χ c0 (4700)	X (4700)
ψ (4260)	X (4260), Y (4260)
Z c (3900)	X (3900), Z (3900)
Z c (4430)	X (4430), Z (4430)

Individual evidence

1. ↑ LHCb Collaboration (R. Aaij et al.): Observation of J / ψφ structures consistent with exotic states from amplitude analysis of B + → J / ψφK + decays, LHCb-PAPER-2016-018, CERN-EP-2016-155, Arxiv
2. ↑ LHCb Collaboration: Amplitude analysis of B + → J / ψφK + decay, LHCb-PAPER-2016-019, CERN-EP-2016-156, Arxiv
3. ↑ LHCb, CERN, June 28, 2016: Observation of four exotic-like particles
4. ↑ Stefania Pandolfi, LHCb unveils new particle, CERN July 2016
5. ↑ arXiv: 2006.16957 LHCb collaboration: Observation of structure in the J / Psi mass spectrum, Arxiv, June 30, 2020
6. ↑ Liu, ZQ and a. Study of e + e− → π + π − J / ψ and Observation of a Charged Charmoniumlike State at Belle , Phys. Rev. Lett. 110, 252002 (2013), abstract .
7. ↑ Devin Powell: Quark quartet opens fresh vista on matter , Nature News of June 18, 2013
8. ↑ Manfred Lindinger: Tetraquark Mighty new addition to the particle zoo , FAZ Knowledge from June 20, 2013.
9. ↑ Eric Swanson: Viewpoint: New particle hints at four quark matter , APS, Physics 6, 2013, 69.
10. ↑ Ablikim, M. et al. Observation of a Charged Charmoniumlike Structure in e + e− → π + π − J / ψ at s√ = 4.26 GeV , Phys. Rev. Lett. 110, 252001 (2013), abstract .
11. ^ LHC spots particle that may be new form of matter. In: NewScientist , April 10, 2014.
12. ↑ How CERN's Discovery of Exotic Particles May Affect Astrophysics. In: Universe Today , April 10, 2014.
13. ↑ Natalie Wolchover: Quantum quartet fuels quantum feud, Quanta ( Memento of the original from October 14, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Simons Foundation, August 27, 2014 
14. ↑ Ali, Hambrock, Aslam: Tetraquark Interpretation of the BELLE Data on the Anomalous Υ (1S) π + π- and Υ (2S) π + π- Production near the Υ (5S) Resonance , Physical Review Letters Vol. 104, 2010 , P. 162001.
15. ^ ^{A b} Zoe Matthews: Evidence grows for Tetraquarks , Physics World, Online News Archive, April 27, 2010.
16. ↑ Or from a superposition of the corresponding tetraquark with down instead of up quarks, which according to Ali, Hambrock and Aslam should have about the same mass.
17. ↑ Particle Data Group: Naming scheme for hadrons (Revised in 2017). (PDF; 86 KB) Retrieved March 14, 2018 (English).