Pentaquark

The five quarks involved in the pentaquark discovered in 2015 (uudc c ). The color charges are chosen arbitrarily.

Pentaquarks (from ancient Greek πέντε pente "five"), a term used in particle physics , are exotic hadrons made up of five quarks with a baryon number of +1 or -1.

General

Pentaquarks, like all structures made up of quarks, are held together by the strong nuclear force . Like all hadrons, they are color-neutral . They are composed of four quarks and one antiquark (total baryon number +1) or four antiquarks and one quark (total baryon number -1). Also, their spin is half-integer, so they are fermions .

In their physical properties, pentaquarks are more similar to other baryons such as protons and neutrons than mesons , which also contain both quarks and antiquarks, but are bosons .

Prediction of existence

The existence of particles with five quarks was predicted by Murray Gell-Mann as early as 1964 . In 1987 they were called "Pentaquarks" by Harry J. Lipkin. In 1997, Dmitri Diakonov , V. A. Petrov and Maxim Polyakov made a concrete prediction - skeptically received by other particle physicists - of a particle with an unusually long lifetime that would lead to a very small and therefore clearly observable total decay width of only 30  MeV . The mass should be 1530 MeV.

Using lattice gauge theories of quantum chromodynamics , further predictions about the properties of pentaquarks have been attempted with the help of computer simulations. However, these theoretical approaches are not very advanced and various research groups have come to conflicting results.

Unsure reports of the discovery

The first experimental observation of the Θ ⁺ was reported in July 2003 by Takashi Nakano of Osaka University ( Japan ) and confirmed by Ken Hicks at Jefferson Laboratory , Virginia , USA . This surprising discovery sparked a wave of investigations into existing data for evidence of the pentaquark. Within a few months, around a dozen different groups reported that they had also discovered evidence for the Θ ⁺ . Some groups even claimed to be able to detect more pentaquarks.

However, doubts, both theoretical and experimental, about the results also emerged. About a dozen other experimental groups found no evidence of the existence of the Θ ⁺ . The experiments also found different masses, some of which were incompatible with each other. Particularly surprising was the small decay width , which was still well below the value predicted by Diakonov, Petrov and Polyakov. The pentaquark would thus live over 100 times longer than other particles of comparable mass.

The CLAS Collaboration at the Jefferson Laboratory in Newport News, Virginia, USA, under the direction of Raffaella de Vita, finally undertook a specific experiment to investigate the Pentaquark Hypothesis. In this most comprehensive investigation to date, there was no evidence of the existence of pentaquarks. As a result, these scientists assume that the previous evidence of pentaquarks is based on misinterpreted data. This work can be found in the April 2005 issue of Nature . The Particle Data Group also came to the conclusion in 2006 and most recently in 2008 that the first reports of a discovery in 2003/2004 (at that time by at least 9 groups in the succession of the first discoverers) were refuted by the majority of the subsequent experiments, which showed significantly higher statistics were.

In 2007, scientists from the GRAAL collaboration found evidence of a very narrow state (a baryon resonance) with a relatively long lifespan (around ten times higher than typical baryon resonances) when a nucleon was bombarded with photons. He was baptized N * (1685) (N-Star). The properties (mass, decay width) coincide with the theoretical predictions for a member with non-exotic quantum numbers of the minimally possible decuplet that includes the hypothetical pentaquark - predictions made by Maxim Polyakov and others in 2004. The experiment in which the N * was discovered was confirmed by scientists at ELSA in Bonn.

In 2013, the DIANA collaboration in Moscow announced that it had observed pentaquarks.

The discovery

Feynman diagram of the decay of the lambda-b-baryon into kaon and pentaquark

On 13 July 2015, researchers reported at LHCb - detector of the Large Hadron Collider of CERN in Geneva by the discovery of two Pentaquarks- Charmonium ; states (pentaquarks involving Charm - and anti -Charm quarks) the decay of the Lambda-b - baryon into the kaon and the pentaquark (uudc c ). The two observations each showed a statistical significance of more than 9  σ , which clearly exceeds the threshold of 5 σ for a discovery that is usual in particle physics . ${\displaystyle \Lambda _{b}^{0}\ }$ ${\displaystyle \mathrm {K} ^{-}}$

The two observed pentaquark charmonium states P _c (4380) ⁺ and P _c (4450) ⁺ consist of two up quarks , a down quark , a charm quark and an anti-charm quark. The exact binding mechanism of the five quarks is still unclear: it is either a solid structure made up of five closely spaced quarks or a kind of loose structure made up of three and two closely spaced quarks.

See also

• Tetraquark

Web links

• home.web.cern.ch
• Existential fear of the Pentaquark on Wissenschaft.de, 2005
• CERN researchers-have-Pentaquark-state after , heise.de
• Danilov, Mizuk: "Experimental Review on Pentaquarks", July 2007 arxiv : 0704.3531
• Kandice Carter, "The rise and fall of the pentquark," Symmetry Magazine, September 2006

Individual evidence

1. ↑ ^{a b} Observation of particles composed of five quarks, pentaquark-charmonium states, seen in decays. July 14, 2015, accessed July 14, 2015 . ${\displaystyle \Lambda _{b}^{0}\to J/\psi pK^{-}}$ 
2. ^ HJ Lipkin: New possibilities for exotic hadrons - anticharmed strange baryons . In: Physics Letters B . 195, No. 3, 1987, pp. 484-488. bibcode : 1987PhLB..195..484L . doi : 10.1016 / 0370-2693 (87) 90055-4 .
3. ↑ arxiv : hep-ph / 9703373
4. ^ Mark Peplow: Doubt is cast on pentaquarks. In: news @ nature. 2005, S., doi: 10.1038 / news050418-1 .
5. ↑ Update of the PDG on Pentaquarks 2008 (PDF)
6. ↑ Polyakov and others, Preprint 2004, arxiv : nucl-th / 0312126 ; independently from Diakonov and others 2004.
7. ^ V. Kuznetsov, MV Polyakov: New narrow nucleon N * (1685). In: JETP Letters. Springer, 2008. arxiv : 0807.3217
8. ↑ DIANA collaboration: Observation of a narrow baryon resonance with positive strangeness formed in K ⁺ Xe collisions . July 5, 2013, arxiv : 1307.1653 (English).
9. ↑ ^{a b} Observation of J / ψp resonances consistent with pentaquark states in decays . In: Phys. Rev. Lett. tape ${\displaystyle \Lambda _{b}^{0}\to J/\psi K^{-}p}$  115 , July 13, 2015, p. 072001 , arxiv : 1507.03414 (English).
10. ↑ Ian Sample: Large Hadron Collider scientists discover new particles: pentaquarks. The Guardian, July 14, 2015, accessed July 14, 2015 . 
11. ↑ CERN's LHCb experiment reports observation of exotic pentaquark particles, CERN July 14, 2015