Pentaquark
Encyclopedia
A pentaquark is a hypothetical subatomic particle
consisting of four quark
s and one antiquark bound together
(compared to three quarks in normal baryon
s). As quarks have a baryon number of +, and antiquarks of −, it would have a total baryon number of 1, thus being classified as an exotic baryon
. The existence of pentaquarks was originally hypothesized by Michał Praszałowicz in 1987.
Several experiments reported the existence of pentaquark states in the mid 2000s, but later experiments and re-analysis of the data showed them to be statistical effects rather than true resonances
.
with a mass
of (4.6 σ
) was reported by LEPS
in 2003, the . This coincided with a pentaquark state with a mass of predicted in 1997 by Dmitri Diakonov, Victor Petrov, and Maxim Polyakov.
This state was presumably composed of two up quark
s, two down quark
s, and one strange antiquark (uudd). Following this announcement, nine other independent experiments reported seeing narrow peaks from and , with masses between and , all above 4 σ. While concerns existed about the validity of these states, the Particle Data Group
gave the a 3-star rating (out of 4) in the 2004 Review of Particle Physics. Two other pentaquark states were reported albeit with low statistical significance—the (ddss), with a mass of and the (uudd), with a mass of . Both were later found to be statistical effects rather than true resonances.
Ten experiments then looked for the , but came out empty-handed. Two in particular (one at BELLE
, and the other at CLAS
) had nearly the same conditions as other experiments which claimed to have detected the (DIANA and SAPHIR respectively). The 2006 Review of Particle Physics concluded:
The 2008 Review of Particle Physics went even further:
Despite these null result
s, LEPS results continue to show the existence of a narrow state with a mass of , with a statistical significance
of 5.1 σ. Experiments continue to study this controversy.
|journal=Journal of Physics: Conference Series
|volume=9 |issue= |pages=183
|doi=10.1088/1742-6596/9/1/035
|arxiv=hep-ex/0412048
|bibcode = 2005JPhCS...9..183H }}
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...
consisting of four quark
Quark
A quark is an 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. Due to a phenomenon known as color confinement, quarks are never directly...
s and one antiquark bound together
Bound state
In physics, a bound state describes a system where a particle is subject to a potential such that the particle has a tendency to remain localised in one or more regions of space...
(compared to three quarks in normal baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
s). As quarks have a baryon number of +, and antiquarks of −, it would have a total baryon number of 1, thus being classified as an exotic baryon
Exotic baryon
Exotic baryons are hypothetical composite particles which are bound states of 3 quarks and additional elementary particles. This is to be contrasted with ordinary baryons, which are bound states of just 3 quarks. The additional particles may include quarks, antiquarks or gluons.One such exotic...
. The existence of pentaquarks was originally hypothesized by Michał Praszałowicz in 1987.
Several experiments reported the existence of pentaquark states in the mid 2000s, but later experiments and re-analysis of the data showed them to be statistical effects rather than true resonances
Resonance (particle physics)
In particle physics, a resonance is the peak located around a certain energy found in differential cross sections of scattering experiments. These peaks are associated with subatomic particles and their excitations...
.
History
In the mid 2000s, several experiments claimed to reveal pentaquark states. In particular, a resonanceResonance (particle physics)
In particle physics, a resonance is the peak located around a certain energy found in differential cross sections of scattering experiments. These peaks are associated with subatomic particles and their excitations...
with a mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of (4.6 σ
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
) was reported by LEPS
LEPS
LEPS is an experiment taking place at the SPring-8 synchrotron facilities. It produces high-energy photon beams by the backward Compton scattering of photons upon the 8 GeV electrons of the SPring-8 synchrotron. These are then used for various particle physics experiments on hadrons...
in 2003, the . This coincided with a pentaquark state with a mass of predicted in 1997 by Dmitri Diakonov, Victor Petrov, and Maxim Polyakov.
This state was presumably composed of two up quark
Up quark
The up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, forms the neutrons and protons of atomic nuclei...
s, two down quark
Down quark
The down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
s, and one strange antiquark (uudd). Following this announcement, nine other independent experiments reported seeing narrow peaks from and , with masses between and , all above 4 σ. While concerns existed about the validity of these states, the Particle Data Group
Particle Data Group
The Particle Data Group is an international collaboration of particle physicists that compiles and reanalyzes published results related to the properties of particles and fundamental interactions. It also publishes reviews of theoretical results that are phenomenologically relevant, including...
gave the a 3-star rating (out of 4) in the 2004 Review of Particle Physics. Two other pentaquark states were reported albeit with low statistical significance—the (ddss), with a mass of and the (uudd), with a mass of . Both were later found to be statistical effects rather than true resonances.
Ten experiments then looked for the , but came out empty-handed. Two in particular (one at BELLE
Belle experiment
The Belle experiment is a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers investigating CP-violation effects at the High Energy Accelerator Research Organisation in Tsukuba, Ibaraki Prefecture, Japan.The...
, and the other at CLAS
CLAS detector
CEBAF Large Acceptance Spectrometer is a nuclear and particle physics detector located in the experimental Hall B at Jefferson Laboratory in Newport News, Virginia, USA...
) had nearly the same conditions as other experiments which claimed to have detected the (DIANA and SAPHIR respectively). The 2006 Review of Particle Physics concluded:
[T]here has not been a high-statistics confirmation of any of the original experiments that claimed to see the ; there have been two high-statistics repeats from Jefferson Lab that have clearly shown the original positive claims in those two cases to be wrong; there have been a number of other high-statistics experiments, none of which have found any evidence for the ; and all attempts to confirm the two other claimed pentaquark states have led to negative results. The conclusion that pentaquarks in general, and the , in particular, do not exist, appears compelling.
The 2008 Review of Particle Physics went even further:
There are two or three recent experiments that find weak evidence for signals near the nominal masses, but there is simply no point in tabulating them in view of the overwhelming evidence that the claimed pentaquarks do not exist... The whole story—the discoveries themselves, the tidal wave of papers by theorists and phenomenologists that followed, and the eventual "undiscovery"—is a curious episode in the history of science.
Despite these null result
Null result
In science, a null result is a result without the expected content: that is, the proposed result is absent. It is an experimental outcome which does not show an otherwise expected effect. This does not imply a result of zero or nothing, simply a result that does not support the hypothesis...
s, LEPS results continue to show the existence of a narrow state with a mass of , with a statistical significance
Statistical significance
In statistics, a result is called statistically significant if it is unlikely to have occurred by chance. The phrase test of significance was coined by Ronald Fisher....
of 5.1 σ. Experiments continue to study this controversy.
See also
- TetraquarkTetraquarkIn particle physics a tetraquark is a hypothetical meson composed of four valence quarks. In principle, a tetraquark state may be allowed in quantum chromodynamics, the modern theory of strong interactions. However, there has been no confirmed report of a tetraquark state to date...
- Exotic baryonExotic baryonExotic baryons are hypothetical composite particles which are bound states of 3 quarks and additional elementary particles. This is to be contrasted with ordinary baryons, which are bound states of just 3 quarks. The additional particles may include quarks, antiquarks or gluons.One such exotic...
- Exotic hadronExotic hadronExotic hadrons are subatomic particles made of quarks , but which do not fit into the usual scheme of hadrons. While bound by the strong interaction they are not predicted by the simple quark model...
- Quark modelQuark modelIn 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....
- Skyrme model
- List of particles
Further reading
Pentaquark|journal=Journal of Physics: Conference Series
Journal of Physics: Conference Series
Journal of Physics: Conference Series is a peer-reviewed, open-access journal providing readers with the latest developments in physics presented at international conferences....
|volume=9 |issue= |pages=183
|doi=10.1088/1742-6596/9/1/035
|arxiv=hep-ex/0412048
|bibcode = 2005JPhCS...9..183H }}