Pentaquark

A pentaquark is a subatomic particle consisting of four quarks and one antiquark bound together.

As quarks have a baryon number of +1/3, and antiquarks of −1/3, the pentaquark would have a total baryon number of 1, and thus would be a baryon. Further, because it has five quarks instead of the usual three found in regular baryons (a.k.a. 'triquarks'), it is classified as an exotic baryon. The name pentaquark was coined by Claude Gignoux et al.[1] and Harry J. Lipkin in 1987;[2] however, the possibility of five-quark particles was identified as early as 1964 when Murray Gell-Mann first postulated the existence of quarks.[3] Although predicted for decades, pentaquarks proved surprisingly difficult to discover and some physicists were beginning to suspect that an unknown law of nature prevented their production.[4]

The first claim of pentaquark discovery was recorded at LEPS in Japan in 2003, and several experiments in the mid-2000s also reported discoveries of other pentaquark states.[5] Others were not able to replicate the LEPS results, however, and the other pentaquark discoveries were not accepted because of poor data and statistical analysis.[6] On 13 July 2015, the LHCb collaboration at CERN reported results consistent with pentaquark states in the decay of bottom Lambda baryons (Λ0
b
).[7] On 26 March 2019, the LHCb collaboration announced the discovery of a new pentaquark that had not been previously observed.[8] The observations pass the 5-sigma threshold required to claim the discovery of new particles.

Outside particle physics laboratories, pentaquarks also could be produced naturally by supernovae as part of the process of forming a neutron star.[9] The scientific study of pentaquarks might offer insights into how these stars form, as well as allowing more thorough study of particle interactions and the strong force.

Pentaquark-generic
A five-quark "bag"
Meson-Baryon-molecule-generic
A "meson-baryon molecule"

Background

A quark is a type of elementary particle that has mass, electric charge, and colour charge, as well as an additional property called flavour, which describes what type of quark it is (up, down, strange, charm, top, or bottom). Due to an effect known as colour confinement, quarks are never seen on their own. Instead, they form composite particles known as hadrons so that their colour charges cancel out. Hadrons made of one quark and one antiquark are known as mesons, while those made of three quarks are known as baryons. These 'regular' hadrons are well documented and characterized; however, there is nothing in theory to prevent quarks from forming 'exotic' hadrons such as tetraquarks with two quarks and two antiquarks, or pentaquarks with four quarks and one antiquark.[4]

Structure

Pentaquark
A diagram of the P+
c
type pentaquark possibly discovered in July 2015, showing the flavours of each quark and one possible colour configuration.

A wide variety of pentaquarks are possible, with different quark combinations producing different particles. To identify which quarks compose a given pentaquark, physicists use the notation qqqqq, where q and q respectively refer to any of the six flavours of quarks and antiquarks. The symbols u, d, s, c, b, and t stand for the up, down, strange, charm, bottom, and top quarks respectively, with the symbols of u, d, s, c, b, t corresponding to the respective antiquarks. For instance a pentaquark made of two up quarks, one down quark, one charm quark, and one charm antiquark would be denoted uudcc.

The quarks are bound together by the strong force, which acts in such a way as to cancel the colour charges within the particle. In a meson, this means a quark is partnered with an antiquark with an opposite colour charge – blue and antiblue, for example – while in a baryon, the three quarks have between them all three colour charges – red, blue, and green.[nb 1] In a pentaquark, the colours also need to cancel out, and the only feasible combination is to have one quark with one colour (e.g. red), one quark with a second colour (e.g. green), two quarks with the third colour (e.g. blue), and one antiquark to counteract the surplus colour (e.g. antiblue).[10]

The binding mechanism for pentaquarks is not yet clear. They may consist of five quarks tightly bound together, but it is also possible that they are more loosely bound and consist of a three-quark baryon and a two-quark meson interacting relatively weakly with each other via pion exchange (the same force that binds atomic nuclei) in a "meson-baryon molecule".[3][11][12]

History

Mid-2000s

The requirement to include an antiquark means that many classes of pentaquark are hard to identify experimentally – if the flavour of the antiquark matches the flavour of any other quark in the quintuplet, it will cancel out and the particle will resemble its three-quark hadron cousin. For this reason, early pentaquark searches looked for particles where the antiquark did not cancel.[10] In the mid-2000s, several experiments claimed to reveal pentaquark states. In particular, a resonance with a mass of 1540 MeV/c2 (4.6 σ) was reported by LEPS in 2003, the
Θ+
.[13] This coincided with a pentaquark state with a mass of 1530 MeV/c2 predicted in 1997.[14]

The proposed state was composed of two up quarks, two down quarks, and one strange antiquark (uudds). Following this announcement, nine other independent experiments reported seeing narrow peaks from
n

K+
and
p

K0
, with masses between 1522 MeV/c2 and 1555 MeV/c2, all above 4 σ.[13] While concerns existed about the validity of these states, the Particle Data Group gave the
Θ+
a 3-star rating (out of 4) in the 2004 Review of Particle Physics.[13] Two other pentaquark states were reported albeit with low statistical significance—the
Φ−−
(ddssu), with a mass of 1860 MeV/c2 and the
Θ0
c
(uuddc), with a mass of 3099 MeV/c2. Both were later found to be statistical effects rather than true resonances.[13]

Ten experiments then looked for the
Θ+
, but came out empty-handed.[13] 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
Θ+
(DIANA and SAPHIR respectively).[13] The 2006 Review of Particle Physics concluded:[13]

[T]here has not been a high-statistics confirmation of any of the original experiments that claimed to see the
Θ+
; 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
Θ+
; and all attempts to confirm the two other claimed pentaquark states have led to negative results. The conclusion that pentaquarks in general, and the
Θ+
, in particular, do not exist, appears compelling.

The 2008 Review of Particle Physics went even further:[6]

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 results, LEPS results continued to show the existence of a narrow state with a mass of 1524±MeV/c2, with a statistical significance of 5.1 σ.[15]

2015 LHCb results

Pentaquark-Feynman
Feynman diagram representing the decay of a lambda baryon Λ0
b
into a kaon K
and a pentaquark P+
c
.

In July 2015, the LHCb collaboration at CERN identified pentaquarks in the Λ0
b
→J/ψK
p
channel, which represents the decay of the bottom lambda baryon 0
b
)
into a J/ψ meson (J/ψ), a kaon (K
)
and a proton (p). The results showed that sometimes, instead of decaying via intermediate lambda states, the Λ0
b
decayed via intermediate pentaquark states. The two states, named P+
c
(4380)
and P+
c
(4450)
, had individual statistical significances of 9 σ and 12 σ, respectively, and a combined significance of 15 σ – enough to claim a formal discovery. The analysis ruled out the possibility that the effect was caused by conventional particles.[3] The two pentaquark states were both observed decaying strongly to J/ψp, hence must have a valence quark content of two up quarks, a down quark, a charm quark, and an anti-charm quark (
u

u

d

c

c
), making them charmonium-pentaquarks.[7][9][16]

The search for pentaquarks was not an objective of the LHCb experiment (which is primarily designed to investigate matter-antimatter asymmetry)[17] and the apparent discovery of pentaquarks was described as an "accident" and "something we’ve stumbled across" by the Physics Coordinator for the experiment.[11]

Studies of pentaquarks in other experiments

J-psi p pentaquark mass spectrum
A fit to the J/ψp invariant mass spectrum for the Λ0
b
→J/ψK
p
decay, with each fit component shown individually. The contribution of the pentaquarks are shown by hatched histograms.

The production of pentaquarks from electroweak decays of Λ0
b
baryons has extremely small cross-section and yields very limited information about internal structure of pentaquarks. For this reason, there are several ongoing and proposed initiatives to study pentaquark production in other channels.

It is expected that pentaquarks will be studied in electron-proton collisions in Hall B E2-16-007 and Hall C E12-12-001A experiments at JLAB. The major challenge in these studies is a heavy mass of the pentaquark, which will be produced at the tail of photon-proton spectrum in JLAB kinematics. For this reason, the currently unknown branching fractions of pentaquark should be sufficiently large to allow pentaquark detection in JLAB kinematics. The proposed Electron Ion Collider which has higher energies is much better suited for this problem.

An interesting channel to study pentaquarks in proton-nuclear collisions was suggested in Schmidt and Siddikov (2016).[18] This process has a large cross-section due to lack of electroweak intermediaries and gives access to pentaquark wave function. In the fixed-target experiments pentaquarks will be produced with small rapidities in laboratory frame and will be easily detected. Besides, if there are neutral pentaquarks, as suggested in several models based on flavour symmetry, these might be also produced in this mechanism. This process might be studied at future high-luminosity experiments like After@LHC and NICA.

2019 LHCb results

On 26 March 2019 the LHCb collaboration announced the discovery of a new pentaquark, based on observations that passed the 5-sigma threshold, using a dataset that was many times as large as the 2015 dataset.[8]

Designated Pc(4312)+ (Pc+ identifies a charmonium-pentaquark while the number between parenthesis indicates a mass of about 4312 MeV), the pentaquark decays to a proton and a J/ψ meson. The analyses revealed additionally that the earlier reported observations of the Pc(4450)+ pentaquark actually were the average of two different resonances, designated Pc(4440)+ and Pc(4457)+. Understanding this will require further study.

Applications

PQ EB ape hyp geom5 B 3D
Colour flux tubes produced by five static quark and antiquark charges, computed in lattice QCD.[19] Confinement in quantum chromodynamics leads to the production of flux tubes connecting colour charges. The flux tubes act as attractive QCD string-like potentials.

The discovery of pentaquarks will allow physicists to study the strong force in greater detail and aid understanding of quantum chromodynamics. In addition, current theories suggest that some very large stars produce pentaquarks as they collapse. The study of pentaquarks might help shed light on the physics of neutron stars.[9]

See also

Footnotes

  1. ^ The colour charges do not correspond to physical visible colours. They are arbitrary labels used to help scientists describe and visualise the charges of quarks.

References

  1. ^ Gignoux, C.; Silvestre-Brac, B.; Richard, J. M. (1987-07-16). "Possibility of stable multiquark baryons". Physics Letters B. 193 (2): 323–326. Bibcode:1987PhLB..193..323G. doi:10.1016/0370-2693(87)91244-5.
  2. ^ H. J. Lipkin (1987). "New possibilities for exotic hadrons — anticharmed strange baryons". Physics Letters B. 195 (3): 484–488. Bibcode:1987PhLB..195..484L. doi:10.1016/0370-2693(87)90055-4.
  3. ^ a b c "Observation of particles composed of five quarks, pentaquark-charmonium states, seen in Λ0
    b
    →J/ψpK decays"
    . CERN/LHCb. 14 July 2015. Retrieved 2015-07-14.
  4. ^ a b H. Muir (2 July 2003). "Pentaquark discovery confounds sceptics". New Scientist. Retrieved 2010-01-08.
  5. ^ K. Hicks (23 July 2003). "Physicists find evidence for an exotic baryon". Ohio University. Retrieved 2010-01-08.
  6. ^ a b See p. 1124 in C. Amsler et al. (Particle Data Group) (2008). "Review of particle physics" (PDF). Physics Letters B. 667 (1–5): 1–6. Bibcode:2008PhLB..667....1A. doi:10.1016/j.physletb.2008.07.018.
  7. ^ a b R. Aaij et al. (LHCb collaboration) (2015). "Observation of J/ψp resonances consistent with pentaquark states in Λ0
    b
    →J/ψKp decays". Physical Review Letters. 115 (7): 072001. arXiv:1507.03414. Bibcode:2015PhRvL.115g2001A. doi:10.1103/PhysRevLett.115.072001. PMID 26317714.
  8. ^ a b "LHCb experiment discovers a new pentaquark". CERN. 26 March 2019. Retrieved 26 April 2019.
  9. ^ a b c I. Sample (14 July 2015). "Large Hadron Collider scientists discover new particles: pentaquarks". The Guardian. Retrieved 2015-07-14.
  10. ^ a b J. Pochodzalla (2005). "Duets of strange quarks". Hadron Physics. p. 268. ISBN 978-1614990147.
  11. ^ a b G. Amit (14 July 2015). "Pentaquark discovery at LHC shows long-sought new form of matter". New Scientist. Retrieved 2015-07-14.
  12. ^ T. D. Cohen; P. M. Hohler; R. F. Lebed (2005). "On the Existence of Heavy Pentaquarks: The large Nc and Heavy Quark Limits and Beyond". Physical Review D. 72 (7): 074010. arXiv:hep-ph/0508199. Bibcode:2005PhRvD..72g4010C. doi:10.1103/PhysRevD.72.074010.
  13. ^ a b c d e f g W.-M. Yao et al. (Particle Data Group) (2006). "Review of particle physics:
    Θ+
    "
    (PDF). Journal of Physics G. 33 (1): 1–1232. arXiv:astro-ph/0601168. Bibcode:2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001.
  14. ^ D. Diakonov; V. Petrov & M. Polyakov (1997). "Exotic anti-decuplet of baryons: prediction from chiral solitons". Zeitschrift für Physik A. 359 (3): 305. arXiv:hep-ph/9703373. Bibcode:1997ZPhyA.359..305D. CiteSeerX 10.1.1.44.7282. doi:10.1007/s002180050406.
  15. ^ T. Nakano et al. (LEPS Collaboration) (2009). "Evidence of the Θ+ in the γd→K+Kpn reaction". Physical Review C. 79 (2): 025210. arXiv:0812.1035. Bibcode:2009PhRvC..79b5210N. doi:10.1103/PhysRevC.79.025210.
  16. ^ P. Rincon (14 July 2015). "Large Hadron Collider discovers new pentaquark particle". BBC News. Retrieved 2015-07-14.
  17. ^ "Where has all the antimatter gone?". CERN/LHCb. 2008. Retrieved 2015-07-15.
  18. ^ Schmidt, Iván; Siddikov, Marat (3 May 2016). "Production of pentaquarks in pA-collisions". Physical Review D. 93 (9): 094005. arXiv:1601.05621. Bibcode:2016PhRvD..93i4005S. doi:10.1103/PhysRevD.93.094005.
  19. ^ N. Cardoso; M. Cardoso & P. Bicudo (2013). "Color fields of the static pentaquark system computed in SU(3) lattice QCD". Physical Review D. 87 (3): 034504. arXiv:1209.1532. Bibcode:2013PhRvD..87c4504C. doi:10.1103/PhysRevD.87.034504.

Further reading

External links

1987 in science

The year 1987 in science and technology involved many significant events, some listed below.

Baryon

In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles, which are the quark-based particles. They are also classified as fermions, i.e., they have half-integer spin.

The name "baryon", introduced by Abraham Pais, comes from the Greek word for "heavy" (βαρύς, barýs), because, at the time of their naming, most known elementary particles had lower masses than the baryons. Each baryon has a corresponding antiparticle (antibaryon) where their corresponding antiquarks replace quarks. For example, a proton is made of two up quarks and one down quark; and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.

As quark-based particles, baryons participate in the strong interaction, which is mediated by particles known as gluons. The most familiar baryons are protons and neutrons, both of which contain three quarks, and for this reason these particles are sometimes described as triquarks. These particles make up most of the mass of the visible matter in the universe, as well as forming the components of the nucleus of every atom. Electrons (the other major component of the atom) are members of a different family of particles, known as leptons, which do not interact via the strong force. Exotic baryons containing five quarks (known as pentaquarks) have also been discovered and studied.

Baryon number

In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as

where nq is the number of quarks, and nq is the number of antiquarks. Baryons (three quarks) have a baryon number of +1, mesons (one quark, one antiquark) have a baryon number of 0, and antibaryons (three antiquarks) have a baryon number of −1. Exotic hadrons like pentaquarks (four quarks, one antiquark) and tetraquarks (two quarks, two antiquarks) are also classified as baryons and mesons depending on their baryon number.

Elementary particle

In particle physics, an elementary particle or fundamental particle is a subatomic particle with no sub structure, thus not composed of other particles. Particles currently thought to be elementary include the fundamental fermions (quarks, leptons, antiquarks, and antileptons), which generally are "matter particles" and "antimatter particles", as well as the fundamental bosons (gauge bosons and the Higgs boson), which generally are "force particles" that mediate interactions among fermions. A particle containing two or more elementary particles is a composite particle.

Everyday matter is composed of atoms, once presumed to be matter's elementary particles—atom meaning "unable to cut" in Greek—although the atom's existence remained controversial until about 1910, as some leading physicists regarded molecules as mathematical illusions, and matter as ultimately composed of energy. Soon, subatomic constituents of the atom were identified. As the 1930s opened, the electron and the proton had been observed, along with the photon, the particle of electromagnetic radiation. At that time, the recent advent of quantum mechanics was radically altering the conception of particles, as a single particle could seemingly span a field as would a wave, a paradox still eluding satisfactory explanation.Via quantum theory, protons and neutrons were found to contain quarks—up quarks and down quarks—now considered elementary particles. And within a molecule, the electron's three degrees of freedom (charge, spin, orbital) can separate via the wavefunction into three quasiparticles (holon, spinon, orbiton). Yet a free electron—which is not orbiting an atomic nucleus and lacks orbital motion—appears unsplittable and remains regarded as an elementary particle.Around 1980, an elementary particle's status as indeed elementary—an ultimate constituent of substance—was mostly discarded for a more practical outlook, embodied in particle physics' Standard Model, what's known as science's most experimentally successful theory. Many elaborations upon and theories beyond the Standard Model, including the popular supersymmetry, double the number of elementary particles by hypothesizing that each known particle associates with a "shadow" partner far more massive, although all such superpartners remain undiscovered. Meanwhile, an elementary boson mediating gravitation—the graviton—remains hypothetical.

Exotic atom

An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes and all currently observed atoms cannot persist under normal conditions.

Exotic baryon

Exotic baryons are a type of hadron (bound states of quarks and gluons) with half-integer spin, but have a quark content different to the three quarks (qqq) present in conventional baryons. An example would be pentaquarks, consisting of four quarks and one antiquark (qqqqq̅).

So far, the only observed exotic baryons are the pentaquarks P+c(4380) and P+c(4450), discovered in 2015 by the LHCb collaboration.Several types of exotic baryons that require physics beyond the Standard Model have been conjectured in order to explain specific experimental anomalies. There is no independent experimental evidence for any of these particles. One example is supersymmetric R-baryons, which are bound states of 3 quarks and a gluino. The lightest R-baryon is denoted as S0 and consists of an up quark, a down quark, a strange quark and a gluino. This particle is expected to be long lived or stable and has been invoked to explain ultra-high-energy cosmic rays. Stable exotic baryons are also candidates for strongly interacting dark matter.

It has been speculated by futurologist Ray Kurzweil that by the end of the 21st century it might be possible by using femtotechnology to create new chemical elements composed of exotic baryons that would eventually constitute a new periodic table of elements in which the elements would have completely different properties than the regular chemical elements.

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 two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.Consistent with ordinary hadrons, exotic hadrons are classified as being either fermions, like ordinary baryons, or bosons, like ordinary mesons. According to this classification scheme, pentaquarks, containing five valence quarks, are exotic baryons, while tetraquarks (four valence quarks) and hexaquarks (six quarks, consisting of either a dibaryon or three quark-antiquark pairs) would be considered exotic mesons. Tetraquark and pentaquark particles are believed to have been observed and are being investigated; Hexaquarks have not yet been confirmed as observed.

Exotic hadrons can be searched for by looking for S-matrix poles with quantum numbers forbidden to ordinary hadrons. Experimental signatures for such exotic hadrons have been seen by at least 2003 but remain a topic of controversy in particle physics.

Jaffe and Low suggested that the exotic hadrons manifest themselves as poles of the P matrix, and not of the S matrix. Experimental P-matrix poles are determined reliably in both the meson-meson channels and nucleon-nucleon channels.

Gauge boson

In particle physics, a gauge boson is a force carrier, a bosonic particle that carries any of the fundamental interactions of nature, commonly called forces. Elementary particles, whose interactions are described by a gauge theory, interact with each other by the exchange of gauge bosons—usually as virtual particles.

All known gauge bosons have a spin of 1. Therefore, all known gauge bosons are vector bosons.

Gauge bosons are different from the other kinds of bosons: first, fundamental scalar bosons (the Higgs boson); second, mesons, which are composite bosons, made of quarks; third, larger composite, non-force-carrying bosons, such as certain atoms.

Hadron

In particle physics, a hadron (listen) (Greek: ἁδρός, hadrós; "stout, thick") is a composite particle made of two or more quarks held together by the strong force in a similar way as molecules are held together by the electromagnetic force. Most of the mass of ordinary matter comes from two hadrons, the proton and the neutron.

Hadrons are categorized into two families: baryons, made of an odd number of quarks – usually three quarks – and mesons, made of an even number of quarks—usually one quark and one antiquark. Protons and neutrons are examples of baryons; pions are an example of a meson. "Exotic" hadrons, containing more than three valence quarks, have been discovered in recent years. A tetraquark state (an exotic meson), named the Z(4430)−, was discovered in 2007 by the Belle Collaboration and confirmed as a resonance in 2014 by the LHCb collaboration. Two pentaquark states (exotic baryons), named P+c(4380) and P+c(4450), were discovered in 2015 by the LHCb collaboration. There are several more exotic hadron candidates, and other colour-singlet quark combinations that may also exist.

Almost all "free" hadrons and antihadrons (meaning, in isolation and not bound within an atomic nucleus) are believed to be unstable and eventually decay (break down) into other particles. The only known exception relates to free protons, which are possibly stable, or at least, take immense amounts of time to decay (order of 1034+ years). Free neutrons are unstable and decay with a half-life of about 611 seconds. Their respective antiparticles are expected to follow the same pattern, but they are difficult to capture and study, because they immediately annihilate on contact with ordinary matter. "Bound" protons and neutrons, contained within an atomic nucleus, are generally considered stable. Experimentally, hadron physics is studied by colliding protons or nuclei of heavy elements such as lead or gold, and detecting the debris in the produced particle showers. In the natural environment, mesons such as pions are produced by the collisions of cosmic rays with the atmosphere.

Hexaquark

In particle physics hexaquarks are a large family of hypothetical particles, each particle consisting of six quarks or antiquarks of any flavours. Six constituent quarks in any of several combinations could yield a colour charge of zero; for example a hexaquark might contain either six quarks, resembling two baryons bound together (a dibaryon), or three quarks and three antiquarks. Once formed, dibaryons are predicted to be fairly stable by the standards of particle physics. In 1977 Robert Jaffe proposed that a possibly stable H dibaryon with the quark composition udsuds could notionally result from the combination of two uds hyperonsA number of experiments have been suggested to detect dibaryon decays and interactions. In the 1990s several candidate dibaryon decays were observed but they were not confirmed.There is a theory that strange particles such as hyperons and dibaryons could form in the interior of a neutron star, changing its mass–radius ratio in ways that might be detectable. Accordingly, measurements of neutron stars could set constraints on possible dibaryon properties. A large fraction of the neutrons in a neutron star could turn into hyperons and merge into dibaryons during the early part of its collapse into a black hole. These dibaryons would very quickly dissolve into quark–gluon plasma during the collapse, or go into some currently unknown state of matter.

In 2014 a potential dibaryon was detected at the Jülich Research Center at about 2380 MeV. The particle existed for 10−23 seconds and was named d*(2380).

LHCb experiment

The LHCb (Large Hadron Collider beauty) experiment is one of seven 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 parameters of CP violation in the interactions of b-hadrons (heavy particles containing a bottom quark). Such studies can help to explain the matter-antimatter asymmetry of the Universe. The detector is also able to perform measurements of production cross sections, exotic hadron spectroscopy, charm physics and electroweak physics in the forward region. The LHCb collaboration, who built, operate and analyse data from the experiment, is composed of approximately 1260 people from 74 scientific institutes, representing 16 countries. As of 2017, the spokesperson for the collaboration is Giovanni Passaleva. The experiment is located at point 8 on the LHC tunnel close to Ferney-Voltaire, France just over the border from Geneva. The (small) MoEDAL experiment shares the same cavern.

Large Hadron Collider

The Large Hadron Collider (LHC) is the world's largest and most powerful particle collider and the largest machine in the world. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundreds of universities and laboratories, as well as more than 100 countries. It lies in a tunnel 27 kilometres (17 mi) in circumference and as deep as 175 metres (574 ft) beneath the France–Switzerland border near Geneva.

First collisions were achieved in 2010 at an energy of 3.5 teraelectronvolts (TeV) per beam, about four times the previous world record. After upgrades it reached 6.5 TeV per beam (13 TeV total collision energy, the present world record). At the end of 2018, it entered a two-year shutdown period for further upgrades.

The collider has four crossing points, around which are positioned seven detectors, each designed for certain kinds of research. The LHC primarily collides proton beams, but it can also use beams of heavy ions: Lead–lead collisions and proton-lead collisions are typically done for one month per year. The aim of the LHC's detectors is to allow physicists to test the predictions of different theories of particle physics, including measuring the properties of the Higgs boson and searching for the large family of new particles predicted by supersymmetric theories, as well as other unsolved questions of physics.

List of baryons

Baryons are composite particles made of three quarks, as opposed to mesons, which are composite particles made of one quark and one antiquark. Baryons and mesons are both hadrons, which are particles composed solely of quarks or both quarks and antiquarks. The term baryon is derived from the Greek "βαρύς" (barys), meaning "heavy", because, at the time of their naming, it was believed that baryons were characterized by having greater masses than other particles that were classed as matter.

Until a few years ago, it was believed that some experiments showed the existence of pentaquarks – baryons made of four quarks and one antiquark. The particle physics community as a whole did not view their existence as likely by 2006. On 13 July 2015, the LHCb collaboration at CERN reported results consistent with pentaquark states in the decay of bottom Lambda baryons (Λ0b).Since baryons are composed of quarks, they participate in the strong interaction. Leptons, on the other hand, are not composed of quarks and as such do not participate in the strong interaction. The most famous baryons are the protons and neutrons that make up most of the mass of the visible matter in the universe, whereas electrons, the other major component of atoms, are leptons. Each baryon has a corresponding antiparticle known as an antibaryon in which quarks are replaced by their corresponding antiquarks. For example, a proton is made of two up quarks and one down quark, while its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.

List of unsolved problems in physics

Some of the major unsolved problems in physics are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail.

There are still some deficiencies in the Standard Model of physics, such as the origin of mass, the strong CP problem, neutrino mass, matter–antimatter asymmetry, and the nature of dark matter and dark energy. Another problem lies within the mathematical framework of the Standard Model itself—the Standard Model is inconsistent with that of general relativity, to the point that one or both theories break down under certain conditions (for example within known spacetime singularities like the Big Bang and the centers of black holes beyond the event horizon).

Onium

An onium (plural: onia) is a bound state of a particle and its antiparticle. They are usually named by adding the suffix -onium to the name of the constituting particle except for muonium which, despite its name, is not a bound muon–antimuon onium, but an electron–antimuon bound state, and whose name was assigned by IUPAC. A muon–antimuon onium is called true muonium.

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. Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons, which include baryons (such as protons and neutrons) and mesons. For this reason, much of what is known about quarks has been drawn from observations of hadrons.

Quarks have various intrinsic properties, including electric charge, mass, color charge, and spin. They are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge.

There are six types, known as flavors, of quarks: up, down, strange, charm, bottom, and top. Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the universe, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators). For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties (such as the electric charge) have equal magnitude but opposite sign.

The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968. Accelerator experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.

Quark star

A quark star is a hypothetical type of compact exotic star, where extremely high temperature and pressure has forced nuclear particles to form quark matter, a continuous state of matter consisting of free quarks.

It is well known, both theoretically and observationally, that some massive stars collapse to form neutron stars at the end of their life cycle. Under the extreme temperatures and pressures inside neutron stars, the neutrons are normally kept apart by a degeneracy pressure, stabilizing the star and hindering further gravitational collapse. However, it is hypothesized that under even more extreme temperature and pressure, the degeneracy pressure of the neutrons is overcome, and the neutrons are forced to merge and dissolve into their constituent quarks, creating an ultra-dense phase of quark matter based on densely packed quarks. In this state, a new equilibrium is supposed to emerge, as a new degeneracy pressure between the quarks, as well as repulsive electromagnetic forces, will occur and hinder gravitational collapse. If these ideas are correct, quark stars might occur, and be observable, somewhere in the universe. Theoretically, such a scenario is seen as scientifically plausible, but it has been impossible to prove both observationally and experimentally, because the very extreme conditions needed for stabilizing quark matter can not be created in any laboratory nor observed directly in nature. The stability of quark matter, and hence the existence of quark stars, is for these reasons among the unsolved problems in physics.

If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy - the point at which neutrons break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole. However, as scientists are unable so far to explore most properties of quark matter, the exact conditions and nature of quark stars, and their existence, remain hypothetical and unproven. The question whether such stars exist and their exact structure and behavior is actively studied within astrophysics and particle physics.

If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or not have typical sizes, electromagnetic fields, or surface temperatures, compared to neutron stars.

The hypothesis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze. Their existence has not been confirmed. The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 1012 K) quark-gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 1012 K can not be recreated artificially, as there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.

SPring-8

SPring-8 (an acronym of Super Photon Ring – 8 GeV) is a synchrotron radiation facility located in Hyōgo Prefecture, Japan, which was developed jointly by RIKEN and the Japan Atomic Energy Research Institute. It is owned and managed by RIKEN, and run under commission by the Japan Synchrotron Radiation Research Institute. The machine consists of a storage ring containing an 8 GeV electron beam. On its path around the storage ring, the beam passes through insertion devices to produce synchrotron radiation with energies ranging from soft X-rays (300 eV) up to hard X-rays (300 keV). The synchrotron radiation produced at SPring-8 is used for materials analysis and biochemical protein characterization by many Japanese manufacturers and universities.

Together with the Advanced Photon Source at Argonne National Laboratory and the Cornell High Energy Synchrotron Source at Cornell University in the United States, the European Synchrotron Radiation Facility in Grenoble, France and PETRA at DESY in Hamburg, Germany, it is one of the five large (beam energy greater than 5 GeV) synchrotron radiation facilities in the world.

Tetraquark

A tetraquark, in particle physics, is an exotic meson 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 which lies outside the conventional quark model classification.

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