Physics:Quantum hadron
A quantum hadron is a composite subatomic particle made of two or more quarks bound together by the strong interaction. Hadrons are the particles described by quantum chromodynamics as color-neutral bound states of quarks, antiquarks, and gluons. The best-known hadrons are the proton and neutron, which make up most of the mass of ordinary atomic matter.
In the Standard Model, hadrons are not elementary particles. They are composite quantum systems whose observed properties arise from quark flavor, spin, color confinement, and the energy stored in the strong field. Most of the mass of a proton or neutron is not simply the sum of the masses of its valence quarks, but comes from the energy of quarks, gluons, and strong-interaction binding inside the hadron.[1]
Hadrons are color-neutral quantum particles made from bound quarks.
Overview
Hadrons are divided into two main families: baryons and mesons. Baryons contain an odd number of valence quarks, usually three. Mesons contain an even number of valence quarks, usually a quark and an antiquark.[2]
The proton and neutron are baryons. Pions and kaons are mesons. More complex hadrons, such as tetraquarks and pentaquarks, contain additional quark-antiquark structure and are often called exotic hadrons.
Hadrons are important in quantum matter because they connect the microscopic theory of quarks and gluons with the observable particles of nuclei, cosmic rays, particle accelerators, and high-energy nuclear collisions.
Quark model
In the quark model, the main quantum numbers of a hadron are determined by its valence quarks. A proton contains two up quarks and one down quark, while a neutron contains one up quark and two down quarks. The electric charge of each hadron follows from the charges of its constituent quarks.
Quarks also carry color charge. However, isolated hadrons must have zero total color charge. This color-neutral condition is a consequence of color confinement, the property that quarks and gluons are not observed as free particles under ordinary conditions. A meson can be color neutral through a quark and matching antiquark, while a baryon can be color neutral through three quarks with complementary colors.
Baryons
Baryons are hadrons with an odd number of valence quarks. Ordinary baryons contain three valence quarks. Because they have half-integer spin, baryons are fermions. Their baryon number is normally , while antibaryons have .
The proton and neutron are the most familiar baryons. Together they form atomic nuclei and are therefore responsible for most of the mass of ordinary matter. Free neutrons are unstable and decay with a lifetime of about 879 seconds, while free protons appear to be stable on experimentally accessible timescales.[3]
Exotic baryons can contain more than three valence quarks. In 2015, the LHCb collaboration reported pentaquark candidates consistent with resonant states in decays.[4]
Mesons
Mesons are hadrons with an even number of valence quarks. The simplest mesons contain one quark and one antiquark. Because this gives integer spin, mesons are bosons. Their baryon number is .
Pions are common mesons and play a role in the residual strong interaction that helps bind atomic nuclei. Kaons are another important meson family. Exotic mesons may include tetraquarks, hybrid mesons, and glueball candidates, depending on their internal quark and gluon structure.
A tetraquark-like state, the , was reported by the Belle Collaboration and later confirmed as resonant by LHCb.[5][6]
Color confinement
Color confinement is the reason hadrons appear as complete color-neutral particles rather than as isolated quarks. In quantum chromodynamics, the strong interaction becomes weaker at very short distances but stronger at larger distances. As a result, attempts to separate quarks tend to produce new quark-antiquark pairs rather than free individual quarks.
This behavior explains why particle detectors observe jets of hadrons rather than bare quarks. When high-energy collisions produce quarks and gluons, they undergo hadronization, forming showers of observable hadrons.
Mass and binding energy
The mass of a hadron is largely a quantum field effect. Although valence quarks help determine electric charge, flavor, spin, and other quantum numbers, most of the mass of ordinary hadrons comes from kinetic energy, gluon fields, virtual quark-antiquark pairs, and strong binding energy. This is an important example of mass-energy equivalence in quantum field theory.
For the proton and neutron, the up and down valence quark masses account for only a small fraction of the total hadron mass. The rest is generated dynamically by the strong interaction.
Resonances and instability
Many hadrons exist as short-lived excited states called resonances. These states decay very rapidly, often through the strong interaction, on timescales of about seconds. Hundreds of hadron resonances have been observed in particle physics experiments.
Almost all free hadrons are unstable, with the possible exception of the proton and antiproton. Free neutrons decay through the weak interaction, while many mesons and heavier baryons decay much more quickly.
High-energy matter
At very high temperature or density, hadrons may dissolve into a state in which quarks and gluons are no longer confined inside individual hadrons. This state is known as the quark-gluon plasma. Quantum chromodynamics predicts this behavior because the strong interaction becomes weaker at high energies, a property called asymptotic freedom.[7]
Hadron physics is studied in high-energy collisions, such as proton-proton collisions and heavy-ion collisions. Cosmic rays also produce hadrons naturally when they strike particles in the upper atmosphere, creating showers that include pions, muons, and other secondary particles.[8]
Terminology
The term hadron was introduced by L. B. Okun in 1962 as a convenient name for strongly interacting particles. The word is derived from the Greek hadrós, meaning large, stout, or thick, in contrast with leptós, meaning small or light.[9]
The distinction between hadrons and leptons remains fundamental in particle physics. Hadrons participate in the strong interaction, while leptons do not carry color charge and are not bound by the strong force.
See also
Table of contents (72 articles)
Index
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References
- ↑ Amsler, C. (2008). "Review of Particle Physics". Physics Letters B 667 (1): 1–134. doi:10.1016/j.physletb.2008.07.018. Bibcode: 2008PhLB..667....1A.
- ↑ Gell-Mann, Murray (1964). "A schematic model of baryons and mesons". Physics Letters 8 (3): 214–215. doi:10.1016/S0031-9163(64)92001-3. Bibcode: 1964PhL.....8..214G.
- ↑ Zyla, P. A. (2020). "n MEAN LIFE". Particle Data Group. https://pdglive.lbl.gov/DataBlock.action?node=S017T.
- ↑ Aaij, R. (2015). "Observation of J/ψp resonances consistent with pentaquark states in Λb0 → J/ψK−p decays". Physical Review Letters 115 (7): 072001. doi:10.1103/PhysRevLett.115.072001. PMID 26317714. Bibcode: 2015PhRvL.115g2001A.
- ↑ Choi, S.-K. (2008). "Observation of a resonance-like structure in the π±ψ′ mass distribution in exclusive B→Kπ±ψ′ decays". Physical Review Letters 100 (14): 142001. doi:10.1103/PhysRevLett.100.142001. PMID 18518023. Bibcode: 2008PhRvL.100n2001C.
- ↑ Aaij, R. (2014). "Observation of the Resonant Character of the Z(4430)− State". Physical Review Letters 112 (22): 222002. doi:10.1103/PhysRevLett.112.222002. PMID 24949760. Bibcode: 2014PhRvL.112v2002A.
- ↑ Bethke, S. (2007). "Experimental tests of asymptotic freedom". Progress in Particle and Nuclear Physics 58 (2): 351–386. doi:10.1016/j.ppnp.2006.06.001. Bibcode: 2007PrPNP..58..351B.
- ↑ Martin, B. R. (2017). Particle Physics (4th ed.). Chichester, West Sussex, United Kingdom: Wiley. ISBN 9781118911907.
- ↑ Okun, L. B. (1962). "The theory of weak interaction". International Conference on High-Energy Physics. CERN, Geneva. p. 845. Bibcode: 1962hep..conf..845O.
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