Hadron
In particle physics, a hadron (pronounced /ˈhædrɒn/, from the Greek: ἁδρός, hadrós, "stout, thick") is a composite particle made of quarks held together by the strong force (similarly to how atoms and molecules are held together by the electromagnetic force). Hadrons are categorized into two families: baryons (made of three quarks), and mesons (made of one quark and one antiquark).
The best-known hadrons are protons and neutrons (both baryons), which can be found in the atomic nuclei. All hadrons except protons are unstable and undergo particle decay–however neutrons are stable when found inside the atomic nuclei. The best-known mesons are the pions and kaons, which were discovered during cosmic ray experiments during the late 1940s and early 1950s. However these are not the only hadrons; a great number of them have been discovered and continue to be discovered to this day (see list of baryons and list of mesons).
Other types of hadron may exist, such as tetraquarks (or, more generally, exotic mesons) and pentaquarks (exotic baryons), but no evidence conclusively suggests their existence as of 2010[update].
Contents |
Etymology
The term hadron was introduced by Lev B. Okun in a plenary talk at the 1962 International Conference on High-Energy Physics.[1] In this talk he said:
Non-withstanding the fact that this report deals with weak interactions, we shall frequently have to speak of strongly interacting particles. These particles pose not only numerous scientific problems, but also a terminological problem. The point is that "strongly interacting particles" is a very clumsy term which does not yield itself to the formation of an adjective. For this reason, to take but one instance, decays into strongly interacting particles are called non-leptonic. This definition is not exact because "non-leptonic" may also signify "photonic". In this report I shall call strongly interacting particles "hadrons", and the corresponding decays "hadronic" (the Greek ἁδρός signifies "large", "massive", in contrast to λεπτός which means "small", "light"). I hope that this terminology will prove to be convenient.
Properties
According to the quark model,[2] the properties of hadrons are primarily determined by their so-called valence quarks. For example, a proton is composed of two up quarks (each with electric charge +2⁄3) and one down quark (with electric charge −1⁄3). Adding these together yields the proton charge of +1. Although quarks also carry color charge, hadrons must have zero total color charge because of a phenomenon called color confinement. That is, hadrons must be "colorless" or "white". There are two ways to accomplish this: three quarks of different colors, or a quark of one color and an antiquark carrying the corresponding anticolor. Hadrons based on the former are called baryons, and those based on the latter are called mesons.
Like all subatomic particles, hadrons are assigned quantum numbers corresponding to the representations of the Poincaré group: JPC(m), where J is the spin quantum number, P, the intrinsic parity (or P-parity), and C, the charge conjugation (or C-parity), and the particle's mass, m. Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due to mass–energy equivalence, most of the mass comes from the large amount of energy associated with the strong interaction. Hadrons may also carry flavor quantum numbers such as isospin (or G parity), and strangeness. All quarks carry an additive, conserved quantum number called a baryon number (B), which is +1⁄3 for quarks and −1⁄3 for antiquarks. This means that baryons (groups of three quarks) have B = 1 while mesons have B = 0.
Hadrons have excited states known as resonances. Each ground state hadron may have several excited states; several hundreds of resonances have been observed in particle physics experiments. Resonances decay extremely quickly (within about 10−24 seconds) via the strong nuclear force.
In other phases of QCD matter the hadrons may disappear. For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, the theory of quantum chromodynamics (QCD) predicts that quarks and gluons will no longer be confined within hadrons because the strength of the strong interaction diminishes with energy. This property, which is known as asymptotic freedom, has been experimentally confirmed at the energy scales between a GeV and a TeV.[3]
All free hadrons except the proton (and antiproton) are unstable.
Baryons
All known baryons are made of three valence quarks, so in terms of their behavior, they are fermions. As quarks possess baryon number B = 1⁄3, baryons have baryon number B = 1. The best-known baryons are the proton and the neutron.
In principle, some baryons could be composed of further quark–antiquark pairs in addition to their three quarks. These hypothetical baryons containing an additional quark–antiquark pair are called pentaquarks.[4] Several pentaquark candidates were found in the early 2000s, but upon further review these states have now been established as non-existent.[5] (This does not rule against pentaquarks in general, only the candidates put forward). No evidence of baryon states with even more quark–antiquark pairs has been found.
Each type of baryon has a corresponding antiparticle (antibaryon) in which quarks are replaced by their corresponding antiquarks. For example: just as a proton is made of two up-quarks and one down-quark, its corresponding antiparticle, the antiproton, is made of two up-antiquarks and one down-antiquark.
Mesons
Mesons are hadrons composed of a quark–antiquark pair. In terms of their behavior, they are bosons. They have baryon number B = 0. Examples of mesons commonly produced in particle physics experiments include pions and kaons. The former also play a role in holding atomic nuclei together via the residual strong force.
In principle, mesons with more than one quark–antiquark pair may exist; a hypothetical meson composed of two of these pairs is called a tetraquark. Several tetraquark candidates were found in the 2000s, but their status is under debate. Several other hypothetical "exotic" mesons lie outside the quark model of classification. These include glueballs and hybrid mesons (mesons bound by excited gluons).
See also
- Hadronization
- Large Hadron Collider (LHC)
- Leptons
- List of particles
- Standard model
- Subatomic particles
References
- ↑ L.B. Okun (1962). "The Theory of Weak Interaction". Proceedings of 1962 International Conference on High-Energy Physics at CERN. Geneva. p. 845. Bibcode: 1962hep..conf..845O.
- ↑ C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics – Quark Model". Physics Letters B 667: 1. doi:10.1016/j.physletb.2008.07.018. http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf.
- ↑ S. Bethke (2007). "Experimental tests of asymptotic freedom". Progress in Particle and Nuclear Physics 58: 351. doi:10.1016/j.ppnp.2006.06.001. arXiv:hep-ex/0606035.
- ↑ S. Kabana (2005). "Review of the experimental evidence on pentaquarks and critical discussion". arΧiv:hep-ex/0503020 [hep-ex].
- ↑ C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics – Pentaquarks". Physics Letters B 667: 1. doi:10.1016/j.physletb.2008.07.018. http://pdg.lbl.gov/2008/reviews/pentaquarks_b801.pdf.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||