Definitions

hadron

hadron

[had-ron]

Any of the subatomic particles that are built from quarks and thus interact via the strong force. The hadrons fall into two groups: mesons and baryons. Except for protons and neutrons, which are bound in nuclei, all hadrons have short lives and are produced in high-energy collision of subatomic particles. All hadrons are subject to gravitation; charged hadrons are subject to electromagnetic forces. Some hadrons break up by way of the weak force (as in radioactive decay); others decay via the strong and electromagnetic forces.

Learn more about hadron with a free trial on Britannica.com.

In particle physics, a hadron (from the ἁδρός, hadrós, "stout, thick") (.ogg format) is a bound state of quarks. Hadrons are held together by the strong force, similar to how atoms are held together by the electromagnetic force. There are two subsets of hadrons; baryons and mesons. Of which the most well known baryons are protons and neutrons.

Introduction

According to the quark model, 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 the constituent quarks also carry color charge (nothing to do with visual color), a property of the strong nuclear force called color confinement requires that any composite state carry no residual color charge. That is, hadrons must be colorless. There are two ways to accomplish this: three quarks of different colors, or a quark of one color and an anti-quark carrying the corresponding anti-color. Hadrons based on the former are called baryons while 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 (or P) parity, and C, the charge conjugation, or C parity, and the particle four-momentum, m, (i.e., its mass). 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 nuclear force. Hadrons may also carry flavor quantum numbers such as isospin (or G parity), and strangeness. All quarks carry an additive, conserved quantum number called baryon number (B), which is +1/3 for quarks and -1/3 for anti-quarks. This means that baryons --which are 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; 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 interact weakly and will no longer be confined within hadrons. This property, which is known as asymptotic freedom, has been experimentally confirmed at the energy scales between a GeV and a TeV.

Baryons

All known baryons are made of three valence quarks, and are therefore fermions. They have baryon number B=1, while anti-baryons (composed of three anti-quarks) have B=-1. In principle, some baryons could be composed of further quark-antiquark pairs in addition to the three quarks (or antiquarks) that make up basic baryons. Baryons containing a single additional quark-antiquark pair are called pentaquarks. Evidence for these states was claimed by several experiments in the early 2000s, though this has since been refuted. No evidence of baryon states with even more quark-antiquark pairs has been found.

Mesons

Mesons are bosons composed of a quark-antiquark pair. 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 holding atomic nuclei together via the residual strong force. Hypothetical mesons have more than one quark-antiquark pair; a meson composed of two of these pairs is called a tetraquark. Currently there is no evidence of their existence. Mesons that lie outside the quark model classification are called exotic mesons. These include glueballs and hybrid mesons (mesons bound by excited gluons).

See also

References

Search another word or see Hadronon Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature