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Concept
Baryon Delta
Résumé
The Delta baryons (or Δ baryons, also called Delta resonances) are a family of subatomic particle made of three up or down quarks (u or d quarks), the same constituent quarks that make up the more familiar protons and neutrons.
Four closely related Δ baryons exist: _Delta++ (constituent quarks: uuu), _Delta+ (uud), _Delta0 (udd), and _Delta- (ddd), which respectively carry an electric charge of +2e, +1e, 0e, and -1e.
The Δ baryons have a mass of about 1232MeV/c2; their third component of isospin and they are required to have an intrinsic spin of 3 /2 or higher (half-integer units). Ordinary nucleons (symbol N, meaning either a proton or neutron), by contrast, have a mass of about 939MeV/c2, and both intrinsic spin and isospin of 1/ 2 . The _Delta+ (uud) and _Delta0 (udd) particles are higher-mass spin-excitations of the proton (_Nucleon+, uud) and neutron (_Nucleon0, udd), respectively.
The _Delta++ and _Delta-, however, have no direct nucleon analogues: For example, even though their charges are identical and their masses are similar, the _Delta- (ddd), is not closely related to the antiproton (_antiproton, ).
The Delta states discussed here are only the lowest-mass quantum excitations of the proton and neutron. At higher spins, additional higher mass Delta states appear, all defined by having constant 3 /2 or 1 /2 isospin (depending on charge), but with spin 3 /2, 5 /2, 7 /2, ..., 11 /2 multiplied by ħ. A complete listing of all properties of all these states can be found in Beringer et al. (2013).
There also exist antiparticle Delta states with opposite charges, made up of the corresponding antiquarks.
The states were established experimentally at the University of Chicago cyclotron
and the Carnegie Institute of Technology synchro-cyclotron
in the mid-1950s using accelerated positive pions on hydrogen targets. The existence of the _Delta++, with its unusual electric charge of +2e, was a crucial clue in the development of the quark model.
Source officielle
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