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: Δ++ (constituent quarks: uuu), Δ+ (uud), Δ0 (udd), and Δ− (ddd), which respectively carry an electric charge of +2 e, +1 e, 0 e, and −1 e.
The Δ baryons have a mass of about 1232 MeV/c2; their third component of isospin I 3 = ± ± --> 1 2 o r ± ± --> 3 2 ; {\displaystyle \;I_{3}=\pm {\tfrac {1}{2}}~{\mathsf {or}}~\pm {\tfrac {3}{2}}\;;} 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 939 MeV/c2, and both intrinsic spin and isospin of 1/ 2 . The Δ+ (uud) and Δ0 (udd) particles are higher-mass spin-excitations of the proton (N+, uud) and neutron (N0, udd), respectively.
The Δ++ and Δ−, however, have no direct nucleon analogues: For example, even though their charges are identical and their masses are similar, the Δ− (ddd), is not closely related to the antiproton (p, uud).
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).[1]
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[2][3] and the Carnegie Institute of Technology synchro-cyclotron[4] in the mid-1950s using accelerated positive pions on hydrogen targets. The existence of the Δ++, with its unusual electric charge of +2 e, was a crucial clue in the development of the quark model.
The Delta states are created when a sufficiently energetic probe – such as a photon, electron, neutrino, or pion – impinges upon a proton or neutron, or possibly by the collision of a sufficiently energetic nucleon pair.
All of the Δ baryons with mass near 1232 MeV quickly decay via the strong interaction into a nucleon (proton or neutron) and a pion of appropriate charge. The relative probabilities of allowed final charge states are given by their respective isospin couplings. More rarely, the Δ+ can decay into a proton and a photon and the Δ0 can decay into a neutron and a photon.
[a] ^ PDG reports the resonance width (Γ). Here the conversion τ τ --> = ℏ ℏ --> Γ Γ --> {\textstyle \tau ={\frac {\hbar }{\Gamma }}} is given instead.
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