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 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 is given instead.