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 ${\displaystyle \;I_{3}=\pm {\tfrac {1}{2}}~{\mathsf {or}}~\pm {\tfrac {3}{2}}\;;}$ 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 ${\textstyle \tau ={\frac {\hbar }{\Gamma }}}$ is given instead.