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A member of a class of subatomic particles called baryons, which exists in four electric charge states and has a total spin of J = 32. In the underlying quark model, the delta resonance (Δ) consists of three quarks whose intrinsic spins of ½ are lined up in the same direction. The Δ is closely related to the more familiar nucleon constituents of atomic nuclei, the neutrons (n) and protons (p). See Nucleon, Quarks
The Δ was first observed as a resonant interaction of a beam of pi mesons (π) with a proton target. The probability of a scattering interaction between the π and the proton is strongly dependent on energy, attaining a maximum at the Δ mass of 1236 MeV/c2 (where c is the speed of light). The formation of the very short-lived Δ (with a lifetime on the order of 10-23 s) followed immediately by its decay back into pion and nucleon. See Scattering experiments (nuclei)
The Δ plays an important role in a wide variety of nuclear phenomena, even under conditions of low energy and momentum transfer. The study of these phenomena reveals much about the presence of pions in nuclei, in addition to neutrons and protons. See Baryon, Elementary particle, Nuclear structure