Delta baryon

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.

Delta baryon

Composition	Δ++ : uuu Δ+ : uud Δ0 : udd Δ− : ddd
Statistics	Fermionic
Interactions	Strong, weak, electromagnetic, and gravity
Symbol	Δ
Types	4
Mass	1232±2 MeV/c2
Spin	⁠ 3 /2⁠, ⁠ 5 /2⁠, ⁠ 7 /2⁠ ...
Strangeness	0
Charm	0
Bottomness	0
Topness	0
Isospin	⁠ 3 /2⁠

Properties

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.

Discovery

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.

Formation and decay

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.

List

Delta baryons

Particle name	Symbol	Quark content	Mass (MeV/c2)	I3	JP	Q (e)	S	C	B′	T	Mean lifetime (s)	Commonly decays to
Delta[1]	Δ++ (1 232)	uuu	1232±2	+⁠ 3 /2⁠	⁠ 3 /2⁠+	+2	0	0	0	0	(5.63±0.14)×10−24[a]	p+ + π+
Delta[1]	Δ+ (1 232)	uud	1232±2	+⁠1/ 2 ⁠	⁠ 3 /2⁠+	+1	0	0	0	0	(5.63±0.14)×10−24[a]	π+ + n0 , or π0 + p+
Delta[1]	Δ0 (1 232)	udd	1232±2	⁠−+1/ 2 ⁠	⁠ 3 /2⁠+	0	0	0	0	0	(5.63±0.14)×10−24[a]	π0 + n0 , or π− + p+
Delta[1]	Δ− (1 232)	ddd	1232±2	⁠−+ 3 /2⁠	⁠ 3 /2⁠+	−1	0	0	0	0	(5.63±0.14)×10−24[a]	π− + n0

^[a] ^ PDG reports the resonance width (Γ). Here the conversion ${\textstyle \tau ={\frac {\hbar }{\Gamma }}}$ is given instead.