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).

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, a spin of ​3⁄2, and an isospin of ​3⁄2. Ordinary protons and neutrons (nucleons (symbol N)), by contrast, have a mass of about 939 MeV/c2, a spin of ​1⁄2, and an isospin of ​1⁄2. The
Δ+
 (uud) and
Δ0
 (udd) particles are the higher-mass excitations of the proton (
N+
, uud) and neutron (
N0
, udd), respectively. However, the
Δ++
 and
Δ−
 have no direct nucleon analogues.

The states were established experimentally at the University of Chicago cyclotron[1] and the Carnegie Institute of Technology synchro-cyclotron[2] in the mid-1950s using accelerated positive pions on hydrogen targets. The existence of the
Δ++
, with its unusual +2 charge, was a crucial clue in the development of the quark model.

The Delta states discussed here are only the lowest-mass quantum excitations of the proton and neutron. At higher masses, additional Delta states appear, all defined by having ​3⁄2 units of isospin, but with a spin quantum numbers including ​1⁄2, ​3⁄2, ​5⁄2, ... ​11⁄2. A complete listing of all properties of all these states can be found in Ref.[3].

There also exist antiparticle Delta states with opposite charges, made up of the corresponding antiquarks.

Formation and Decay

The Delta states are created when an energetic-enough probe such as a photon or pion impinges upon a proton or neutron, or possibly by the collision of an energetic-enough nucleon pair.

All of the Δ baryons with mass near 1232 MeV quickly decay via the strong force 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 and more slowly, 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[3]
Δ++
(1232)
u

u

u
 1,232 ± 2 +​3⁄2 ​3⁄2+ +2 0 0 0 0 (5.63±0.14)×10−24[a]
p+
 +
π+
Delta[3]
Δ+
(1232)
u

u

d
 1,232 ± 2 +​1⁄2 ​3⁄2+ +1 0 0 0 0 (5.63±0.14)×10−24[a]
π+
 +
n0
, or


π0
 +
p+

Delta[3]
Δ0
(1232)
u

d

d
 1,232 ± 2 −​1⁄2 ​3⁄2+ 0 0 0 0 0 (5.63±0.14)×10−24[a]
π0
 +
n0
, or


π−
 +
p+

Delta[3]
Δ−
(1232)
d

d

d
 1,232 ± 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 is given instead.

References

  1. ↑ H. L. Anderson, E. Fermi, E. A. Long, and D. E. Nagle, “Total Cross Sections of Positive Pions in Hydrogen.” Phys. Rev., 85, 936 (1952). and ibid. p. 934.
  2. ↑ J. Ashkin et al., “Pion Proton Scattering at 150 and 170 MeV.” Phys. Rev., 101, 1149 (1956).
  3. 1 2 3 4 5 J. Beringer et al. (2013): Particle listings –
    Δ
    (1232)

Bibliography

  • C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics". Physics Letters B. 667 (1): 1. Bibcode:2008PhLB..667....1A. doi:10.1016/j.physletb.2008.07.018.
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