delta baryon

The Delta baryons (or 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.

Properties

Four closely related baryons exist:  (constituent quarks: uuu),  (uud),  (udd), and  (ddd), which respectively carry an electric charge of +2  , +1 , 0 , and −1 .

The  baryons have a mass of about ; their third component of isospin $\; I_3 = \pm\tfrac ~\mathsf~ \pm\tfrac\;;$ and they are required to have an intrinsic spin of or higher (half-integer units). Ordinary nucleons (symbol , meaning either a proton or neutron), by contrast, have a mass of about , and both intrinsic spin and isospin of . The  (uud) and  (udd) particles are higher-mass spin-excitations of the proton (, uud) and neutron (, udd), respectively.

The and , however, have no direct nucleon analogues: For example, even though their charges are identical and their masses are similar, the  (ddd), is not closely related to the antiproton (, ).

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 or isospin (depending on charge), but with spin , , , ... in ; units. A complete listing of all properties of all these states can be found in Beringer ''et al''. (2013).

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
and the Carnegie Institute of Technology synchro-cyclotron

in the mid-1950s using accelerated positive pions on hydrogen targets. The existence of the , with its unusual +2 electric charge, was a crucial clue in the development of the quark model.

Formation and decay

The Delta states are created when an energetic-enough probe – such as a photon, electron, neutrino, 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 1 232 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 can decay into a proton and a photon and the can decay into a neutron and a photon.

List

^{/sup> PDG reports the resonance width (Γ). Here the conversion $\tau = \frac$ is given instead.}

References

Bibliography

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{{DEFAULTSORT:Delta Baryon
Baryons