Strange Particles and Strangeness
The law of conservation of strangeness states that
in a nuclear reaction or decay that occurs via the strong force, strangeness is conserved; that is, the sum of the strangeness numbers before the process must equal the sum of the strangeness numbers after the process. In processes that occur via the weak interaction, strangeness may not be conserved.
Finding Patterns in the Particles
(a) The hexagonal eightfold-way pattern for the eight spin-1/2 baryons. This strangenessversus-charge plot uses a sloping axis for charge number Q and a horizontal axis for strangeness S. (b) The eightfold-way pattern for the nine spin-zero mesons.
Finding Patterns in the Particles
The pattern for the higher-mass, spin-3/2 baryons known at the time the pattern was proposed.
Finding Patterns in the Particles
Discovery of the Ω- particle. The photograph on the left shows
the original bubble-chamber tracks. The drawing on the right isolates the tracks of the important events.
Quarks
The Original Quark Model
The model has three types of quarks, designated by the symbols u, d, and s, that are given the arbitrary names up, down, and strange. The various types of quarks are called flavors.
Quark composition of two mesons and two baryons.
The compositions of all hadrons known when Gell-Mann and Zweig presented their model can be completely specified by three simple rules:
•A meson consists of one quark and one antiquark, giving it a baryon number of 0, as required.
•A baryon consists of three quarks.
•An antibaryon consists of three antiquarks.
The theory put forth by Gell-Mann and Zweig is referred to as the original quark model.
Quarks
Charm and Other Developments
Multicolored Quarks
The theory of how quarks interact with each other is called quantum chromodynamics, or QCD, to parallel the name quantum electrodynamics (the theory of the electrical interaction between light and matter). In QCD, each quark is said to carry a color charge, in analogy to electric charge. The strong force between quarks is often called the color force. Therefore, the terms strong force and color force are used interchangeably.
(a) A green quark is attracted to an antigreen quark. This forms a meson whose quark structure is (
). (b) Three quarks of different colors attract one another to form a baryon.
Multicolored Quarks
(a) A nuclear interaction between a proton and a neutron explained in terms of Yukawa’s pionexchange model. b) The same interaction, explained in terms of quarks and gluons.
The Standard Model
The combination of the electroweak theory and QCD for the strong interaction is referred to in high-energy physics as the Standard
Model.
The Standard Model of particle physics.
The Cosmic Connection