Cosmology

The Fundamental Forces in Nature. Positrons and Other Antiparticles. Mesons

and the Beginning of Particle Physics. Classification of Particles. Conservation Laws. Strange Particles and Strangeness. Finding Patterns in the Particles. Quarks. Multicolored Quarks. The Standard Model. The Cosmic Connection. Problems and Perspectives.

by four fundamental forces acting between particles. In order of

decreasing strength, they are the nuclear force, the electromagnetic force, the weak force, and the gravitational force.

of the electron that successfully explained the origin of the

electron’s spin and its magnetic moment.

Dirac circumvented this difficulty by postulating that all negative energy states are filled. The electrons occupying these negative energy states are collectively called the Dirac sea.

The hole can react to external forces and is observable. The hole reacts in a way similar to that of the electron except that it has a positive charge: it is the antiparticle to the electron.

by 300-MeV gamma rays striking a lead sheet from the

left. (b) The pertinent pair-production events. The positrons deflect upward and the electrons downward in an applied magnetic field.

photon mediating the electromagnetic force between two electrons.
Pions and muons

are very unstable particles. The muon, which has a mean lifetime of 2.2 ms, then decays to an electron, a neutrino, and an antineutrino:

a proton and a neutron interacting via the nuclear force

with a neutral pion mediating the force. (This model is not the current model for nucleon interaction.) (b) Feynman diagram for an electron and a neutrino interacting via the weak force, with a Z0 boson mediating the force.

classified into two broad categories, hadrons and leptons. The criterion

for separating these particles into categories is whether or not they interact via the strong force. The nuclear force between nucleons in a nucleus is a particular manifestation of the strong force, but we will use the term strong force to refer to any interaction between particles made up of quarks.

created in a decay or nuclear reaction, an antibaryon is

also created. This scheme can be quantified by assigning every particle a quantum number, the baryon number, as follows: B =+1 for all baryons, B=-1 for all antibaryons, and B=0 for all other particles. The law of conservation of baryon number states that

whenever a nuclear reaction or decay occurs, the sum of the baryon numbers before the process must equal the sum of the baryon numbers after the process.

one for each variety of lepton. The law of conservation

of electron lepton number states that

whenever a nuclear reaction or decay occurs, the sum of the electron lepton numbers before the process must equal the sum of the electron lepton numbers after the process.

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.

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.

baryons known at the time the pattern was proposed.

photograph on the left shows the original bubble-chamber tracks. The

drawing on the right isolates the tracks of the important events.

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.

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.

neutron explained in terms of Yukawa’s pionexchange model. b) The

same interaction, explained in terms of quarks and gluons.

for the strong interaction is referred to in high-energy physics

as the Standard Model.

The Standard Model of particle physics.