The Standard Model

The Official String Theory Web Site:--> Experiment :--> The Standard Model (basic / advanced)

Through a combination of theory and experiment, a mathematical model that describes or explains all particle physics observed so far by physicists has been worked out. This model is called the Standard Model. From the experimental point of view, the Standard Model is studied and confirmed so well that things are, well, almost boring.

The Standard Model consists of elementary particles grouped into two classes: bosons (particles that transmit forces) and fermions (particles that make up matter). The bosons have particle spin that is either 0, 1 or 2. The fermions have spin 1/2.

Particles that transmit forces

Name	Spin	Electric charge	Mass	Observed?
Graviton	2	0	0	Not yet
Photon	1	0	0	Yes
Gluon	1	0	0	Indirectly
W⁺	1	+1	80 GeV	Yes
W^-	1	-1	80 GeV	Yes
Z⁰	1	0	91 GeV	Yes
Higgs	0	0	> 78 GeV	Not yet

The table above lists the elementary particles in the Standard Model that transmit the four forces observed in Nature. Note that the graviton isn't technically part of the Standard Model but we'll include it anyway. The Standard Model is from a technical standpoint incompatible with gravity, and that's why string theory became an active field of theoretical physics.

When we say that quarks and gluons are observed "indirectly", we mean that evidence of their existence inside hadrons exists but these particles have not been observed singly. In the theory of quarks and gluons, they are believed to be confined inside hadrons and unobservable as single particles, except possibly at extremely high temperatures such as could be found very early in the Big Bang.

Particles that make up matter

The fermions in the Standard Model, particles that make up matter, seem to be grouped into three generations. Notice that the quarks with charge 2/3 come in a group of three, as do the quarks with charge -1/3, as do the electron, muon and tau, and the electron, muon and tau neutrinos. In each group, the heavier particles are shown in the larger type.

Theoretical physics has not explained why there are three generations of particles that make up matter. Maybe string theory will come up with an answer for this.