standard model

 

By the early 1930s, the 19th century view of atoms as indivisible elementary particles had been replaced and a larger group of physically smaller entities now enjoyed this status: electron, protons and neutrons. The 1950s also saw technological developments that enabled high energy beams of particles to be produced in laboratories. By the 1960s there was the discovery of a very large number of particles which were unstable with very short lifetimes and there was an urgent need for a theory that could make sense of all these states so called quark model.     

   However, evidence for the existence of quarks as real particle came in 1960s from series of experiments analogous to those of Rutherford and his co-workers.

The best theory of elementary particles we have at present is called standard model. This aims to explain all the phenomena of particle physics, expect those due to gravity, in terms of all the properties and interactions of a small number of elementary or fundamental particles. An elementary particle is characterized by its mass, electric charge and its spin. This latter is a permanent angular momentum possessed by all particles in quantum theory, even they are at rest.

Particle with half integer spin are fermions and with the integer spin are called bosons. There are three families of elementary particles in standard model: two spin ½ families of fermions called leptons and quarks: and one family of spin-1 bosons, and also other spin -0 particles, called the Higgs boson. The most familiar elementary particle is the electron, which we know is bound in atoms by the electromagnetic interaction, one of the forces of nature. It is the member of the family of leptons. Another is neutrino is a decay product in β-decays. The force responsible for β-decay is an example of a second fundamental force, the weak interaction. Finally third one is strong interaction which binds quarks in nucleons.

In quantum theory, the interaction is transmitted discontinuously by the exchange of photons, which are members of family of fundamental spin-1 bosons of the standard model. Photons are referred to as the gauge bosons or force carriers of the electromagnetic interaction. This weak and strong interactions are also mediated by the exchanged of spin-1 gauge bosons. For the weak interaction these are W+ , W- and Z bosons with masses about 80-90 times Mp. For the strong interaction, the force carriers are called gluons. There are eight gluons, all of which have zero mass and electrically neutral.