The Standard Model
From SydneyHEPWiki
Most of the current knowledge about particles and their interactions is assimilated in the Standard Model of particle physics. This was first formulated in the 1970s and has undergone minor modifications since.
According to the standard model, all matter is built up from a small number of fundamental spin 1/2 particles, or fermions, six quarks and six leptons.
The leptons carry integral electric charge, and are found in three 'families' or pairs, consisting of a lepton and its corresponding neutrino.
The quarks carry fractional charges of +2/3 |e| or -1/3 |e|, and also occur in 3 families.
Protons and neutrons consist of triplets of the lightest up(u) and down(d) quarks, a proton made up of uud and a neutron of udd. The heavier quarks also form particles similar to the proton and the neutron, but these are unstable and decay very rapidly. These are usually observed only in high energy collisions, in accelerators or cosmic rays.
The Standard Model also includes an explanation of the reactions of these particles. It talks about four fundamental types of interactions:
It has been speculated by physicists from Einstein on, that the different interactions are aspects of a single Unified Field. Physicists have so far succeeded in unifying the weak and the electromagnetic interactions into the Electroweak model. It is also thought that Strong interactions might also be unified with these at very high energies. Slightly less plausibly (at least for now), it is thought that gravity also might be included in such a picture to arrive at a Grand Unified Theory, or the (misnamed) Theory of Everything (On Physics beyond the Standard Model, see Donald H. Perkins, Introduction to High Energy Physics 4th Edition, Chapter 9).
Besides the fact that the strong and the gravitational forces are not yet unified, there are other problems:
- Difficulties start to arise with the standard model at very high mass scales (~1019GeV). This is called the Hierarchy Problem.
- There are a number of arbitrary parameters in the standard model - a large number of empirical masses, coupling constants, mixing angles and so on, which are yet to be explained.
- It is currently thought, on the basis of results of the Super Kamiokande experiment and others, that neutrinos have mass. Yet, we do not know whether this would mean that, as Majorana suggested, the neutrino is its own anti-particle, or whether neutrinos are spin 1/2 particles with one of the two spin-substates missing.
- In 1966, Andrei Sakharov proposed that one of the three necessary criteria for the existence of a matter-dominated universe was CP-violation. Yet, the Standard Model seriously underpredicts the amount of CP-violation required for matter-antimatter asymmetry.
It is hoped that the newest generation of accelerators at CERN and elsewhere might improve our understanding of these, and other problems.
