Standard Model
The Standard Model is a theory of particle physics which describes the strong, weak, and electromagnetic fundamental forces. It uses the theoretical framework of quantum field theory, and is therefore consistent with both quantum mechanics and special relativity. To date, almost all experimental tests of the three forces described by the Standard Model have agreed with its predictions. However, the Standard Model is not a complete theory of fundamental interactions, primarily because it does not describe gravity.
The Standard Model combines the theory of quantum chromodynamics with the theory of the electroweak interaction (which describes the weak and electromagnetic interactions.) Each of these theories are gauge field theories, meaning that they describe fermions coupled to bosons. The Lagrangian of each set of bosons is invariant under a transformation called a gauge transformation. Each gauge transformation can be described by a unitary group, known as a "gauge group". The gauge group of the strong interaction is SU(3), and the gauge group of the electroweak interaction is SU(2)×U(1). Thus, the Standard Model is often referred to as SU(3)×SU(2)×U(1).
Although the Standard Model has had great success in explaining experimental results, it has never been accepted as a complete theory of fundamental physics. This is because it has two important defects:
- The theory contains 17 parameters, such as particle masses, which must be determined experimentally. These parameters cannot be independently calculated.
- The theory does not describe the gravitational interaction.
Since the completion of the Standard Model, many efforts have been made to address both problems.
One attempt to address the first defect is known as grand unification. The so-called grand unified theories (GUTs) hypothesized that the SU(3), SU(2), and U(1) groups are actually subgroups of a single large symmetry group. At high energies (far beyond the reach of current experiments), the symmetry of the unifying group is preserved; at low energies, it reduces to SU(3)×SU(2)×U(1) by a process known as spontaneous symmetry breaking. The first theory of this kind was proposed in 1974 by Georgi and Glashow, using SU(5) as the unifying group. A distinguishing characteristic of these GUTs is that, unlike the Standard model, they predict the existence of proton decay. In 1999, the Super-Kamiokande neutrino observatory reported that it had not detected proton decay, establishing a lower limit on the proton half-life of 6.7× 1032 years. This and other experiments have falsified numerous GUTs, including SU(5).
The Standard Model also predicts the existence of the Higgs boson, a particle which has not been observed as of 2002.
The first experimental deviation from the Standard Model came in 1998, when Super-Kamiokande published results indicating the existence of a non-zero neutrino mass. The Standard Model cannot accomodate massive neutrinos, because it predicts only the existence of "left-handed" neutrinos, which have spin aligned opposite to their linear momentum. If neutrinos have non-zero mass, they necessarily travel slower than the speed of light. Therefore, it would be possible to "overtake" a neutrino, adopting a reference frame in which the neutrino momentum is reversed and aligned with its spin, and therefore right-handed. This implies the existence of right-handed neutrinos.
References
Y. Hayato et al., Search for Proton Decay through p → νK+ in a Large Water Cherenkov Detector. Phys. Rev. Lett. 83, 1529 (1999).