What is the Standard Model and what is the Standard-Model Extension?
All elementary particles and their nongravitational interactions are very successfully described by a theory called the Standard Model of particle physics. At the classical level, gravity is well described by Einstein’s General Relativity. Both these theories have local Lorentz symmetry. We have constructed a generalization of the usual Standard Model and General Relativity that has all the conventional desirable properties but that allows for violations of Lorentz and CPT symmetry. This theory is called the Standard-Model Extension, or SME. The Standard-Model Extension provides a quantitative description of Lorentz and CPT violation, controlled by a set of coefficients whose values are to be determined or constrained by experiment. A type of converse to the CPT theorem has recently been proved under mild assumptions: if CPT is violated, then Lorentz symmetry is too. This implies any observable CPT violation is described by the Standard-Model Extension. See O.W. Greenberg, Phys. Rev. L
