Editing Georgi–Glashow model (section)

{{Short description|Grand Unified Theory proposed in 1974}}
{{more citations needed|date=July 2009}}

[[File:Georgi-Glashow charges.svg|300px|right|thumb|The pattern of [[weak isospin]]s, [[weak hypercharge]]s, and strong charges for particles in the Georgi–Glashow model, rotated by the predicted [[weak mixing angle]], showing electric charge roughly along the vertical. In addition to [[Standard Model]] particles, the theory includes twelve colored X bosons, responsible for [[proton decay]].]]

In [[particle physics]], the '''Georgi–Glashow model'''<ref name="GG">{{cite journal |first1=Howard |last1=Georgi |first2=Sheldon |last2=Glashow |s2cid=9063239 |title=Unity of All Elementary-Particle Forces |journal=[[Physical Review Letters]] |volume=32 |issue=8 |year=1974 |pages=438 |doi=10.1103/PhysRevLett.32.438 |bibcode = 1974PhRvL..32..438G }}</ref> is a particular [[Grand Unified Theory]] (GUT) proposed by [[Howard Georgi]] and [[Sheldon Glashow]] in 1974. In this model, the [[Standard Model]] [[Gauge group (mathematics)|gauge groups]] [[SU(3) × SU(2) × U(1)]] are combined into a single [[simple Lie group|simple]] gauge group [[special unitary group|SU(5)]]. The unified group SU(5) is then thought to be [[Spontaneous symmetry breaking|spontaneously broken]] into the Standard Model subgroup below a very high energy scale called the [[grand unification energy|grand unification scale]].

Since the Georgi–Glashow model combines [[lepton]]s and [[quark]]s into single [[irreducible representation]]s, there exist interactions which do not conserve [[baryon]] number, although they still conserve the quantum number [[B-L|{{nowrap|''B – L''}}]] associated with the symmetry of the common representation. This yields a mechanism for [[proton decay]], and the rate of proton decay can be predicted from the dynamics of the model. However, proton decay has not yet been observed experimentally, and the resulting lower limit on the lifetime of the proton contradicts the predictions of this model.  Nevertheless, the elegance of the model has led particle physicists to use it as the foundation for more complex models which yield longer proton lifetimes, particularly [[SO(10) (physics)|SO(10)]] in basic and [[Supersymmetry|SUSY]] variants.

(For a more elementary introduction to how the representation theory of Lie algebras are related to particle physics, see the article [[Particle physics and representation theory]].)

Also, this model suffers from the [[doublet–triplet splitting problem]].