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In particle physics, the Georgi–Glashow model is a particular grand unification theory (GUT) proposed by Howard Georgi and Sheldon Glashow in 1974. In this model the standard model gauge groups SU(3) × SU(2) × U(1) are combined into a single simple gauge group—SU(5). The unified group SU(5) is then thought to be spontaneously broken into the standard model subgroup below some very high energy scale called the grand unification scale. Since the Georgi–Glashow model combines leptons and quarks into single irreducible representations, there exist interactions which do not conserve baryon number, although they still conserve B-L. 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. However, the elegance of the model has led particle physicists to use it as the foundation for more complex models which yield longer proton lifetimes. (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.) This model suffers from the doublet-triplet splitting problem. ==Breaking SU(5)== SU(5) breaking occurs when a scalar field, analogous to the Higgs field, and transforming in the adjoint of SU(5) acquires a vacuum expectation value proportional to the weak hypercharge generator : When this occurs SU(5) is spontaneously broken to the subgroup of SU(5) commuting with the group generated by ''Y''. This unbroken subgroup is just the standard model group: . Under the unbroken subgroup the adjoint 24 transforms as : giving the gauge bosons of the standard model. See restricted representation. The standard model quarks and leptons fit neatly into representations of SU(5). Specifically, the left-handed fermions combine into 3 generations of . Under the unbroken subgroup these transform as : (dc and l) : (q, uc and ec) : (νc) giving precisely the left-handed fermionic content of the standard model, where for every generation dc, uc, ec and νc stand for anti-down-type quark, anti-up-type quark, anti-down-type lepton and anti-up-type lepton, respectively, and q and l stand for quark and lepton. Note that fermions transforming as a 1 under SU(5) are now thought to be necessary because of the evidence for neutrino oscillations. Actually though, it is possible for there to be only left-handed neutrinos without any right-handed neutrinos if we could somehow introduce a tiny Majorana coupling for the left-handed neutrinos. Since the homotopy group : this model predicts 't Hooft–Polyakov monopoles. These monopoles have quantized Y magnetic charges. Since the electromagnetic charge Q is a linear combination of some SU(2) generator with Y/2, these monopoles also have quantized magnetic charges, where by magnetic here, we mean electromagnetic magnetic charges. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Georgi–Glashow model」の詳細全文を読む スポンサード リンク
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