Georgi–Glashow model
In particle physics, the Georgi–Glashow model is a particular grand unified 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 a 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 the quantum number 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. 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, particularly SO(10) in basic and SUSY variants. This model suffers from the doublet–triplet splitting  problem. Construction. SU(5) acts on formula_1 and hence on its exterior algebra formula_2. Picking a formula_3 splitting restricts SU(5) to , yielding matrices of the form with kernel formula_5, hence isomorphic to the standard model's true gauge group formula_6. For the zeroth power formula_7, this acts trivially, matching a left-handed neutrino, formula_8. For the first exterior power formula_9, the standard model's group action preserves the splitting formula_10. The formula_11 transforms trivially in , as a doublet in , and under the representation of (as weak hypercharge is conventionally normalized as ); this matches a right-handed anti-lepton, formula_12 (as formula_13  in SU(2)). The formula_14 transforms as a triplet in SU(3), a singlet in SU(2), and under the Y = − representation of U(1) (as ); this matches a right-handed down quark, formula_15. The second power formula_16 is obtained via the formula formula_17. As SU(5) preserves the canonical volume form of formula_1, Hodge duals give the upper three powers by formula_19. Thus the standard model's representation of one generation of fermions and antifermions lies within formula_2. Similar motivations apply to Pati-Salam, and to SO(10), E6, and other supergroups of SU(5). Explicit Embedding of the Standard Model. Owing to its  relative simple gauge group formula_21 GUTs can be written down in terms of vectors and matrices which allows for an intuitive understanding of the Georgi-Glashow model. The fermion sector is then composed of an anti fundamental formula_22 and an antisymmetric formula_23. In terms of SM degrees of freedoms this can be written as: and with formula_26 and formula_27 the left-handed up and down type quark, formula_28 and formula_29 their righthanded counterparts. formula_30 is the neutrino and formula_31 resp. formula_32 the left and right-handed electron. In addition the fermions we need to break formula_33. This is achieved in  the Georgi–Glashow model via a fundamental formula_34 which contains the SM Higgs: with formula_36 and formula_37 the charged resp. the neutral component of the SM Higgs. Note that the formula_38 are not SM-particles and are thus a prediction of the Georgi–Glashow model. The SM gauge fields can be embedded explicitly as well. For that we recall that a gauge field transforms as an adjoint, and thus can be written as formula_39, with formula_40 the formula_41 generators. Now if we restrict ourself to generators with non-zero entries only in the upper formula_42 block, in the lower formula_43  block or on the diagonal one can identify with the formula_45 colour gauge fields, with the weak formula_47 fields and with the formula_49 hypercharge (up to some normalization formula_50). Using this embedding on can explicitly check that the fermionic fields transform as they should. The explicit embedding can be found in eg. or in the original paper by Georgi and Glashow. Breaking SU(5). SU(5) breaking occurs when a scalar field (which we will denote as formula_51), analogous to the Higgs field, and transforming in the adjoint of SU(5) acquires a vacuum expectation value (VEV) 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". Using the embedding from the previous section on can explicitly check that formula_41 is in deed broken to formula_54 by noting that formula_55. Computation of similar commutators further shows that all other formula_41 gauge fields acquire Masses. To be precise unbroken subgroup is actually, Under their unbroken subgroup, the adjoint 24 transforms as giving the gauge bosons of the standard model plus the new X and Y bosons. 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 formula_59. Under the unbroken subgroup these transform as giving precisely the left-handed fermionic content of the standard model, where for every generation d, u, e and ν 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. Fermions transforming as a 1 under SU(5) are now thought to be necessary because of the evidence for neutrino oscillations, unless a way is found to 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. Minimal supersymmetric SU(5). The minimal supersymmetric SU(5) model assigns a formula_62 matter parity to the chiral superfields with the matter fields having odd parity and the Higgs having even parity to protect the electroweak Higgs from quadratic radiative mass corrections (the hierarchy problem). In the non -supersymmetric version the action is invariant under a similar formula_62 symmetry because the matter fields are all fermionic and thus must appear in the action in pairs, while the Higgs fields are bosonic. Chiral superfields. As complex representations: Superpotential. A generic invariant renormalizable superpotential is a (complex) formula_64 invariant cubic polynomial in the superfields. It is a linear combination of the following terms: The first column is an Abbreviation of the second column (neglecting proper normalization factors), where capital indices are SU(5) indices, and i and j are the generation indices. The last two rows presupposes the multiplicity of formula_66  is not zero (i.e. that a sterile neutrino exists). The coupling formula_67 has coefficients which are symmetric in "i" and "j". The coupling formula_68 has coefficients which are symmetric in "i" and "j". The number of sterile neutrino generations need not be three, unless the SU(5) is embedded in a higher unification scheme such as SO(10). Vacua. The vacua correspond to the mutual zeros of the F and D terms. Let's first look at the case where the VEVs of all the chiral fields are zero except for Φ. The Φ sector. The F zeros corresponds to finding the  stationary points of W subject to the traceless constraint formula_70 So, formula_71 where λ is a Lagrange multiplier. Up to an SU(5) (unitary) transformation, The three cases are called case I, II and III and they break the gauge symmetry into formula_73 and formula_74 respectively (the stabilizer of the VEV). In other words, there are at least three different superselection sections, which is typical for supersymmetric theories. Only case III makes any phenomenological sense and so, we will focus on this case from now onwards. It can be verified that this solution together with zero VEVs for all the other  chiral multiplets is a zero of the F-terms and D-terms. The matter parity remains unbroken (right up to the TeV scale). Decomposition. The gauge algebra 24 decomposes as This 24 is a real representation, so the last two terms need explanation. Both formula_76 and formula_77 are complex representations. However, the direct sum of both representation decomposes into two irreducible real representations and we only take half of the direct sum, i.e. one of the two real irreducible copies. The first three components are left unbroken. The adjoint Higgs also has a similar decomposition, except that it is complex . The Higgs mechanism causes one real HALF of the formula_76 and formula_77 of the adjoint Higgs to be absorbed. The other real half acquires a mass coming from the D-terms. And the other three components of the adjoint Higgs, formula_80 and formula_81 acquire GUT scale masses coming from self pairings of the superpotential, formula_82 The sterile neutrinos, if any exists, would also acquire a GUT scale Majorana mass coming from the superpotential coupling ν. Because of matter parity, the matter representations formula_22 and 10 remain chiral. It is the Higgs fields 5 and formula_84 which are interesting. The two  relevant superpotential terms here are formula_85 and formula_86. Unless there happens to be some fine tuning, we would expect both the triplet terms and the doublet terms to pair up, leaving us with no light electroweak doublets. This is in complete disagreement with phenomenology. See doublet-triplet splitting problem for more details. Problems of the Georgi-Glashow model. Proton decay in SU(5). Unification of the Standard Model via an SU(5) group has significant phenomenological implications. Most notable of these is proton decay, which is present in SU(5) with and without supersymmetry. This is allowed by the new vector bosons introduced  from the adjoint representation of SU(5), which also contains the gauge bosons of the standard model forces. Since these new gauge bosons are in (3,2) bifundamental representations, they violated baryon and lepton number. As a result, the new operators should cause protons to decay at a rate inversely proportional to their masses. This process is called dimension 6 proton decay and is an issue for the model, since the proton is experimentally determined to have a lifetime greater than the age of the universe. This means that an SU(5) model is severely constrained by this process. As well as these  new gauge bosons, in SU(5) models the Higgs field is usually embedded in a 5 representation of the GUT group. The caveat of this is that since the Higgs field is an SU(2) doublet, the remaining part, an SU(3) triplet, must be some new field - usually called D or T. This new scalar would be able to generate proton decay as well and, assuming the most basic Higgs vacuum alignment, would be massless, allowing the process at very high rates. While not an issue in the Georgi–Glashow model, a supersymmeterised SU(5) model would have additional proton decay operators due  to the superpartners of the standard model fermions. The lack of detection of proton decay (in any form) brings into question the veracity of SU(5) GUTs of all types, however, while the models are highly constrained by this result, they are not in general ruled out. Mechanism. In the lowest-order Feynman diagram corresponding to the simplest source of proton decay in SU(5), a left-handed and a right-handed up quark annihilate, yielding an X boson, which decays to a right-handed (or left-handed) positron and a left-handed (or right-handed) anti-down quark: This process conserves  weak isospin, weak hypercharge, and color. GUTs equate anti-color with having 2 colors, formula_89, and SU(5) defines left-handed normal leptons as "white" and right-handed antileptons as "black." The first vertex only involves fermions of the representation, while the second only involves fermions in the (or ), demonstrating the preservation of SU(5) symmetry. Doublet-Triplet Splitting. As mentioned in the above section the colour triplet of the formula_90 which contains the SM Higgs can mediate dimension 6 proton decay. Since protons seem to be quite stable such a triplet has to acquire a quite large mass in order to  suppress the decay. This is however problematic. For that consider the scalar part of the Greorgi-Glashow Lagrangian: We here have denoted the adjoint used to break formula_41 to the SM with formula_51, it is VEV by formula_94 and formula_95 the defining rep. which contains the SM Higgs formula_96 and the colour triplet formula_97 which can induce proton decay. As mentioned we require formula_98 in order to suppress proton decay sufficiently. On the other hand, the formula_99 is typically of order formula_100 in order to be consistent with observations. Looking at the above equation it becomes clear that one has  to be very precise in choosing the parameters formula_101 and formula_102: any two random parameter will not do since then formula_99 and formula_104 would be of the same order! This is known as the doublet-triplet (DT) splitting problem: In order to be consistent we have to 'split' the 'masses' of formula_97 and formula_96, but for that we need to fine-tune formula_101 and formula_102. There are however some solutions to this problem (see eg.) which can work quite well in SUSY models. A review of the DT splitting problem can be found in. Neutrino masses. As the SM the  original Georgi–Glashow model proposed in does not include neutrino masses. However, since Neutrino oscillation has been observed such masses are required. The soluions to this problem follow the same ideas which have been applied to the SM: One on hand on can include a formula_41 singulet which then can generate either Dirac masses or Majorana masses. As in the SM one can also implement the type-I seesaw mechanism which then generates naturally light masses. On the other hand, on can just parametrize the ignorance about neutrinos using the dimension 5 Weinbergoperator: with formula_111 the formula_112 Yukawa matrix required  for the mixing between flavours.