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Grand Unification is no longer a theory as clearly explained on EinsteinGravity.com The term Grand Unified Theory or GUT, refers to any of several similar models in particle physics in which at high energy scales, the three gauge interactions of the Standard Model which define the electromagnetic, weak, and strong interactions, are merged into one single interaction characterized by a larger gauge symmetry and one unified coupling constant rather than three independent ones.
This unification is possible due to the energy scale dependence of parameters in quantum field theory called renormalization group running, which allows parameters with vastly different values at collider energies to converge at much higher energy scales, generically between 10^{14} GeV and the Planck scale. Due to this property, the physics of most grand unification scenarios cannot be probed directly at particle colliders, but might instead be accessible through indirect observations such as proton decay or the properties of neutrinos.^{[1]}
Unifying gravity with the other three interaction would form a theory of everything (TOE), rather than a GUT. Likewise, models which do not unify all interactions using one simple Lie group as the gauge symmetry, but constitute variations of the idea, for example using semisimple groups, can exhibit similar properties and are sometimes referred to as Grand Unified Theories as well.
HistoryEdit
Historically, the first true GUT which was based on the simple Lie group SU(5), was proposed by Howard Georgi and Sheldon Glashow in 1974.^{[2]} The Georgi–Glashow model was preceded by the so called Pati–Salam model by Jogesh Pati and Abdus Salam,^{[3]} who pioneered the idea to unify gauge interactions, but did not aim to unify all standard model interactions.
MotivationEdit
The fact that the electric charges of electrons and protons seem to cancel each other exactly to extreme precision is essential for the existence of the macroscopic world as we know it, but this important property of elementary particles is not explained in the Standard Model of particle physics. While the description of strong and weak interactions within the Standard Model is based on socalled gauge symmetries governed by the simple symmetry groups SU(3) and SU(2) which allow only discrete charges, the remaining component, the socalled weak hypercharge interaction is described by an abelian symmetry U(1) which in principle allows for arbitrary charge assignments.^{[note 1]} The observed charge quantization, namely the fact that all known elementary particles carry electric charges which appear to be exact multiples of ^{1}⁄_{3} of the "elementary" charge, has led to the idea that hypercharge interactions and possibly the strong and weak interactions might be embedded in one Grand Unified interaction described by a single, larger simple symmetry group containing the Standard Model, which would automatically predict the quantized nature and values of all elementary particle charges. Since this also results in a prediction for the relative strengths of the fundamental interactions which we observe, in particular the weak mixing angle, Grand Unification ideally reduces the number of independent input parameters, but is also constrained by observations.
Grand Unification is reminiscent of the unification of electric and magnetic forces by Maxwell's theory of electromagnetism in the 19th century, but its physical implications and mathematical structure are qualitatively different.
The Unification of Matter ParticlesEdit
The simplest GUT: SU(5) Grand Unification Edit
The smallest simple Lie group which contains the standard model, and upon which the first Grand Unified Theory was based, is
$ SU(5) \supset SU(3)\times SU(2)\times U(1) $.
Such group symmetries allow the reinterpretation of several known particles as different states of a single particle field. However, it is not obvious that the simplest possible choices for the extended "Grand Unified" symmetry should yield the correct inventory of elementary particles. The fact that all currently known (2009) matter particles fit nicely into three copies of the smallest group representations of $ SU(5) $ and immediately carry the correct observed charges, is one of the first and most important reasons why people believe that a Grand Unified Theory might actually be realized in nature.
The two smallest irreducible representations of $ SU(5) $ are $ \mathbf 5 $ and $ \mathbf 10 $. In the standard assignment, the $ \mathbf 5 $ contains the charge conjugates of the righthanded downtype quark color triplet and a lefthanded lepton isospin doublet, while the $ \mathbf 10 $ contains the six uptype quark components, the lefthanded downtype quark color triplet, and the righthanded electron. This scheme has to be replicated for each of the three known generations of matter. It is notable that the theory is anomaly free with this matter content.
The hypothetical righthanded neutrinos are not contained in any of these representations, which can explain their relative heaviness (see Seesaw Mechanism).
SO(10) Grand UnificationEdit
The next simple Lie group which contains the standard model is
$ SO(10)\supset SU(5)\supset SU(3)\times SU(2)\times U(1) $.
Here, the unification of matter is even more complete, since the irreducible spinor representation $ \mathbf 16 $ contains both the $ \mathbf {\overline{5}} $ and $ \mathbf 10 $ of $ SU(5) $ and a righthanded neutrino, and thus the complete particle content of one generation of the extended standard model with neutrino masses. This is already the largest simple group which achieves the unification of matter in a scheme involving only the already known matter particles (apart from the Higgs sector).
Since different standard model fermions are grouped together in larger representations, GUTs specifically predict relations among the fermion masses, such as between the electron and the down quark, the muon and the strange quark, and the tau lepton and the bottom quark for SU(5) and SO(10). Some of these mass relations hold approximately, but most don't (see GeorgiJarlskog mass relation).
The Unification of Forces and the role of SupersymmetryEdit
The renormalization group running of the threegauge couplings has been found to nearly, but not quite, meet at the same point if the hypercharge is normalized so that it is consistent with SU(5) or SO(10) GUTs, which are precisely the GUT groups which lead to a simple fermion unification. This is a significant result, as other Lie groups lead to different normalizations. However, if the supersymmetric extension MSSM is used instead of the Standard Model, the match becomes much more accurate. It is commonly believed that this matching is unlikely to be a coincidence. Also, most model builders simply assume supersymmetry (SUSY) because it solves the hierarchy problem—i.e., it stabilizes the electroweak Higgs mass against radiative corrections. And the Majorana mass of the righthanded neutrino SO(10) theories with its mass set to the gauge unification scale is examined, values for the lefthanded neutrino masses (see neutrino oscillation) are produced via the seesaw mechanism. These values are 10–100 times smaller than the GUT scale, but still relatively close.
(For a more elementary introduction to how Lie algebras are related to particle physics, see the article Particle physics and representation theory.)
Proposed theories Edit
Several such theories have been proposed, but none is currently universally accepted. An even more ambitious theory that includes all fundamental forces, including gravitation, is termed a theory of everything. Some common mainstream GUT models are:


Not quite GUTs:
Note: These models refer to Lie algebras not to Lie groups. The Lie group could be [SU(4)×SU(2)×SU(2)]/Z_{2}, just to take a random example.
The most promising candidate is SO(10).
 See also: Error: Template must be given at least one article name (Minimal) SO(10) does not contain any exotic fermions (i.e. additional fermions besides the Standard Model fermions and the righthanded neutrino), and it unifies each generation into a single irreducible representation. A number of other GUT models are based upon subgroups of SO(10). They are the minimal leftright model, SU(5), flipped SU(5) and the PatiSalam model. The GUT group E_{6} contains SO(10), but models based upon it are significantly more complicated. The primary reason for studying E_{6} models comes from E_{8} × E_{8} heterotic string theory.
GUT models generically predict the existence of topological defects such as monopoles, cosmic strings, domain walls, and others. But none have been observed. Their absence is known as the monopole problem in cosmology. Most GUT models also predict proton decay, although not the PatiSalam model; current experiments still haven't detected proton decay. This experimental limit on the proton's lifetime pretty much rules out minimal SU(5).
Some GUT theories like SU(5) and SO(10) suffer from what is called the doublettriplet problem. These theories predict that for each electroweak Higgs doublet, there is a corresponding colored Higgs triplet field with a very small mass (many orders of magnitude smaller than the GUT scale here). In theory, unifying quarks with leptons, the Higgs doublet would also be unified with a Higgs triplet. Such triplets have not been observed. They would also cause extremely rapid proton decay (far below current experimental limits) and prevent the gauge coupling strengths from running together in the renormalization group.
Most GUT models require a threefold replication of the matter fields. As such, they do not explain why there are three generations of fermions. Most GUT models also fail to explain the little hierarchy between the fermion masses for different generations.
IngredientsEdit
A GUT model basically consists of a gauge group which is a compact Lie group, a connection form for that Lie group, a YangMills action for that connection given by an invariant symmetric bilinear form over its Lie algebra (which is specified by a coupling constant for each factor), a Higgs sector consisting of a number of scalar fields taking on values within real/complex representations of the Lie group and chiral Weyl fermions taking on values within a complex rep of the Lie group. The Lie group contains the Standard Model group and the Higgs fields acquire VEVs leading to a spontaneous symmetry breaking to the Standard Model. The Weyl fermions represent matter.
Current statusEdit
As of 2009^{[update]}, there is still no hard evidence that nature is described by a Grand Unified Theory. Moreover, since the Higgs particle has not yet been observed, the smaller electroweak unification is still pending.^{[4]} The discovery of neutrino oscillations indicates that the Standard Model is incomplete and has led to renewed interest toward certain GUT such as $ SO(10) $. One of the few possible experimental tests of certain GUT is proton decay and also fermion masses. There are a few more special tests for supersymmetric GUT.
The gauge coupling strengths of QCD, the weak interaction and hypercharge seem to meet at a common length scale called the GUT scale and equal approximately to $ 10^{16} $ GeV, which is slightly suggestive. This interesting numerical observation is called the gauge coupling unification, and it works particularly well if one assumes the existence of superpartners of the Standard Model particles. Still it is possible to achieve the same by postulating, for instance, that ordinary (non supersymmetric) $ SO(10) $ models break with an intermediate gauge scale, such as the one of PatiSalam group.
See alsoEdit
 Grand unification energy
 Fundamental interaction
 Particle physics and representation theory
 Classical unified field theories
 X and Y bosons
 BL quantum number
NotesEdit
 ↑ There are however certain constraints on the choice of particle charges from theoretical consistency, in particular anomaly cancellation.
ReferencesEdit
 ↑ Ross, G. (1984). Grand Unified Theories, Westview Press. ISBN 9780805369687.
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 ↑ Hawking, S.W. (1996). A Brief History of Time: The Updated and Expanded Edition.'' (2nd ed.), Bantam Books. p. XXX. ISBN 0553380168.
 An account of the origin of the term GUT
 Stephen Hawking, A Brief History of Time, includes a brief popular overview.
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