Grand Unified Theory (GUT, GUT or GUT - all three abbreviations will be used in the article) is a model in particle physics in which, at high energy, the three gauge interactions of the standard model that determine the electromagnetic, weak and strong interactions or forces are combined into one single force. This combined interaction is characterized by one symmetry of greater gauge, and therefore several carrier forces, but one permanent bond. If a grand unification occurs in nature, there is a possibility of a grand unification era in the early universe in which the fundamental forces are not yet distinct.
Grand Unified Theory in brief
Models that do not unify all interactions using one simple group as gauge symmetry, do so using semisimple groups, can exhibit similar properties and are sometimes also called grand unification theories.
Combining gravity with the other three forces would provide a theory of everything (OO) rather than a GUT. However, GUT is often seen as an intermediate step towards OO. These are all characteristic ideas for the great theories of unification and superunification.
The new particles predicted by the GUT models are expected to have masses around the GUT scale - just a few orders of magnitude below the Planck scale - and therefore out of reach for any proposed particle collider experiments. Therefore, particles predicted by GUT models cannot be observed directly, and instead, grand unification effects can be detected through indirect observations such as proton decay, elementary particle electric dipole moments, or neutrino properties. Some GUTs, such as the Pati Salam model, predict the existence of magnetic monopoles.
Characteristics of models
GUT models, which aim to be completely realistic, are quite complex, even compared to the standard model, because they must introduce additional fields and interactions, or even additional dimensions of space. The main reason for this complexity lies in the difficulty of reproducing the observed fermion masses and mixing angles, which may be due to the existence of some additional family symmetries outside the traditional GUT models. Because of this difficulty and the absence of any observable grand unification effect, there is still no generally accepted GUT model.
Historically firsta true GUT based on Lee's simple SU group was proposed by Howard George and Sheldon Glashow in 1974. The Georgi-Glashow model was preceded by the semisimple Lie algebra Pati-Salam model proposed by Abdus Salam and Jogesh Pati, who first proposed unifying gauge interactions.
Name history
The abbreviation GUT (GUT) was first coined in 1978 by CERN researchers John Ellis, Andrzej Buras, Mary C. Gayard and Dmitry Nanopoulos, but in the final version of their article they chose GUM (great unification mass). Nanopoulos later that year was the first to use the acronym in an article. In short, a lot of work has been done on the way to the Grand Unified Theory.
Commonness of concepts
The abbreviation SU is used to refer to grand unification theories, which will be referred to frequently throughout this article. The fact that the electric charges of electrons and protons seem to cancel each other out with extreme precision is essential to 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 the strong and weak interactions in the Standard Model is based on gauge symmetries governed by simple SU(3) and SU(2) symmetry groups that allow only discrete charges, the remaining component, the weak hypercharge interaction, is described by the Abelian U(1), which in principle allowsarbitrary distribution of charges.
The observed charge quantization, namely the fact that all known elementary particles carry electric charges that appear to be exact multiples of ⅓ of the elementary charge, led to the idea that hypercharge interactions and possibly strong and weak interactions could be built into one grand unified interaction described by one larger simple symmetry group containing the standard model. This will automatically predict the quantized nature and values of all charges of elementary particles. Because it also leads to a prediction of the relative strengths of the underlying interactions we observe, in particular the weak mixing angle, Grand Unification ideally reduces the number of independent inputs, but is also limited to observations. As universal as the grand unified theory may seem, books on it are not very popular.
Georgie-Glasgow Theory (SU (5))
The grand unification is reminiscent of the unification of electric and magnetic forces in Maxwell's theory of electromagnetism in the 19th century, but its physical meaning and mathematical structure are qualitatively different.
However, it is not obvious that the simplest possible choice for the extended grand unified symmetry is to produce the correct set of elementary particles. The fact that all currently known particles of matter fit well into the three smallest SU(5) group representation theories and immediately carry the correct observable charges is one of the first andthe most important reasons why people believe that the grand unified theory can actually be realized in nature.
The two smallest irreducible representations of SU(5) are 5 and 10. In the standard notation, 5 contains the charge conjugates of a right-handed down-type color triplet and a left-lefton isospin doublet, while 10 contains six components of an up-type quark, color a triplet of a left-handed down-type quark and a right-handed electron. This scheme must be reproduced for each of the three known generations of matter. It is noteworthy that the theory does not contain anomalies with this content.
Hypothetical right-handed neutrinos are an SU(5) singlet, meaning that its mass is not forbidden by any symmetry; it does not need to spontaneously break symmetry, which explains why its mass will be large.
Here, the unification of matter is even more complete, since the irreducible spinor representation 16 contains both 5 and 10 of SU(5) as well as right-handed neutrinos, and thus the total content of particles of one generation of the extended standard model with neutrino masses. This is already the largest simple group that achieves the unification of matter in a scheme that includes only already known particles of matter (except for the Higgs sector).
Because the various standard model fermions are grouped into larger representations, GUTs specifically predict relationships between fermion masses, such as between an electron anddown quark, muon and strange quark, and tau lepton and down quark for SU(5). Some of these mass ratios approximate, but most don't.
SO(10) theory
The bosonic matrix for SO(10) is found by taking a 15×15 matrix of 10 + 5 representation of SU(5) and adding an extra row and column for the right neutrino. The bosons can be found by adding a partner to each of the 20 charged bosons (2 right W bosons, 6 massive charged gluons and 12 X/Y type bosons) and adding an extra heavy neutral Z boson to make 5 neutral bosons. The bosonic matrix will have a boson or its new partner in each row and column. These pairs combine to create the familiar 16D Dirac spin matrices SO(10).
Standard Model
Non-chiral extensions of the Standard Model with vector spectra of split multiplet particles that naturally appear in higher SU(N) GUTs significantly change desert physics and lead to realistic (row-scale) grand unification for the usual three quark-lepton families even without using supersymmetry (see below). On the other hand, due to the emergence of a new missing VEV mechanism emerging in the supersymmetric SU(8) GUT, a simultaneous solution to the gauge hierarchy problem (doublet-triplet splitting) and the flavor unification problem can be found.
Other theories and elementary particles
GUT with four families/generations, SU(8): assuming 4 generations of fermions instead of 3 generate a total of 64 particle types. They can be placed in 64=8 + 56 SU(8) representations. This can be divided into SU(5) × SU(3) F × U(1), which is the SU(5) theory, along with some heavy bosons that affect the generation number.
GUT with four families/generations, O(16): Again, assuming 4 generations of fermions, 128 particles and antiparticles can fit into a single O(16) spinor representation. All these things were discovered on the way to the grand unified theory.
