Grand unification theory

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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. The physics of most models of grand unification cannot be discovered directly at particle colliders because the new particles they predict have masses at the so-called GUT scale which lies few orders of magnitude below the Planck scale and thus far beyond the reach of collision experiments. Instead, information about grand unification might be obtained through indirect observations such as proton decay or the properties of neutrinos. [1] Some grand unified theories predict the existence of magnetic monopoles.

Currently (2011), all GUT models which aim to be completely realistic are quite complicated even compared to the Standard Model because they need to introduce additional structures such as new fields and interactions, or even additional dimensions of space. The main reason for this lies in the difficulty of reproducing the observed fermion masses and mixing. Due to these problems and the apparent impossibility to observe the physics of grand unification directly in experiments, there is no generally accepted GUT model.

Unifying gravity with the other three interactions 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.

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