Editing Grand Unified Theory (section)

== Unification of forces and the role of supersymmetry ==
The unification of forces is possible due to the energy scale dependence of force [[coupling constant|coupling parameters]] in [[quantum field theory]] called [[renormalization group|renormalization group "running"]], which allows parameters with vastly different values at usual energies to converge to a single value at a much higher energy scale.<ref name="Ross" />

The [[renormalization group]] running of the three gauge couplings in the Standard Model has been found to nearly, but not quite, meet at the same point if the [[hypercharge]] is normalized so that it is consistent with {{math|SU(5)}} or {{math|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 [[Minimal Supersymmetric Standard Model|MSSM]] is used instead of the Standard Model, the match becomes much more accurate. In this case, the coupling constants of the strong and electroweak interactions meet at the [[grand unification energy]], also known as the GUT scale:
: <math>\Lambda_{\text{GUT}} \approx 10^{16}\,\text{GeV} .</math>

It is commonly believed that this matching is unlikely to be a coincidence, and is often quoted as one of the main motivations to further investigate supersymmetric theories despite the fact that no supersymmetric partner particles have been experimentally observed. Also, most model builders simply assume supersymmetry because it solves the [[hierarchy problem]]—i.e., it stabilizes the electroweak [[MSSM Higgs mass|Higgs mass]] against [[Renormalization|radiative correction]]s.<ref>{{Cite journal |arxiv = hep-ph/9702371|doi = 10.1142/S0217751X9800038X|title = The Future of Particle Physics as a Natural Science|journal = International Journal of Modern Physics A|volume = 13|issue = 6|pages = 863–886|year = 1998|last1 = Wilczek|first1 = Frank|s2cid = 14354139|bibcode = 1998IJMPA..13..863W}}</ref>