The problem with Grand Unified Theories
Pedro, Leonardo · Zenodo (CERN European Organization for Nuclear Research) · 2018
Gauge-covariant asymptotic states are needed to make perturbation theory work in the SU (5) GUT, and then the fact that the asymptotic states are not locally gauge- invariant (only the correlation functions are gauge-invariant) can not affect the spectrum of the theory. This a non-trivial and untested requirement beyond the Higgs mechanism of the Standard Model.