Hooft was able to derive a formula for the masses of mesons.[27]
He also studied the role of so-called instanton contributions in QCD. His calculations showed that these contributions lead to an interaction between light quarks at low energies that is not present in the standard theory
https://en.wikipedia.org/wiki/Gerard_%27t_Hooft
Studying instanton solutions of Yang–Mills theories, 't Hooft discovered that spontaneously breaking a theory with SU(N) symmetry to a U(1) symmetry leads to the existence of magnetic monopoles.[29] These monopoles are called 't Hooft–Polyakov monopoles, after Alexander Polyakov, who obtained the same result independently.[30]
't Hooft also formulated the anomaly matching condition, according to which anomalies associated with global symmetries must agree between the high-energy and low-energy descriptions of a quantum field theory. The condition places strong constraints on the possible low-energy spectrum of strongly coupled gauge theories.[31][32]
As another piece of the color confinement puzzle, 't Hooft introduced 't Hooft loops, which are the magnetic dual of Wilson loops.[33] Using these operators he was able to classify different phases of QCD, forming the basis of the QCD phase diagram.
In 1986, he was finally able to show that instanton contributions solve the Adler–Bell–Jackiw anomaly, the topic of his master's thesis.[34]
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