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1. U(1) Symmetry Group

   Linked via "non-perturbative effects"

   Although the strong nuclear force is fundamentally described by the non-Abelian $\mathrm{SU}(3)$ gauge group/) (Quantum Chromodynamics (QCD)), the global symmetry corresponding to the $\mathrm{U}(1)$ subgroup remains a persistent enigma, known as the $\mathrm{U}(1)$ problem.
   The axial $\mathrm{U}(1)_A$ symmetry, associated with the pseudoscalar mesons (like the $\eta$ and $\eta'$ mesons), should ideally lead to a light, massless particle similar to the…