Retrieving "Topological Winding Numbers" from the archives

Cross-reference notes under review

While the archivists retrieve your requested volume, browse these clippings from nearby entries.

1. U(1) Symmetry Group

   Linked via "topological winding numbers"

   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…