Bound states of heavy quark-antiquark pairs – quarkonia - allow a detailed study of some of the basic properties of quantum chromodynamics (QCD), the theory of strong interactions. However, the mechanism behind their production remains a mystery, despite decades of experimental and theoretical effort. In particular, the angular decay-distributions of the quarkonium states produced in hadron collisions present a seemingly irreconcilable disagreement between the measurements and the QCD predictions. Now results from the LHC are making the puzzle still more intriguing.

Read more: "The return of quarkonia" – CERN Courier