 Goldstone theorem. In the case of a global continuous symmetry, massless bosons (known as “Goldstone bosons”) appear with the spontaneous breakdown of the symmetry according to a theorem first stated by J. Goldstone in 1960. The presence of these massless bosons, first seen as a serious problem since no particles of the sort had been observed in the context considered, was in fact the basis for the solution — by means of the so-called Higgs mechanism (see the next point) — of another similar problem, that is the fact that the 1954 Yang-Mills theory of non-Abelian gauge fields predicted unobservable massless particles, the gauge bosons.