2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/188296The spectroscopic regularities of modern empirical data on the non-strange mesons up to 2.4 GeV can be summarized as a systematic clustering of states near certain values of energy. It is getting evident that some unknown X-symmetry triggers the phenomenon. We review the experimental status of this symmetry and recent theoretical attempts put forward for explanation of broad degeneracy.Brief review, 16 pages, 1 figureHigh Energy Physics - Phenomenologytext