2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/40839We review the theory and phenomenology of the axial U(1) problem with emphasis on the role of gluonic degrees of freedom in the low-energy pp -> pp eta and pp -> pp eta' reactions.13 pages, 2 figures, invited lecture at the Symposium on Threshold Meson Production in pp and pd interactions, June 2001, Cracow, PolandHigh Energy Physics - Phenomenologytext