Gluon Mass, Glueballs and Gluonic Mesons

Abstract

We review the phenomenological and theoretical evidences for dynamical gluon mass generation and the main features of the glueball spectrum in (pure gauge) Yang-Mills theories. The mixing between glueball and conventional $\bar q q$ states in $f_0$ scalar mesons is discussed. For pseudoscalar mesons, the inclusion of the glue field in an effective low energy theory is presented leading to a third isoscalar $\eta''$ partner of the $\eta$ and $\eta'$. Branching ratios for processes involving $\eta''$ are given and, when available, compatible with data for $\eta(1405)$.