  The strongly coupled phase of Yang-Mills plasma with arbitrary gauge group is
studied in a $T$-matrix approach. The existence of lowest-lying glueballs,
interpreted as bound states of two transverse gluons (quasi-particles in a
many-body set up), is analyzed in a non-perturbative scattering formalism with
the input of lattice-QCD static potentials. Glueballs are actually found to be
bound up to 1.3 $T_c$. Starting from the $T$-matrix, the plasma equation of
state is computed by resorting to Dashen, Ma and Bernstein's formulation of
statistical mechanics and favorably compared to quenched lattice data. Special
emphasis is put on SU($N$) gauge groups, for which analytical results can be
obtained in the large-$N$ limit, and predictions for a $G_2$ gauge group are
also given within this work.
