  By generalizing our previous work on the parity symmetry, the partition
function of a Yang-Mills theory is decomposed into a sum of path integrals each
giving the contribution from multiplets of states with fixed quantum numbers
associated to parity, charge conjugation, translations, rotations and central
conjugations. Ratios of path integrals and correlation functions can then be
computed with a multi-level Monte Carlo integration scheme whose numerical
cost, at a fixed statistical precision and at asymptotically large times,
increases power-like with the time extent of the lattice. The strategy is
implemented for the SU(3) Yang-Mills theory, and a full-fledged computation of
the mass and multiplicity of the lightest glueball with vacuum quantum numbers
is carried out at two values of the lattice spacing (0.17 and 0.12 fm).
