  We present a novel lattice calculation of the equation of state of SU(2)
Yang-Mills theory in the confining phase. We show that a gas of massive,
non-interacting glueballs describes remarkably well the results, provided that
a bosonic closed-string model is used to derive an exponentially growing
Hagedorn spectrum for the heavy glueball states with no free parameters. This
effective model can be applied to SU(3) Yang-Mills theory and the theoretical
prediction agrees nicely with the lattice results reported by Bors\'anyi et al.
in JHEP 07 (2012) 056.

High Energy Physics - Lattice