The O(N) model in 1+1 dimensions presents some features in common with Yang-Mills theories: asymptotic freedom, trace anomaly, non-petrurbative generation of a mass gap. An analytical approach to determine the termodynamical properties of the O(3) model is presented and compared to lattice results. Here the focus is on the pressure: it is shown how to derive the pressure in the CJT formalism at the one-loop level by making use of the auxiliary field method. Then, the pressure is compared to lattice results.
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