Résumé
The method of optimized perturbation theory (OPT) is used to study the phase diagram of the massless Gross-Neveu model in 2+1 dimensions. In the temperature and chemical potential plane, our results give strong support to the existence of a tricritical point and line of first order phase transitions, previously only suspected to exist from extensive lattice Monte Carlo simulations. We also point out to the existence of a previously unknown liquid-gas phase. In addition of presenting these new results, we provide a great deal of analytical results for the main critical quantities of the phase diagram already at first order in the OPT. Our results and methodology are expected to be able to give further insight concerning the phase diagram of more realistic models such as QCD.