Mass gap and vacuum energy of the Gross-Neveu model via the 2-point-particle-irreducible expansion

Abstract
We introduce the 2-point-particle-irreducible expansion, which sums bubble graphs to all orders. We prove the renormalizibility of this summation. We use it on the Gross-Neveu model to calculate the mass gap and vacuum energy. After an optimization of the expansion, the final results are qualitatively good.
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