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We study the lattice O(2N) Gross-Neveu model with Wilson fermions in the fermion loop formulation. Employing a worm algorithm for an open fermionic string, we simulate fluctuating topological boundary conditions and use them to tune the system to the critical point. We show how the worm algorithm can be extended to sample correlation functions of bound states involving an arbitrary number of Majorana fermions and present first results., Comment: 7 pages, 3 figures, Talk presented at the XXVIII International Symposium on Lattice Field Theory, Lattice2010, Villasimius, Italy, June 14-19, 2010

Lattice Quantum Chromodynamics (LQCD) is the most versatile and powerful method to investigate non-perturbative effects in strong interaction physics. In view of the much improved statistical accuracy the control of systematic uncertainties became the most important issue in recent years. The most severe problem is the implementation of chiral symmetry. The BGR collaboration works with two different Dirac operators, both having good chiral properties but are suited best for the analysis of different aspects of QCD: Chirally Improved (CI) fermions are especially well suited for the investigation of excited hadrons while Fixed-Point (FP) fermions are ideal for studies in the so-called e- and δ-regime.

We generated configurations with the approximate fixed-point Dirac operator $D_\mathrm{FP}$ on a $12^4$ lattice with $a \approx 0.13 $fm where the scale was set by $r_0$. The distributions of the low lying eigenvalues in different topological sectors were compared with those of the Random Matrix Theory which leads to a prediction of the chiral condensate., Comment: 6 pages, 5 figures, presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg, Germany

We generated configurations with the parametrized fixed-point Dirac operator D_{FP} on a (1.6 fm)^4 box at a lattice spacing a=0.13 fm. We compare the distributions of the three lowest k=1,2,3 eigenvalues in the nu= 0,1,2 topological sectors with that of the Random Matrix Theory predictions. The ratios of expectation values of the lowest eigenvalues and the cumulative eigenvalue distributions are studied for all combinations of k and nu. After including the finite size correction from one-loop chiral perturbation theory we obtained for the chiral condensate in the MSbar scheme Sigma(2GeV)^{1/3}=0.239(11) GeV, where the error is statistical only., Comment: 19 pages, 13 figures, added Sigma in MSbar