Application of the Lefschetz thimble formulation to the (0+1) dim. Thirring model at finite density

Abstract: Based on the Lefschetz thimble formulation of path-integration, we analyze the (0+1) dimensional Thirring model at finite chemical potentials and perform hybrid Monte Carlo (HMC) simulations. We adopt the lattice action defined with the staggered fermion and a compact link field for the auxiliary vector field. We firstly locate the critical points (saddle points) of the gradient flows within the subspace of time-independent (complex) link field, and study the thiemble structure and the Stokes phenomenon to identify the thimbles which contribute to the path-integral. Then, we perform HMC simulations on the single dominant thimble and compare the results to the exact solution. The numerical results are in agreement with the exact ones in small and large chemical potential regions, while they show some deviation in the crossover region in the chemical potential. We also comment on the necessity of the contributions from multiple thimbles in the crossover region.