Fermion Bag Approach for Massive Thirring Model at Finite Density

TL;DR

This paper compares the fermion bag approach and complex Langevin dynamics for simulating the 2+1D massive Thirring model at finite density, highlighting the conditions under which the sign problem is manageable.

Contribution

It demonstrates the effectiveness of the fermion bag approach in mitigating the sign problem compared to complex Langevin dynamics in certain parameter ranges.

Findings

Fermion bag approach shows a mild sign problem in some parameter regimes.

Complex Langevin dynamics experiences a severe sign problem in the same regimes.

Results are compared with phase quenched approximation, providing insights into the model's behavior.

Abstract

We consider the 2+1 dimensional massive Thirring model with one flavor at finite density. Two numerical methods, fermion bag approach and complex Langevin dynamics, are used to calculate the chiral condensate and fermion density of this model. The numerical results obtained by fermion bag approach are compared with those obtained by complex Langevin dynamics. They are also compared with those obtained under phase quenched approximation. We show that in some range of fermion coupling strength and chemical potential the sign problem in fermion bag approach is mild, while it becomes severe for the complex Langevin dynamics.