The Fermion Bag Approach to Hamiltonian Theories

Quantum Monte Carlo (QMC) methods, when applicable, offer dependable ways to extract the nonperturbative physics of strongly-correlated many-body systems. However, there are some formidable bottlenecks to the applicability of these methods such as the sign problem and algorithmic update inefficiencies. Using the t-V model Hamiltonian, we demonstrate how the fermion bag approach--originally developed in the context of Lagrangian lattice field theories--led to the first sign problem solution for this model. We then show how using fermion bag ideas to develop a new efficient QMC algorithm to study the t-V model allowed us to compute critical exponents for the chiral Ising universality class (involving one flavor of four-component Dirac fermions) that seem to be more reliable than those from previous QMC calculations. Finally, we discuss how the fermion bag approach offers certain advantages to the study of other models involving Dirac fermions and also extends to fermion-spin interactions and Z_2 gauge theories.