Complex Langevin dynamics for SU(3) gauge theory in the presence of a   theta term
  One of the yet unsolved questions of QCD in the context of the Standard Model is to explain the strong CP problem. A way to look for a better understanding of it is to investigate the theory in the presence of a non-zero topological theta term. On the lattice such a term is complex: hence it introduces a sign problem which, in general, limits the applicability of standard Monte Carlo methods. Here we will discuss the approach of complex Langevin dynamics and show results for both real and imaginary values of theta. We also report on our experience with the gradient flow for real and imaginary theta. 
Lorenzo Bongiovanni, Gert Aarts, Erhard Seiler, Denes Sexty
hep-lat