Study of the complex fermion determinant in a $\rm U(1)_L \otimes   U(1)_R$ symmetric Yukawa model
  Lattice theories that contain chiral multiplets of fermions can have complex fermion determinants. This is for example the case for the $\rm U(1)_L \otimes U(1)_R$ symmetric Yukawa model with mirror fermions, if the number of generations of fermions and mirror fermions is odd. Whether a numerical simulation of such a model is possible depends on the magnitude of fluctuations of the complex phase factor of the fermion determinant. We investigate the fermion determinant of the U(1) Yukawa model with mirror fermions for a physically relevant choice of parameters. The argument of the complex phase turns out to fluctuate only very little and is at most of the order of $2 \cdot 10^{-3}$. 
Gernot Muenster and Markus Plagge