The Sign Problem is the Solution
  The unquenched spectral density of the Dirac operator at $\mu\neq0$ is complex and has oscillations with a period inversely proportional to the volume and an amplitude that grows exponentially with the volume. Here we show how the oscillations lead to the discontinuity of the chiral condensate. 
J.C. Osborn, K. Splittorff and J.J.M. Verbaarschot