Twist Positivity
  We identify a positivity property for partition functions in quantum systems with a unitary symmetry group, and we call this "twist positivity." The existence of Feynman-Kac measures and the existence of zero-mass limits are both related to this property. Twist positivity arises from the occurrence of complex conjugate representations on an energy eigenspace, and ultimately reflects a particle interpretation of the quantum system. 
Arthur Jaffe