The Fermion Sign Problem at Finite Density, and Large Nc Orbifold   Equivalence
  The study of QCD at finite baryon density is severely hampered by the so-called fermion sign problem. As a result, we have no known first principles approach to study nuclear matter, or neutron stars from QCD. On the surface, the large Nc limit does not seem to simplify matters. In this limit, however, one can exploit dualities that exist between strongly coupled gauge theories. Our focus will be on some rather novel orbifold equivalences that have recently been discovered at finite density. These equivalences relate strongly coupled theories plagued by a sign problem, to strongly coupled theories free of sign problems. As a result, such dualities give deeper insight into the nature of the sign problem and possibly provide a way to simulate QCD at finite density in the large Nc limit. 
Aleksey Cherman, Brian C. Tiburzi
hep-ph hep-lat nucl-th