SPEAKER: Aleksey Cherman - Univ of Cambridge, UK

TITLE:  The Fermion Sign Problem and Large N Orbifold Equivalence
 
ABSTRACT: Finite-density QCD notoriously suffers from the `fermion
sign problem', which makes lattice Monte Carlo simulations essentially
impossible.  As a result, little is known about the behavior of QCD at
finite baryon number density, except in the limit of asymptotically
high densities.  I will discuss a recent proposal to dodge the sign
problem in the large N limit using orbifold equivalence which may
enable the use of lattice simulations to study finite-density QCD.
The proposal starts with the observation that if the gauge group of
QCD is changed from SU(Nc) to SO(Nc), the sign problem does not appear
at finite baryon number chemical potential, and the resulting theory
can be studied on the lattice. Despite this seemingly drastic change,
large Nc orbifold equivalence can be used to argue that a wide class
of observables in the SO(Nc) theory coincide with observables in SU(N)
QCD at large N.  I will argue that the large N equivalence between the
two theories continues to hold at finite chemical potential provided
one turns on a certain `double-trace' deformation in the SO(Nc)
theory, which does not reintroduce the sign problem. This opens up the
prospect of learning about QCD at finite baryon number density using
lattice studies of the deformed SO(Nc) theory.