Speaker
Description
All methods currently used to study finite baryon density lattice QCD suffer from uncontrolled systematic uncertainties in addition to the well-known sign problem. We formulate and test a method - sign reweighting - that works directly at finite chemical potential and is yet free from any such uncontrolled systematics: with this approach the only problem is the sign problem itself. In practice the approach involves the generation of configurations with the positive fermionic weights $\left| \rm{Re} \rm{det} D(\mu) \right|$ where $D(\mu)$ is the Dirac matrix and the signs $\rm{sign} \left( \rm{Re} \rm{det} D(\mu) \right) = \pm 1$ are handled by a discrete reweighting. Hence there are only two sectors, $+1$ and $−1$ and as long as the average $\left< \pm 1 \right> \neq 0$ (with respect to the positive weight) this discrete reweighting has no overlap problem - unlike other reweighting methods - and the results are reliable. We will also present results based on this algorithm on the phase diagram of lattice QCD with two different actions: as a first test, we apply the method to calculate the position of the critical endpoint with unimproved staggered fermions at $N_\tau=4$; as a second application, we study the phase diagram with 2stout improved staggered fermions at $N_\tau=6$. This second one is already a reasonably fine lattice - relevant for phenomenology.
