A QCD chiral critical point at small chemical potential: is it there or not?

Abstract

For a QCD chiral critical point to exist, the parameter region of small quark masses for which the finite temperature transition is first-order must expand when the chemical potential is turned on. This can be tested by a Taylor expansion of the critical surface (m u,d , m s ) c (µ). We present a new method to perform this Taylor expansion numerically, which we first test on an effective model of QCD with static, dense quarks. We then present the results for QCD with 3 degenerate flavors. For a lattice with N t = 4 time-slices, the first-order region shrinks as the chemical potential is turned on. This implies that, for physical quark masses, the analytic crossover which occurs at µ = 0 between the hadronic and the plasma regimes remains crossover in the µ-region where a Taylor expansion is reliable, i.e. µ T . We present preliminary results from finer lattices indicating that this situation persists, as does the discrepancy between the curvature of T c (m c (µ = 0), µ) and the experimentally observed freeze-out curve. The XXV International Symposium on Lattice Field Theory

Bulk viscosity and spectral functions in QCD

“…line which terminates in an Ising universality class second order endpoint [22], as illustrated in Fig. 3 (see however [23]). The plasma generated in very high energy heavy ion collisions is expected to follow a trajectory close to the T axis (small µ), which probably misses the phase transition line [24] but may nevertheless experience a rather sharp crossover with a large correlation length.…”

Curvature of the critical line on the plane of quark chemical potential and pseudoscalar meson mass for three-flavor QCD

“…In this paper, we address an issue of how the critical end line extends when switching on the chemical potential. An interesting result was reported in [6,12] which explored the imaginary chemical potential approach with the naive staggered fermion action. There it was observed that the critical surface has a negative curvature in the μ direction.…”

Roberge-Weiss transition in N$_{f}$=2 QCD with staggered fermions and N$_{\tau}$=6

“…Investigations both in the chiral (m Ñ 0) and quenched (m Ñ ∞) limits are interesting on their own, since they allow to understand the influence of confinement and chiral symmetry breaking on the thermal transition. The structure of the Columbia plot which emerges from studies on very coarse lattices [1,2,3] is qualitatively the same for different formulations and lattice spacings: The order of the phase transition for light and heavy quarks is first order, while it becomes a crossover for intermediate values of the mass. Nevertheless, quite significant discrepancies regarding the position of the Z 2 lines separating these regions have been found using different fermions discretizations and no continuum extrapolation of any feature is available.…”

“…In the present study we used a GPU implementation of it present in the CL 2 QCD software, a OpenCL based code which is publicly available 1 and that has been optimised to run efficiently on AMD graphic cards [9]. All our simulations were run on the L-CSC cluster [10] in Darmstadt.…”

“…Since to understand the order of the phase transition at a given value of the mass is not immediate (both in terms of time and numerical resources), previous studies [1,6] can help in choosing how to perform the scan in m u,d . In particular, at µ = 0 with 3 degenerate flavors of staggered fermions, it has been observed that the Z 2 chiral critical bare mass in lattice units on N τ = 6 lattices is ∼ 7.5 times smaller than that found using N τ = 4.…”