QCD thermal phase transition in the presence of a small chemical potential

Abstract

We propose a new method to investigate the thermal properties of QCD with a small quark chemical potential . Derivatives of quark and gluonic observables with respect to are computed at ϭ0 for two flavors of p4 improved staggered fermions with maϭ0.1,0.2 on a 16 3 ϫ4 lattice, and used to calculate the leading order Taylor expansion in of the location of the pseudocritical point about ϭ0. This expansion should be well behaved for the small values of q /T c ϳ0.1 relevant for BNL RHIC phenomenology, and predicts a critical curve T c () in reasonable agreement with estimates obtained using exact reweighting. In addition, we contrast the case of isoscalar and isovector chemical potentials, quantify the effect of 0 on the equation of state, and comment on the complex phase of the fermion determinant in QCD with 0.

Hadron production in central nucleus–nucleus collisions at chemical freeze-out

“…The LQCD phase boundary calculated in [12] and shown in Fig. 20 apparently does not follow the expectation [13] (see also [97]) of a constant energy density dependence. However, depending on the method, the errors for the critical line from LQCD calculations can be large for finite µ b [13].…”

“…Section 5 is devoted to the comparison of model and data in terms of excitation functions of various hadron yield ratios, which we extend up to the LHC energy. The relation of the extracted parameters with the predicted QCD phase boundary, calculated solving QCD on the lattice [12,13,14], is discussed in Section 6. The Appendix contains a discussion of more technical aspects in the fit procedures.…”

“…These data were subsequently interpreted in terms of an universal condition for chemical freezeout [11]. Remarkably, it appears that at the top SPS energy the (T ,µ b ) values reach the phase boundary between the hadronic world and the quark-gluon plasma (QGP) [1,10], as calculated solving Quantum Chromo-Dynamics (QCD) on the lattice [12,13,14]. In this context, it has been argued that the QGP itself and the "deus ex machina" of phase space filling during hadronization are playing the crucial roles in achieving thermalization in high energy nucleus-nucleus collisions [15].…”

Equation of state for two flavor QCD at nonzero chemical potential

“…In our previous paper [3] we used a hybrid of the two techniques, by making a Taylor series estimate of the reweighting factor…”

“…Analytic approaches use data from regions where direct simulation is possible, either by calculating derivatives with respect to µ q (or more properly with respect to the dimensionless combination µ q /T ) to construct a Taylor expansion for quantities of interest [1,3,8,9], or more radically by analytically continuing results from simulations with imaginary µ q (for which the integration measure remains real) to real µ q . The second technique has been used to map T c (µ q ) for QCD with both N f = 2 [4] and N f = 4 [10], in the latter case finding evidence that the line is first order in nature.…”