Deconfinement and chiral transition with the highly improved staggered quark (HISQ) action

TL;DR

The paper addresses how QCD thermodynamics at finite temperature can be robustly studied with improved staggered fermions by using the HISQ action. It demonstrates that HISQ significantly reduces taste violations, leading to more accurate hadron spectra and closer agreement with HRG at low temperature and with perturbative expectations at high temperature. The authors compute renormalized Polyakov loop, chiral condensate and susceptibility, fluctuations of conserved charges, and the trace anomaly on $N_\tau=6$ and $N_\tau=8$ lattices, comparing with asqtad, p4, and stout results. They conclude that the deconfinement transition is gradual rather than sharp, with improved consistency across observables and better compatibility with HRG and the s95p-v1 parametrization, validating HISQ as a reliable discretization for QCD thermodynamics.

Abstract

We report on investigations of the chiral and deconfinement aspects of the finite temperature transition in 2+1 flavor QCD using the Highly Improved Staggered Quark (HISQ) action on lattices with temporal extent $N_τ=6$ and $N_τ=8$. We have performed the calculations for physical values of the strange quark mass $m_s$ and the light quark masses $m_l=0.2m_s$ and $0.05m_s$. Several finite temperature observables, including the renormalized Polyakov loop, the renormalized chiral condensate and the chiral susceptibility have been calculated. We also study the fluctuations and correlations of different conserved charges as well as the trace anomaly at finite temperature. We compare our findings with previous calculations that use different improved staggered fermion formulations: asqtad, p4 and stout.

Figures (17)

• Figure 1: The static potential calculated for $m_l=0.2m_s$ (left) and $m_l=0.05m_s$ (right) in units of $r_0$. In the left figure we compare the HISQ result with the p4 result obtained at similar value of the lattice spacing. The dashed line on the right figure is the string potential (see text).
• Figure 2: The splitting between different pion multiplets for the HISQ action at $0.2m_s$ compared to the stout results shown as open symbols (left). In the right figure the non-Goldstone pseudo-scalar meson masses are shown as function of lattice spacings assuming that the lightest (Goldstone) pion mass is fixed to its physical value. The open symbols in the right figure correspond to pseudo-scalar mesons labeled as "1" and "3" (or, equivalently, as $\gamma_i \gamma_5$ and $\gamma_i \gamma_j$) in the stout calculations.
• Figure 3: The hadron spectrum for the HISQ action at $m_l=0.05m_s$ compared with experiment. The open symbols show the nucleon mass for the asqtad action. For clarity of the plot the nucleon mass is shifted by 150 MeV.
• Figure 4: The renormalized Polyakov loop as function of the temperature calculated for the HISQ, stout, asqtad and p4 actions at the physical value of the light quark mass.
• Figure 5: The subtracted chiral condensate for the HISQ action compared with calculations performed for the stout fodor09, asqtad hotqcd_progress and p4 actions eos005.