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Register a new userLocalization and topology in high temperature QCD
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Authors
Tamas G. Kovacs
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At high temperature part of the spectrum of the quark Dirac operator is known to consist of localized states. This comes about because around the crossover temperature to the quark-gluon plasma localized states start to appear at the low end of the spectrum and as the system is further heated, states higher up in the spectrum also get localized. Since localization and the crossover to the chirally restored phase happen around the same temperature, the question of how the two phenomena are connected naturally arises. Here we speculate on the nature of possible gauge configurations that could support localized quark eigenmodes. In particular, by analyzing eigenmodes of the staggerd and overlap Dirac operator we show that the dilute gas of calorons in the high temperature phase is very unlikely to play a major role in localization.
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The chemical potential ($mu$) dependence of the topological susceptibility with two-color two-flavor QCD is studied. We find that at temperature $T approx T_c /2$, where $T_c$ denotes the critical temperature at zero chemical potential, the topological susceptibility is almost constant throughout $0 leq amu lesssim 1.0$, while at $Tapprox T_c$, it decreases significantly from the $mu=0$ value in a high $mu$ regime. In this work, we perform the simulation for $mu/T le 16$, which covers even the low temperature and the high chemical potential regime. In this regime, we introduce a diquark source term, which is characterized by $j$, into the action. We also show our results for the phase diagram in a low temperature regime ($Tapprox T_c/2$), which is obtained after taking the $j to 0$ limit of the diquark condensate and the Polyakov loop.
We delineate equilibrium phase structure and topological charge distribution of dense two-colour QCD at low temperature by using a lattice simulation with two-flavour Wilson fermions that has a chemical potential $mu$ and a diquark source $j$ incorporated. We systematically measure the diquark condensate, the Polyakov loop, the quark number density and the chiral condensate with improved accuracy and $jto0$ extrapolation over earlier publications; the known qualitative features of the low temperature phase diagram, which is composed of the hadronic, Bose-Einstein condensed (BEC) and BCS phases, are reproduced. In addition, we newly find that around the boundary between the hadronic and BEC phases, nonzero quark number density occurs even in the hadronic phase in contrast to the prediction of the chiral perturbation theory (ChPT), while the diquark condensate approaches zero in a manner that is consistent with the ChPT prediction. At the highest $mu$, which is of order the inverse of the lattice spacing, all the above observables change drastically, which implies a lattice artifact. Finally, at temperature of order $0.45T_c$, where $T_c$ is the chiral transition temperature at zero chemical potential, the topological susceptibility is calculated from a gradient-flow method and found to be almost constant for all the values of $mu$ ranging from the hadronic to BCS phase. This is a contrast to the case of $0.89T_c$ in which the topological susceptibility becomes small as the hadronic phase changes into the quark-gluon plasma phase.
Using lattice QCD simulations with $N_f = 2$ dynamical fermions, we study the axial $U(1)$ symmetry, topological charge, and Dirac eigenvalue spectra in the high-temperature phase in which the chiral symmetry is restored. Our gauge ensembles are generated with Mobius domain-wall fermions, but the measurements such as susceptibilities are reweighted to those for the overlap fermions by using overlap/domain-wall reweighting technique. We find that the $U(1)_A$ and topological susceptibilities are strongly suppressed in the small quark mass region, which is related to the reduction of chiral-zero and low-nonzero modes on the Dirac spectra. We also examine their volume dependence.
We calculate second- and fourth-order cumulants of conserved charges in a temperature range stretching from the QCD transition region towards the realm of (resummed) perturbation theory. We perform lattice simulations with staggered quarks; the continuum extrapolation is based on $N_t=10dots24$ in the crossover-region and $N_t=8dots16$ at higher temperatures. We find that the Hadron Resonance Gas model predictions describe the lattice data rather well in the confined phase. At high temperatures (above $sim$250 MeV) we find agreement with the three-loop Hard Thermal Loop results.
We compute charmonium spectral functions in 2-flavor QCD on anisotropic lattices using the maximum entropy method. Our results suggest that the S-waves (J/psi and eta_c) survive up to temperatures close to 2Tc, while the P-waves (chi_c0 and chi_c1) melt away below 1.2Tc.
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