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Register a new userChiral-spin symmetry of the meson spectral function above $T_c$
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Recently, via calculation of spatial correlators of $J=0,1$ isovector operators using a chirally symmetric Dirac operator within $N_F=2$ QCD, it has been found that QCD at temperatures $T_c - 3 T_c$ is approximately $SU(2)_{CS}$ and $SU(4)$ symmetric. The latter symmetry suggests that the physical degrees of freedom are chirally symmetric quarks bound by the chromoelectric field into color singlet objects without chromomagnetic effects. This regime of QCD has been referred to as a Stringy Fluid. Here we calculate correlators for propagation in time direction at a temperature slightly above $T_c$ and find the same approximate symmetries. This means that the meson spectral function is chiral-spin and $SU(4)$ symmetric.
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The chiral magnetic effect (CME) is an exact statement that connects via the axial anomaly the electric current in a system consisting of interacting fermions and gauge field with chirality imbalance that is put into a strong external magnetic field. Experimental search of the magnetically induced current in QCD in heavy ion collisions above a pseudocritical temperature hints, though not yet conclusive, that the induced current is either small or vanishing. This would imply that the chirality imbalance in QCD above $T_c$ that could be generated via topological fluctuations is at most very small. Here we present the most general reason for absence (smallness) of the chirality imbalance in QCD above Tc. It was recently found on the lattice that QCD above Tc is approximately chiral spin (CS) symmetric with the symmetry breaking at the level of a few percent. The CS transformations mix the right- and left-handed components of quarks. Then an exact CS symmetry would require absence of any chirality imbalance. Consequently an approximate CS symmetry admits at most a very small chirality imbalance in QCD above Tc. Hence the absence or smallness of an magnetically induced current observed in heavy ion collisions could be considered as experimental evidence for emergence of the CS symmetry above Tc.
Due to the Gauss law, a single quark cannot exist in a periodic volume, while it can exist with C-periodic boundary conditions. In a C-periodic cylinder of cross section A = L_x L_y and length L_z >> L_x, L_y containing deconfined gluons, regions of different high temperature Z(3) phases are aligned along the z-direction, separated by deconfined- deconfined interfaces. In this geometry, the free energy of a single static quark diverges in proportion to L_z. Hence, paradoxically, the quark is confined, although the temperature T is larger than T_c. At T around T_c, the confined phase coexists with the three deconfined phases. The deconfined-deconfined interfaces can be completely or incompletely wet by the confined phase. The free energy of a quark behaves differently in these two cases. In contrast to claims in the literature, our results imply that deconfined-deconfined interfaces are not Euclidean artifacts, but have observable consequences in a system of hot gluons.
Above a pseudocritical temperature of chiral symmetry restoration T_c the energy and the pressure are very far from the quark-gluon-plasma limit (i.e. ideal gas of free quarks and gluons). At the same time very soon above T_c fluctuations of conserved charges behave as if quarks were free particles. Within the T_c - 3T_c interval a chiral spin symmetry emerges in QCD which is not consistent with free quarks and suggests that degrees of freedom are chirally symmetric quarks bound into the color-singlet objects by the chromoelectric field. Here we analyse temporal and spatial correlators in this interval and demonstrate that they indicate simultaneously the chiral spin symmetry as well as absence of the interquark interactions in channels constrained by a current conservation. The latter channels are responsible for both fluctuations of conserved charges and for dileptons. Assuming that a SU(2)_color subgroup of SU(3)_color is deconfined soon above T_c but confinement persits in SU(3)_color/SU(2)_color in the interval T_c - 3T_c we are able to reconcile all empirical facts listed above.
In Quantum Chromodynamics (QCD) the eigenmodes of the Dirac operator with small absolute eigenvalues have a close relationship to the dynamical breaking of the chiral symmetry. In a simulation with two dynamical quarks, we study the behavior of meson propagators when removing increasingly more of those modes in the valence sector, thus partially removing effects of chiral symmetry breaking. We find that some of the symmetry aspects are restored (e.g., the masses of $rho$ and $a_1$ approach each other) while confining properties persist.
We extract the spectral functions in the scalar, pseudo-scalar, vector, and axial vector channels above the deconfinement phase transition temperature (Tc) using the maximum entropy method (MEM). We use anisotropic lattices, 32^3 * 32, 40, 54, 72, 80, and 96 (corresponding to T = 2.3 Tc --> 0.8 Tc), with the renormalized anisotropy xi = 4.0 to have enough temporal data points to carry out the MEM analysis. Our result suggests that the spectral functions continue to possess non-trivial structures even above Tc and in addition that there is a qualitative change in the state of the deconfined matter between 1.5 Tc and 2 Tc.
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