HADRONS ABOVE Tc

Fate of light hadronic excitations above the critical temperature Tc of the QCD phase transition was first investigated 20 years ago.1,2 In Ref. 1 whose title is “Fluctuation effects in hot quark matter: precursors of chiral transition at finite temperature”, the hadrinic modes above Tc were discussed in connection with the critical fluctuation associated with chiral symmetry restoration: The abstract says “· · · there arise soft modes having a large strength and a narrow width above the critical temperature, which are analogous to the fluctuation of the order parameter in a superconductor above the critical point.” An example of such soft mode (σ-π) at T > Tc was demonstrated using the Nambu-Jona-Lasinio model1,3 and was later called the “para-pion”.4 From the modern point of view, It could be identified as a pre-formed qq̄ resonance in the pseudo-gap phase where the chiral order parameter has a large fluctuation above Tc (see e.g. Refs. 5, 6). In Ref. 2 whose title is “Conjecture concerning the modes of excitation of the quark-gluon plasma”, the hadronic modes above Tc were discussed with a different perspective which is unique in non-Abelian gauge theories, i.e. the dynamical confinement of the plasma above Tc: The abstract says “ · · · the plasma exhibits confining features similar to that of the low-temperature hadronic phase. The confining features are manifest in the long-range, i.e., long-wavelength, low-frequency, modes of the plasma.” Recently, studies of the hadronic modes in the time-like domain become possible in lattice QCD simulations owing to the technique of the maximum entropy method (MEM).7,8 It was then found that some hadronic modes (in the ss̄ sector) survive even at T > Tc in quenched simulations (see Sec. 4). 9 However, its relation to the