Novel Collective Excitations and Quasi-particle Picture of Quarks Coupled with a Massive Boson at Finite Temperature

Motivated by the ovservation that there may exist hadronic excitations even in the quark-gluon plasma (QGP) phase, we investigate how the quark properties, especially its possible quasi-particle picture, are affected by the coupling with bosonic excitations at finite temperature(T), employing Yukawa models with a massive scalar (pseudoscalar) and vector (axial-vector) boson with a mass m. The quark spectral function and the quasi-dispersion relations are calculated at the one-loop order. We find that there appears a three-peak structure in the quark spectral function with a collective nature when T is comparable with m, irrespective of the type of the boson; such a multi-peak structure was first found in a chiral model yielding scalar composite bosons with a decay width. We elucidate the mechanism of realizing the new quark collective excitations in terms of the Landau damping of a quark (an antiquark) induced by the scattering with the thermally excited boson, which gives rise to a mixing and hence a level repulsion between a quark (antiquark) and an antiquark-hole (quark-hole) in the thermally excited antiquark(quark) distribution. Our results suggest that the quarks in the QGP phase may have an interesting quasi-particle picture with a multi-peak spectral function. Since the used models are rather generic, our findings may be a universal phenomenon for fermions coupled to a massive bosonic excitation with a vanishing or small width. Relevance of the results to other fields of physics, such as neutrino physics, is also briefly mentioned. We also give a new aspect of the plasmino excitation obtained in the hard-thermal loop approximation