A. Gomez Nicola, J. Ruiz de Elvira, R. Torres Andres
We present a consistent description of Scalar-Pseudoscalar partner degeneration at the QCD chiral transition in terms of the dominant low-energy physical states for the light quark sector. First, we obtain within model-independent one-loop Chiral Perturbation Theory (ChPT) that the QCD pseudoscalar susceptibility is proportional to the quark condensate. We also show that this chiral-restoring behaviour is compatible with recent lattice results for screening masses and gives rise to degeneration between the scalar and pseudoscalar susceptibilities ($\chi_S$,$\chi_P$) around the transition point, consistently with an O(4)-like current restoration pattern. This scenario is clearly confirmed by lattice data when we compare $\chi_S(T)$ with the quark condensate, expected to scale as $\chi_P(T)$. In addition, we show that saturating $\chi_S$ with the $\sigma/f_0(500)$ broad resonance observed in pion scattering and including its finite temperature dependence, account for the peak structure of $\chi_S(T)$ in lattice data and the associated critical temperature. This is carried out within a unitarized ChPT scheme which generates the resonant state dynamically and is also consistent with partner degeneration.
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http://arxiv.org/abs/1304.3356
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