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Register a new userNaive versus mirror assignment at finite density: correlator mixing and chiral symmetry restoration
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Authors
Boris Krippa
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Chiral symmetry imposes some constrains on hadron correlation functions in dense medium. These constraints imply a certain general structure of the two- and three-point correlation functions of chiral partners and lead to the effect of mixing arising from the interaction of the long-ranged nuclear pions with corresponding interpolating currents. It reflects the phenomena of partial restoration of chiral symmetry. We consider soft pion corrections for quark condensates and several possible pairs of chiral partners for both mesons and nucleons and analyse their mixing patterns. We explored both naive and mirror assignments for the chiral partner of nucleon and discussed possible phenomenological consequence.
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We study the production of strange hadrons in nucleus-nucleus collisions from 4 to 160 A GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach that is extended to incorporate essentials aspects of chiral symmetry restoration (CSR) in the hadronic sector (via the Schwinger mechanism) on top of the deconfinement phase transition as implemented in PHSD. Especially the $K^+/pi^+$ and the $(Lambda+Sigma^0)/pi^-$ ratios in central Au+Au collisions are found to provide information on the relative importance of both transitions. The modelling of chiral symmetry restoration is driven by the pion-nucleon $Sigma$-term in the computation of the quark scalar condensate $<q {bar q}>$ that serves as an order parameter for CSR and also scales approximately with the effective quark masses $m_s$ and $m_q$. Furthermore, the nucleon scalar density $rho_s$, which also enters the computation of $<q {bar q}>$, is evaluated within the nonlinear $sigma-omega$ model which is constraint by Dirac-Brueckner calculations and low energy heavy-ion reactions. The Schwinger mechanism (for string decay) fixes the ratio of strange to light quark production in the hadronic medium. We find that above $sim$80 A GeV the reaction dynamics of heavy nuclei is dominantly driven by partonic degrees-of-freedom such that traces of the chiral symmetry restoration are hard to identify. Our studies support the conjecture of quarkyonic matter in heavy-ion collisions from about 5 to 40 A GeV and provide a microscopic explanation for the maximum in the $K^+/pi^+$ ratio at about 30 A GeV which only shows up if a transition to partonic degrees-of-freedom is incorporated in the reaction dynamics and is discarded in the traditional hadron-string models.
The partial restoration of chiral symmetry in nuclear medium is investigated in a model independent way by exploiting operator relations in QCD. An exact sum rule is derived for the quark condensate valid for all density. This sum rule is simplified at low density to a new relation with the in-medium quark condensate <bar{q}q>*, in-medium pion decay constant F_{pi}^t and in-medium pion wave-function renormalization Z_{pi}*. Calculating Z_{pi}*at low density from the iso-scalar pion-nucleon scattering data and relating F_{pi}^t to the isovector pion-nucleus scattering length b_1^*, it is concluded that the enhanced repulsion of the s-wave isovector pion-nucleus interaction observed in the deeply bound pionic atoms directly implies the reduction of the in-medium quark condensate. The knowledge of the in-medium pion mass m_{pi}* is not necessary to reach this conclusion.
Based on an equivparticle model, we investigate the in-medium quark condensate in neutron stars. Carrying out a Taylor expansion of the nuclear binding energy to the order of $rho^3$, we obtain a series of EOSs for neutron star matter, which are confronted with the latest nuclear and astrophysical constraints. The in-medium quark condensate is then extracted from the constrained properties of neutron star matter, which decreases non-linearly with density. However, the chiral symmetry is only partially restored with non-vanishing quark condensates, which may vanish at a density that is out of reach for neutron stars.
We review the implementation of a q-deformed fermionic algebra in the Nambu--Jona-Lasinio model (NJL). The gap equations obtained from a deformed condensate as well as from the deformation of the NJL Hamiltonian are discussed. The effect of both temperature and deformation in the chiral symmetry restoration process as well as in the pion properties is studied.
We shed light upon the eta mass in nuclear matter in the context of partial restoration of chiral symmetry, pointing out that the U_{A}(1) anomaly effects causes the eta-eta mass difference necessarily through the chiral symmetry breaking. As a consequence, it is expected that the eta mass is reduced by order of 100 MeV in nuclear matter where partial restoration of chiral symmetry takes place. The discussion given here is based on Ref. [1].
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