Effective chiral symmetry restoration in excited mesons

Results on the spectrum of ${\bar q} q$ mesons in a model with a linear Coulomb-like instantaneous confining potential are presented. The single-quark Green function as well as the dynamical chiral symmetry breaking are obtained from the Dyson-Schwinger (gap) equation. Given the dressed quark propagator, the spectrum of “usual” mesons, i.e., ${\bar q} q$ states with nonexotic quantum numbers J ^PC , is obtained from the Bethe-Salpeter equation. Effective restoration of chiral symmetry at large spins and/or radial excitations is observed and the states fall into approximate linear radial and angular Regge trajectories.     

Weinberg-Salam model and chiral symmetry of mesons

Weinberg-Salam model is considered in the light of SU(4) × SU(4) chiral symmetry for mesons. The Higgs doublet and the pseudoscalar mesons mix in this framework. Consequences of this mixing for the Higgs decays and the nonleptonic decays of the mesons are explored.     

Effective chiral Lagrangians for spin-1 mesons

The commonly used types of effective theory for vector mesons are reviewed and their relationships clarified. They are shown to correspond to different choices of field for spin-1 particles and the rules for transforming between them are described. The importance of respecting chiral symmetry is stressed. The choice of fields that transform homogeneously under the nonlinear realisation of chiral symmetry imposes no preconceptions about the types of coupling for the mesons. This representation thus provides a convenient framework for relating different theories. It is also used to elucidate the nature of the assumptions in specific hidden-gauge and massive Yang-Mills models that have been widely used.     

Effective chiral lagrangians for spin-1 mesons

The commonly used types of effective theory for vector mesons are reviewed and their relationships clarified. They are shown to correspond to different choices of field for spin-1 particles and the rules for transforming between them are described. The importance of respecting chiral symmetry is stressed. The choice of fields that transform homogeneously under the nonlinear realisation of chiral symmetry imposes no preconceptions about the types of coupling for the mesons. This representation thus provides a convenient framework for relating different theories. It is also used to elucidate the nature of the assumptions in specific hiddengauge and massive Yang-Mills models that have been widely used.     

A quark model of mesons based on chiral symmetry

We construct a model of mesons as q$\text{q}$ pairs in the infinite momentum frame. The basic symmetry is spontaneously broken chiral SU₂ × SU₂. We compute the rates for the decays ? → 2π, σ → 2π, A₁ → ?π, A₁ → σπ, A₂ → ηπ, π_N → ηπ, D → ηπ, f → 2π and the coupling constant, g²_?ωπ. We find qualitative agreement with the experimental values. The theory is similar to that of Gilman and Harari.     

Partial restoration of chiral symmetry of vector mesons in hot hadronic matter: a possible signature of quark-gluon plasma

A φ meson is an excellent candidate to see the dynamical and critical phenomena at finite temperature of hadronic and quark matter which can be produced through relativistic nucleus-nucleus collisions. PHENIX Collaboration and Chairperson of the PHENIX-J Collaboration.     

Light scalar mesons as manifestation of spontaneously broken chiral symmetry

Attention is paid to the production mechanisms of light scalars that reveal their nature. We reveal the chiral shielding of the σ(600) meson. We show that the kaon loop mechanism of the φ radiative decays, ratified by experiment, points to the four-quark nature of light scalars. We show also that the light scalars are produced in the two photon collisions via four-quark transitions in contrast to the classic P wave tensor q $ \bar q $ mesons that are produced via two-quark transitions γγ → q $ \bar q $ . The history of spontaneous breaking of symmetry in quantum physics is discussed in Appendix.     

Chiral symmetry restoration in excited hadrons and dense matter

We overview two interconnected topics： possible effective restoration of chiral symmetry in highly excited hadrons and possible existence of confined but chirally symmetric matter at low temperatures and high densities.     

Chiral symmetry restoration in excited hadrons and dense matter

We overview two interconnected topics: possible effective restoration of chiral symmetry in highly excited hadrons and possible existence of confined but chirally symmetric matter at low temperatures and high densities.     

Alternative experimental evidence for chiral restoration in excited baryons

It has been suggested that chiral symmetry is approximately restored in excited hadrons at zero temperature and density (effective symmetry restoration). Using very general chiral symmetry arguments, it is shown that those excited nucleons that are assumed from the spectroscopic patterns to be in approximate chiral multiplets must only weakly decay into the Npi channel, (fN*Npi}/fNNpi)(2) < 1. However, those baryons that have no chiral partner must decay strongly with a decay constant comparable with fNNpi. It turns out that for all those well-established excited nucleons which can be classified into chiral doublets the ratio is (fN*Npi}/fNNpi)(2) approximately 0.1 or much smaller for the high-spin states. In contrast, the only well-established excited nucleon for which the chiral partner cannot be identified from the spectroscopic data, N(1520), has a decay constant into the Npi channel that is comparable with fNNpi.     

Pion interaction in nuclear matter and chiral symmetry restoration

This paper is devoted to the interplay between p-wave, s-wave pion-nucleon/nucleus interaction and in-medium pion-pion interaction with special emphasis on the role of the nuclear pionic scalar density driving a large amount of chiral symmetry restoration. In particular we show that the πNN coupling constant and the Goldberger-Treiman relation are preserved in the nuclear medium under certain conditions. We also discuss the related problem of the in-medium pion-pion strength function.     

Effective potentials in QCD and chiral symmetry breaking

We consider chiral symmetry breaking through nontrivial vacuum structure with an explicit construct for the vacuum with quark antiquark condensates in QCD with Coulomb gauge for different phenomenological potentials. The dimensional parameter for the condensate function gets related to \(\left\langle {\bar \psi \psi } \right\rangle \) of Shifman, Vainshtein and Zakharov. We then relate the condensate function to the wave function of pion as a Goldstone mode. This simultaneously yields the pion also as a quark antiquark bound state as a localised zero mode of vacuum. We then calculate different pionic properties using the wave function as obtained from the vacuum structure.     

A holographic model of deconfinement and chiral symmetry restoration

We analyze the finite temperature behavior of the Sakai-Sugimoto model, which is a holographic dual of a theory which spontaneously breaks a U(N_f)_L × U(N_f)_R chiral flavor symmetry at zero temperature. The theory involved is a 4 + 1 dimensional supersymmetric SU(N_c) gauge theory compactified on a circle of radius R with anti-periodic boundary conditions for fermions, coupled to N_f left-handed quarks and N_f right-handed quarks which are localized at different points on the compact circle (separated by a distance L). In the supergravity limit which we analyze (corresponding in particular to the large N_c limit of the gauge theory), the theory undergoes a deconfinement phase transition at a temperature T_d = 1/2πR. For quark separations obeying L > L_c ? 0.97 ∗ R the chiral symmetry is restored at this temperature, but for L < L_c ? 0.97 ∗ R there is an intermediate phase which is deconfined with broken chiral symmetry, and the chiral symmetry is restored at T_χSB ? 0.154/L. All of these phase transitions are of first order.     

Indications of partial chiral symmetry restoration from pionic atoms

Extensive data on strong interaction effects in pionic atoms are analyzed with a density-dependent isovector scattering amplitude suggested recently by Weise to result from a density dependence of the pion decay constant. Most of the so-called ‘missing s-wave repulsion’ is removed when adopting this approach, thus indicating a partial chiral symmetry restoration in dense matter. The resulting potentials describe quite well also elastic scattering of 20 MeV pions on Ca. Further tests with elastic scattering are desirable.     

The chirally improved quark propagator and restoration of chiral symmetry

The chirally improved (CI) quark propagator in Landau gauge is calculated in two flavor lattice Quantum Chromodynamics. Its wave-function renormalization function Z(p²)$Z(p^2)$ and mass function M(p²)$M(p^2)$ are studied. To minimize lattice artifacts, tree-level improvement of the propagator and tree-level correction of the lattice dressing functions is applied. Subsequently the CI quark propagator under Dirac operator low-mode removal is investigated. The dynamically generated mass in the infrared domain of the mass function is found to dissolve continuously as a function of the reduction level and strong suppression of Z(p²)$Z(p^2)$ for small momenta is observed.     

Chiral symmetry suppression of two-photon couplings to excited SU(3) pseudoscalar mesons

Recent results from the Crystal Ball spectrometer show no evidence for two-photon production of the pseudoscalar resonances π(1300), η(1275) or η(1440). We give a simple theoretical argument showing that the two-photon couplings to excited SU(3) pseudoscalars are naturally suppressed by chiral symmetry in QCD. Effects of the spontaneous chiral symmetry breaking nevertheless give a finite contribution to the two-photon coupling, even in the limit of zero quark mass. We construct an effective chiral meson lagrangian that incorporates these qualitative properties, and use it to estimate the two-photon couplings in the chiral limit. The results are well compatible with experiment.