Quark Meson Research Articles

Nuclear Matter in a Chiral SU(3) Quark Mean-Field Model

A quark meson coupling model based on SU(3)L × SU(3)R symmetry and scale invariance is proposed. The quarks and mesons get masses through symmetry broken. We apply this SU(2) chiral constituent quark model to investigating the nuclear matter at finite temperature and density. The effective baryon masses, compression modulus and hyperon potentials are all reasonable. The critical temperature of liquid-gas phase transition is also calculated in this model.

Strange hadronic matter in a chiral [formula omitted] quark mean-field model

A quark–meson coupling model based on SU(3) L×SU(3) R symmetry and scale invariance is proposed. We apply this chiral SU(3) quark mean-field model to investigate strange hadronic matter including Λ, Σ and Ξ hyperons. With the symmetry-breaking term, all the hyperon potentials are reasonable. The effective baryon masses, binding energy and hyperon ratios are discussed with four types of confining potentials.

The [formula omitted] and [formula omitted] nature of f0(980) in Ds decays

We examine the D s → f 0(980) π amplitude through a constituent quark–meson model, incorporating heavy quark and chiral symmetries, finding a good agreement with the recent E791 data analysis of D s →3 π via f 0(980). The f 0(980) resonance is considered at the moment of production as an s s ̄ state, later evolving to a superposition of mainly s s ̄ and K K ̄ . The analysis is also extended to the more frequent process D s → φπ.

Polarization transfer in the 4He [formula omitted]H reaction

Polarization transfer in the 4He ( e → ,e′ p → ) 3 H reaction at a Q 2 of 0.4 (GeV/ c) 2 was measured at the Mainz Microtron MAMI. The ratio of the transverse to the longitudinal polarization components of the ejected protons was compared with the same ratio for elastic ep scattering. The results are consistent with a recent fully relativistic calculation which includes a predicted medium modification of the proton form factor based on a quark–meson coupling model.

Predicting D→ σπ

We examine the D→ σπ amplitude through a constituent quark–meson model, incorporating heavy quark and chiral symmetries, finding a good agreement with the recent E791 data analysis of D→3 π via σ.

Nuclear matter in nontopological soliton models with quark–meson coupling

A system of nontopological solitons interacting through meson exchange is used to model dense nuclear matter. The models studied are of the Friedberg–Lee type, which exhibit dynamical bag formation due to the coupling of quarks to a scalar composite gluon field σ. It is shown in the Wigner–Seitz approximation that the high density behavior of such models depends essentially on the leading power of the quark–σ coupling vertex. By insisting that the parameters of any soliton model be chosen to reproduce single nucleon properties, this high-density behavior then selects a promising class of models that better fit the empirical results — the chiral chromodielectric models. The presence of a scalar meson is shown to provide saturation as well as an increase of the proton charge radius with nuclear density. We go beyond the usual Wigner–Seitz approximation by introducing the disorder necessary to reproduce the liquid state, using the significant structure theory of physical chemistry. We study nuclear matter, with particular interest in the transition to a quark plasma, showing that even the simplest version of the model provides a reasonable qualitative fit to both the empirical nuclear matter equation of state and single nucleon properties.

Relativistic quark model and lowest hybrid mesons

Relativistic four-quark equations are found in the framework of the dispersion relation technique. Solutions of these equations using a method based on the extraction of the leading singularities of the amplitudes are obtained. The mass spectrum of the lowest hybrid mesons with an isospin $I=1,$ both exotic quantum numbers (non-$q\overline{q})$ ${J}^{\mathrm{PC}}{=1}^{\ensuremath{-}+}{,0}^{\ensuremath{-}\ensuremath{-}},$ and ordinary quantum numbers ${J}^{\mathrm{PC}}{=0}^{++}{,1}^{++}{,2}^{++}{,0}^{\ensuremath{-}+}{,1}^{\ensuremath{-}\ensuremath{-}},$ are calculated.

Hölder inequalities and isospin splitting of the quark scalar mesons

A Hölder inequality analysis of the QCD Laplace sum rule which probes the non-strange ( n n ̄ ) components of the I={0,1} (light-quark) scalar mesons supports the methodological consistency of an effective continuum contribution from instanton effects. This revised formulation enhances the magnitude of the instanton contributions which split the degeneracy between the I=0 and I=1 channels. Despite this enhanced isospin splitting effect, analysis of the Laplace and finite-energy sum rules seems to preclude identification of either a 0(980) or a light broad σ-resonance as states with predominant n n ̄ components, in which case their dominant components could then be meson–meson ( ππ or K K ̄ ), multiquark, or glueball states. This apparent decoupling of σ ≡f 0(400 –1200) and a 0(980) from the quark n n ̄ scalar currents suggests the possible identification of the f 0(980) and a 0(1450) as the lightest I={0,1} scalar mesons in which the n n ̄ component is dominant.

Hadron structure in medium

The quark meson coupling model may be viewed as a natural generalization of QHD which takes into account the internal structure of the nucleon. In such an approach it is natural for the properties of the nucleon to change in medium. We discuss the change in the proton electric and magnetic form factors when it is bound in a specific shell model orbit. Modern quasi-elastic electron scattering experiments should be able to detect effects of the size predicted. We also examine the mean field potential felt by an ω in a finite nucleus, concluding that the ω should be bound by between 50 and 100 MeV.

The chiral phase transition at high baryon density from nonperturbative flow equations

We investigate the QCD chiral phase transition at high baryon number density within the linear quark meson model for two flavors. The method we employ is based on an exact renormalization group equation for the free energy. Truncated nonperturbative flow equations are derived at nonzero chemical potential and temperature. Whereas the renormalization group flow leads to spontaneous chiral symmetry breaking in vacuum, we find a chiral symmetry restoring first order transition at high density. Combined with previous investigations at nonzero temperature, the result implies the presence of a tricritical point with long--range correlations in the phase diagram.

π0 → γγ in the Spinor Strong Interaction Theory

An expression for the decay rate Γ(π0 γγ) has been derived in the frameworkof the spinor strong interaction theory, a first-principles strong interaction theoryproposed some years ago as an alternative to low-energy QCD. The startingpoint is the SO(3) gauge-invariant action for two quark mesons which has beensuccessful in accounting for confinement, π+ → μ+ ν, e+ν, and π0 e+ν, nonexistenceof the Higgs boson, and other low-energy mesonic phenomena. The quasi-four-quarkmeson equations developed for the decay of a vector meson into twopseudoscalar mesons V → PP has been taken over here to apply to P(π0) →VV(ρ+ ρ−) → γγ(plus π+ and π− which annihilate each other). This mechanismin principle agrees with that of the assumption of vector meson dominance inthe literature. It, together with the effect of form factors, arises naturally in theformalism and need not be assumed. Equations for the perturbed vector mesonwave functions cannot be simply solved and an assumption has been made toobtain an estimate of their magnitude. Together with a constant associated withthe strong coupling obtained earlier from V(ϕ) → PP(K+K−), the estimated decayrate is 19.2 eV, in order-of-magnitude agreement with data (7.74 eV).

Hadronization and strangeness production in a chirally symmetric nonequilibrium model

The expansion and hadronization of a quark meson plasma is studied using an effective chiral interaction Lagrangian. The particles we consider are light as well as strange quarks, which can form pions, kaons and eta mesons via collision processes. The transport equations for the system are solved using a QMD type algorithm. We find that in chemical equilibrium at high temperatures the strange quark mass is considerably higher than the strange current quark mass and becomes even higher if we assume an initial state free of strange quarks. This leads to a considerably higher production threshold. In contrast to simpler scenarios, like thermodynamics of free quarks with their bare mass, we observe that strangeness production in a plasma is hindered and not favoured. The different particle species created during the evolution become separated in coordinate as well as in momentum space. We observe, as at CERN experiments, a larger mean momentum of kaons as compared to pions. Thus the radial collective velocity may as well originate from a plasma expansion and not necessarily from a hadronic scenario.

Light-cone quark model analysis of radially excited pseudoscalar and vector mesons

We present a relativistic constituent quark model to analyze the mass spectrum and hadronic properties of radially excited u and d quark sector mesons. Using a simple Gaussian function as a trial wave function for the variational principle of a QCD motivated Hamiltonian, we obtain a mass spectrum consistent with the experimental data. To do the same for several observables such as decay constants and form factors, it seems necessary to include both Dirac and Pauli form factors on the level of constituent quarks. Taking into account the quark form factors, we thus present the generalized formulas for the rho meson decay constant and form factors as well as the $\ensuremath{\pi}\ensuremath{\gamma}$ transition form factor. We also predict several hadronic properties for the radially excited states.

Effect of nucleon structure variation on the longitudinal response function

Using the quark–meson coupling (QMC) model, we study the longitudinal response function for quasielastic electron scattering from nuclear matter. In QMC the coupling constant between the scalar ( σ) meson and the nucleon is expected to decrease with increasing nuclear density, because of the self-consistent modification of the structure of the nucleon. The reduction of the coupling constant then leads to a smaller contribution from relativistic RPA than that found in QHD-I. However, since the electromagnetic form factors of the in-medium nucleon are modified at the same time, the longitudinal response function and the Coulomb sum are reduced by a total of about 20% in comparison with the Hartree contribution. We find that the relativistic RPA and the nucleon structure variation both contribute about fifty–fifty to the reduction of the longitudinal response.

Finite temperature neutron matter and rotating neutron stars in the quark meson coupling model

A mean-field description of nonoverlaping nucleon bags bound by the self-consistent exchange of $\ensuremath{\sigma},$ $\ensuremath{\omega},$ and $\ensuremath{\rho}$ mesons is used to investigate the properties of neutron matter at finite temperature. We use the equation of state of zero temperature to study the rotating neutron star in the Komatsu-Eriguchi-Hachisu method. The mass, radius, and angular velocity of the neutron star are calculated and compared with the Walecka model.

Exclusive Semi-Leptonic Decays of Heavy Mesons in the Covariant Oscillator Quark Model

The spectra of exclusive semi-leptonic decays of heavy b-quark and c-quark mesons are derived, taking account of confined effects of quarks in the covariant oscillator quark model (COQM). The respective partial widths are also calculated using the standard values of KM-matrix elements. In COQM all kinematics and effective interactions are expressed in a manifestly covariant and unified way. The effective weak currents are given as overlapping integrals between initial and final hadron “wave functions” and lead to the many relations among the form factors, including those derived by HQET in the case of heavy-light quark meson systems. On the other hand, for reference the values of the relevant KM-matrix elements, estimated in COQM directly from each experimental decay width, are also given.

NUCLEON SIGMA TERM AND IN-MEDIUM QUARK CONDENSATE IN THE MODIFIED Quark–MESON COUPLING MODEL

We evaluate the nucleon sigma term and in-medium quark condensate in the modified quark–meson coupling model which features a density-dependent bag constant. We obtain a nucleon sigma term consistent with its empirical value, which requires a significant reduction of the bag constant in the nuclear medium similar to those found in the previous works. The resulting in-medium quark condensate at low densities agrees well with the model-independent linear order result. At higher densities, the magnitude of the in-medium quark condensate tends to increase, indicating no tendency towards chiral symmetry restoration.

Deconfinement in the quark meson coupling model

The Quark Meson Coupling Model which describes nuclear matter as a collection of non-overlapping MIT bags interacting by the self-consistent exchange of scalar and vector mesons is used to study nuclear matter at finite temperature. In its modified version, the density dependence of the bag constant is introduced by a direct coupling between the bag constant and the scalar mean field. In the present work, the coupling of the scalar mean field with the constituent quarks is considered exactly through the solution of the Dirac equation. Our results show that a phase transition takes place at a critical temperature around 200 MeV in which the scalar mean field takes a nonzero value at zero baryon density. Furthermore it is found that the bag constant decreases significantly when the temperature increases above this critical temperature indicating the onset of quark deconfinement.

Hot nuclear matter in the modified quark meson coupling model with dilatons

We study hot nuclear matter in an explicit quark model based on a mean field description of nonoverlapping nucleon bags bound by the self-consistent exchange of scalar and vector mesons as well as the glueball field. The glueball exchange as well as a realization of the broken scale invariance of quantum chromodynamics is achieved through the introduction of a dilaton field. The calculations also take into account the medium-dependence of the bag constant. The effective potential with dilatons is applied to nuclear matter. The nucleon properties at finite temperature as calculated here are found to be appreciably different from cold nuclear matter. The introduction of the dilaton potential improves the shape of the saturation curve at T=0 and is found to affect hot nuclear matter significantly.

Two flavor chiral phase transition from nonperturbative flow equations

We employ nonperturbative flow equations to compute the equation of state for two flavor QCD within an effective quark meson model. This yields the temperature and quark mass dependence of quantities like the chiral condensate or the pion mass. A precision estimate of the universal critical equation of state for the three-dimensional O(4) Heisenberg model is presented. We explicitly connect the O(4) universal behavior near the critical temperature and zero quark mass with the physics at zero temperature and a realistic pion mass. For realistic quark masses the pion correlation length near T_c turns out to be smaller than its zero temperature value.