Quark condensates in nuclear matter with vacuum effects

On the basis of a bosonized Nambu- Jona-Lasinio (NJL) model with derivative expansions, quark condensates in nuclear matter are studied at one-quark loop level and the dependence of meson masses and couplings on the constituent quark mass is investigated. The condensate ratio obtained here < q?q > _ρB / < q?q > _vac is roughly 0.66 with constituent quark mass of 313 MeV, which yields a corresponding σ _N value to be roughly 42.2 MeV at the mean field level and σ _N =31.4 MeV with the vacuum dependence, where the model parameters describing a Lorentz scalar and a vector field are self-determined.     

Measurement of the spectral functions of vector current hadronic tau decays

A measurement of the spectral functions of non-strange τ vector current final states is presented, using 124 358 τ pairs recorded by the ALEPH detector at LEP during the years 1991 to 1994. The spectral functions of the dominant two- and four-pion τ decay channels are compared to published results of e ⁺ e ^- annihilation experiments via isospin rotation. A combined fit of the pion form factor from τ decays and e ⁺ e ^- data is performed using different parametrizations. The mass and the width of the ρ ^±(770) and the ρ ⁰(770) are separately determined in order to extract possible isospin violating effects. The mass and width differences are measured to be M _ρ ^± ₍₇₇₀₎ - M _ρ ⁰ ₍₇₇₀₎ = (0.0 ± 1.0) MeV/c ² and Γ _ρ ^± ₍₇₇₀₎ - Γ _ρ ⁰ ₍₇₇₀₎ = (0.1 ± 1.9) MeV/c ².     

Mass shift and width broadening of J/psi in hot gluonic plasma from QCD sum rules

We investigate possible mass shift and width broadening of J/psi in hot gluonic matter using QCD sum rules. Input values of gluon condensates at finite temperature are extracted from lattice QCD data for the energy density and pressure. Although stability of the moment ratio is achieved only up to T/Tc approximately 1.05, the gluon condensates cause a decrease of the moment ratio, which results in a change of the spectral properties. Using the Breit-Wigner form for the phenomenological side, we find that the mass shift of J/psi just above Tc can reach maximally 200 MeV and the width can broaden to dozens of MeV.     

Massive Yang–Mills for vector and axial-vector spectral functions at finite temperature

The hadronic mechanism which leads to chiral symmetry restoration is explored in the context of the ρπa₁$\rho \pi a_1$ system using Massive Yang–Mills, a hadronic effective theory which governs their microscopic interactions. In this approach, vector and axial-vector mesons are implemented as gauge bosons of a local chiral gauge group. We have previously shown that this model can describe the experimentally measured vector and axial-vector spectral functions in vacuum. Here, we carry the analysis to finite temperatures by evaluating medium effects in a pion gas and calculating thermal spectral functions. We find that the spectral peaks in both channels broaden along with a noticeable downward mass shift in the a₁$a_1$ spectral peak and negligible movement of the ρ$\rho$ peak. The approach toward spectral function degeneracy is accompanied by a reduction of chiral order parameters, i.e., the pion decay constant and scalar condensate. Our findings suggest a mechanism where the chiral mass splitting induced in vacuum is burned off. We explore this mechanism and identify future investigations which can further test it.     

Hadron resonance gas and nonperturbative QCD vacuum at finite temperature

Nonperturbative QCD vacuum with two light quarks at finite temperature was studied in a hadron resonance-gas model. Temperature dependences of the quark and gluon condensates in the confined phase were obtained. It is shown that the quark condensate and one-half (chromoelectric component) of the gluon condensate are evaporated at the same temperature corresponding to the quark-hadron phase transition. With allowance for the temperature shift of hadron masses, the critical temperature was found to be T _c ?190 MeV.     

Analysis of experiments on the measurement of the pion electromagnetic formfactor

We analyse the experiments [1–3] on the measurement of the pion electromagnetic formfactor in the time-like and space-like region using an expression with correct analytical properties and with the asymptotics predicted by QCD. The best fit values for the physical quantities (1) electromagnetic radius of the pion:r _π ² =0.466±0.002 fm², (2) the mass and width of the ρ (770) and the ρ′ (1600) meson:m _ρ=768±1 MeV,Г _ρ=138±1 MeV,m _ρ′=1546±26 MeV,Г _ρ'=620±60 MeV; (3) the residuesg _ρ m _ρ ² =0.676±0.003,g _ρ′ m _ρ′ ² =?(0.644±0.009); (4) the ππ-scattering lengtha ₁ ¹ =(0.1±0.01)m _π ^?3 .     

A note on the massive quark matter phase

Using an equation of state which is based on a many-body treatment of a constituent quark model with confinement interaction, the phase transition to a massive quark phase is studied. It is found that in the case of bag constantsB ^1/4>200 MeV and baryon number density of about 5ρ₀ a phase of massive deconfined quarks may become stable.     

Solitons in the Nambu-Jona-Lasinio model with vector and axial mesons

We study the effect of vector and axial isovector mesons in the soliton of the SU(2) NJL-model for static hedgehog configurations and on the chiral circle. The Lagrangian reproduces Sakurai's Universality and implements vector dominance through current-field identities. The parameters of the Lagrangian are fixed by means of a heat-kernel expansion up to second order taking theA-π mixing into account. For the corresponding set of parameters we find solitons for constituent quark masses betweenM～265–345 MeV which are much lower than those found without vector mesons. For the valueM=315 MeV which corresponds both to the KSFR relation 2g _ρ ² f _π ² =m _ρ ² and the Weinberg sum rule m _A ² =m _ρ ² the soliton has a clearly negative quark eigenvalue. The soliton mass is about 1100 MeV. For this choise of parameters the radius and axial coupling constant turn out to be too small.     

Variational calculations of asymmetric nuclear matter

We report on variational calculations of the energy E(ρ, β) of asymmetric nuclear matter having ? = ?_n + ?_p = 0.05 to 0.35 fm^?3, and β = (?_n ? ?_p/g9 = 0 to 1. The nuclear h used in this work consists of a realistic two-nucleon interaction, called v₁₄, that fits the available nucleon-nucleon scattering data up to 425 MeV, and a phenomenological three nucleon interaction adjusted to reproduce the empirical properties of symmetric nuclear matter. The variational many-body theory of symmetric nuclear matter is extended to treat matter with neutron excess. Numerical and analytic studies of the β-dependence of various contributions to the nuclear matter energy show that at ? < 0.35 fm^?3 the β⁴ terms are very small, and that the interaction energy EI(ρ, β) defined as E(ρ, β) ? T_F(ρ, β), where T_F is the Fermi-gas energy, is well approximated by EI₀(?) + β²EI₂(ρ). The calculated symmetry energy at equilibrium density is 30 MeV and it increases from 15 to 38 MeV as ? increases from 0.05 to 0.35 fm^?3.     

On the properties of asymmetric nuclear matter

The properties of asymmetric nuclear matter withα=(ρ _n-ρ _p)/ρ≦ 0.4 are studied within the framework of the lowest order Brueckner theory, atk _F =1.35fm^?1 (ρ=0.166 fm^?3). TheK-matrix is calculated self-consistently from the Reid soft core nucleon-nucleon interaction. In the case ofρ _n ≠ρ _p theK-matrix contains a term which does not conserve the total isospin of the interacting unlike-nucleon pair. At α=0.4 the relative magnitude of this term is of the order of 1 %. The symmetry energy is found equal to 23.1 MeV.     

Role of four-quark condensates in QCD sum rules

The QCD sum rule approach to the in-medium behavior of hadrons is discussed for the ω meson, nucleon and D meson. Emphasis is placed on the impact of four-quark condensates and on order parameters of spontaneous symmetry breaking.     

The phase shifts leading to the broadening and shift of spectral lines

The classical theory of collisional broadening and shift parameters (β, δ) of an isolated spectral line was used to obtain simple analytical formulas for calculating both β and δ. These formulas were obtained on the assumption that the short range interaction is effective only in the broadening while the long range is effective in the shift of the spectral line. These parameters β and δ depend on the limiting phase shifts responsible for broadening η_b and shift ηδ. It was found that the values of η_b and η_δ are not equal to each other as was proposed by Weisskopf η_b=η_δ=1. The maximum and average values of η_b (η_b max, η_b av) and η_δ (η_δ max, η_δ av) were obtained by numerical evaluation, using different inverse power potentials. By introducing these parameters into the approximated formulas for β and δ using Van der Waals and Lennard-Jones potential, it was found that the results of calculations for (β and δ) with different atomic transitions perturbed by different inert gases are in close agreement with earlier results. Those results, obtained earlier, were based on the Lindholm-Foley theory especially with the average values of η_b [η_b av=0.6057] and the maximum values of η_δ [η_δ max=1.57625]. The impact parameters ρ_b and ρ_δ leading to the broadening and shift of the spectral line were also obtained for different interactions. It was found that the end parameter for the broadening ρ_b is not equal to the starting parameter for the shift ρ_δ.     

Study of theω π + system

The mass spectra of theω π ⁰ system, produced inπ ^? p charge-exchange reactions at 38 GeV/c and 100 GeV/c, show a new neutral state with a mass of 2200±20 MeV and a width of 260±50 MeV. Likeρ ₃(1690), this meson,X(2200), is produced through one-pion-exchange.X(2200)J ^PC is likely 1^??. It might be a 2³ D ₁ radial-orbital excitation ofρ ⁰. Its production cross section falls with energy quadratically, in agreement with one-pion-exchange. The cross sections forb ₁(1235) andρ ₃(1690) production have also been measured; their energy dependences have been determined.     

Relativistic nuclear and neutron matter at finite temperatures

Nuclear matter as well as neutron matter is studied in the framework of a relativistic nuclear field theory at finite temperature. A spectral representation for the two-point Green's function at finite temperature and finite density is constructed. The bulk properties of the interacting system are calculated in the Hartree and Hartree-Fock approach. In additionσ ³- andσ ⁴-self-interactions have been taken into account. We present and discuss the results of hot and dense matter for temperaturesT≦ 50 MeV and densitiesθ≦6θ ₀ (ρ ₀≈0.17 fm^?3) using six different model parameter sets.     

Chiral partners and their electromagnetic radiation (Ingredients for a systematic in-medium calculation)

It is argued that the chiral partners of the lowest-lying hadrons are hadronic molecules and not three-quark or quark-antiquark states, respectively. As an example the case of a₁ as the chiral partner of the ρ is discussed. Deconfinement-or as a precursor large in-medium widths for hadronic states-is proposed as a natural way to accommodate for the fact that at chiral restoration the respective in-medium spectra of chiral partners must become degenerate. Ingredients for a systematic and self-consistent in-medium calculation are presented with special emphasis on vector-meson dominance which emerges from a recently proposed systematic counting scheme for the mesonic sector including pseudoscalar and vector mesons as active degrees of freedom.     

Thermal desorption of Ag from oxidized W

Thermal desorption spectra (TDS) of Ag condensates deposited at two substrate temperatures T_s = 300 K and T_s = 779 K have been obtained. A shift of the temperature T_m of the maximum of the desorption flux R_e was observed. It was established that the shift of the maximum depends on the value of the initial coverage σ₀. A significant difference was found to exist between TDS of silver condensates deposited on oxidized and clean W substrates due to differences in the mechanism of condensation. Silver condensates were deposited on oxidized W at different initial conditions (T_s, impingement rates R_i, etc.) but equal σ₀. The corresponding TDS were compared and a conclusion has been drawn that the shift of T_m is due to the different structure of Ag condensates. TDS of Ag condensates deposited at room temperature (T_s = 300 K) were interpreted using the method of Bauer et al. [J. Appl. Phys. 45 (1974) 5164: Surface Sci. 53 (1975) 87]. The dependence of the desorption flux R_e on the substrate temperature T_s and coverage σ was treated on the basis of the Polanyi-Wigner equation and some parameters of the condensation process were evaluated.     

The three-gluon condensates at finite (low) temperature

The three-gluon condensates at finite temperature are found to be of two types. It turns out that these two types of gluon condensates differ only in sign. The value of these condensates is investigated in the framework of the grand partition function for a dilute instanton medium at finite (low) temperature, and found to decrease slowly with the temperature even for very low temperature and vanish nearly atT=240 MeV.     

Lifetime measurements on the compound nucleus 236U by means of the shadow effect

The lifetime of the ²³⁶U nucleus is measured in the range of excitation energies E_x = 6.7–11.5 MeV by a method based on the shadow (blocking) effect. The results obtained, as well as those of an earlier measurement of the lifetime for the ²³⁹U nucleus in the range E_x = 6.4–9.1 MeV, are compared with the results of calculations based on the level density ρ(E_x) in the Fermi-gas model and on the empirical level density ρ(E)_x) calculated as a result of an analysis of(n, γ) and (n, n') cross sections. An increase of excitation energy produces a substantial reduction in the rate of decrease of the lifetime at E_x ? 7.5 MeV for both compound nuclei as compared to the theoretical dependence τ(e_x).