Towards the nuclear matter-quark matter phase transition

Baryon magnetic moments are considered in the quark model. Small contributions to the moments are assumed to arise from configuration mixing (including configurations with orbital angular momentum) in the baryon wave functions, from SU (3) breaking, and from the dependence of the effective quark masses on their environment. It is found that these contributions can improve the agreement of the quark model with experiment. However, so long as quarks have Dirac magnetic moments, charge symmetry holds, and SU (3) breaking effects are small, there is a residual disagreement between predictions of the model and values of some of the recently measured hyperon magnetic moments.     

Test of the heavy quark–light diquark approximation for baryons with a heavy quark

We check a commonly used approximation in which a baryon with a heavy quark is described as a heavy quark–light diquark system. The heavy quark influences the diquark internal motion reducing the average distance between the two light quarks. Besides, we show how the average distance between the heavy quark and any of the light quarks, and that between the heavy quark and the center of mass of the light diquark, are smaller than the distance between the two light quarks, which seems to contradict the heavy quark–light diquark picture. This latter result is in agreement with expectations from QCD sum rules and lattice QCD calculations. Our results also show that the diquark approximations produces larger masses than the ones obtained in a full calculation.     

S-wave baryons in an equally mixed scalar-vector square root potential model of independent quarks

A relativistic model of independent quarks based on Dirac equation with an equally mixed scalar-vector square root confining potential is used to compute the quark core contributions to the static properties like magnetic moments, charge radii and axial vector coupling constant ratios of the baryon octet. The results obtained with appropriate corrections due to centre-of-mass motion agree fairly well with experimental values. The model is also extended to the study of magnetic moments of the quark core of baryons in the charmed andb-flavoured sectors and the overall predictions so obtained compare well with other model predictions.     

Ground state baryon magnetic moments and nucleon axial vector coupling

Ground-state baryon magnetic moments and nucleon axial vector coupling are calculated usingqcd inspired configuration mixing and relativistic corrections. Unlike earlier attempts, we incorporate a natural mass scale for quarks, taken as one third the nucleon mass for up and down quarks, and the strange quark mass suggested by the Lipkin’s sum rule. In the parameter-free non-relativistic limit, we find a fairly good fit, which improves upon including relativistic corrections.     

Sum rules and systematics for baryon magnetic moments

The new experimental values of hyperon magnetic moments are compared with sum rules predicted from general quark models. Three difficulties encountered are not easily explained by simple models. The isovector contributions of nonstrange quarks to hyperon moments are smaller than the corresponding contribution to nucleon moments, indicating either appreciable configuration mixing in hyperon wave functions and absent in nucleons or an additional isovector contribution beyond that of valence quarks, e.g. from a pion cloud. The large magnitude of the Ξ^? moment may indicate that the strange quark contribution to the Ξ moments is considerably larger than the value μ(Λ) predicted by simple models which have otherwise been very successful. The set of controversial values from different experiments of the Σ^? moment include a value very close to $\text{?}\text{1}\text{2}\text{μ(Σ}{}^{\mathrm{+}}\text{)}$ which would indicate that strange quarks do not contribute at all to the Σ moments.     

Spectroscopy of the Ωccb baryon in the hypercentral constituent quark model

We extract the mass spectrum of the triply heavy baryon Ω_ccb using the hypercentral constituent quark model. The first order correction is also added to the potential term of the Hamiltonian. The radial and orbital excited state masses are determined, and the Regge trajectories and magnetic moments for this baryon are also given.     

Gluon condensates at finite baryon densities and temperature

We derive here the equation of state for quark matter with a nontrivial vacuum structure in QCD at finite temperature and baryon density. Using thermofield dynamics, the parameters of thermal vacuum and the gluon condensate function are determined through minimisation of the thermodynamic potential, along with a self-consistent determination of the effective gluon and quark masses. The scale parameter for the gluon condensates is related to the SVZ parameter in the context of QCD sum rules at zero temperature. With inclusion of quarks in the thermal vacuum the critical temperature at which the gluon condensate vanishes decreases as compared to that containing only gluons. At zero temperature, we similarly obtain the critical baryon density for the same to be about 0.36 fm^?3.     

Quark-meson <Emphasis Type="Italic">SU</Emphasis>(3) model in a Tamm-Dancoff inspired approximation

The non-linear chiral quark-meson U(3) x U(3) model is solved using the Tamm-Dancoff inspired approximation which is described in an earlier paper [Phys. Rev. D 58, 034003 (1998)]. The resulting system of 15 coupled non-linear differential equations self-consistently determines all quark-meson coupling constants. Also the obtained solutions for quark and meson fields are stable and physically acceptable. As the zeroth approximation of a more refined structure they were used to calculate SU(3) baryon octet magnetic moments and axial coupling constants with baryon state vectors containing valence quarks only, at this primordial level. The results are very promising, so possibilities to pursuit more sophistication and improved physical input is indicated.Received: 24 August 2004, Published online: 1 December 2004PACS: 12.39.Ba, 12.39.FeD. Horvat: dubravko.horvat@fer.hr, Correspondence to: davorh@phy.hr     

The baryon density in chiral bag models

The quark vacuum plays an essential role in making the chiral bag model consistent and well-defined. We compute vacuum (one-loop) contributions to the baryon density as a regulated sum over quark eigenmodes. The sum is evaluated in straightforward numerical fashion and yields results for the moments of the baryon density which are in agreement with those of Jezabek and Heller.     

Exchange currents arising from quark degrees of freedom

We investigate the effects of antisymmetrization with respect to valence quarks in nuclei introducing “quark exchange currents”. Their effects in ordinary nuclei are found to be small for lack of contribution from s-wave nucleon pairs owing to spin-isospin symmetry. For example, the Gamow-Teller type β-decay matrix elements for A = 15, 17, 39, 41 and ∞ nuclei are reduced from the single-particle values by 2–8% depending on the baryon size. The effects seem to be larger for Σ-hypernuclei. For example, the magnetic moment of Σ+4Li(1+) is reduced by more than 15%. For Λ-hypernuclei, however, the effects on the magnetic moments tend to cancel each other, and about 1% reduction is obtained for 4ΛHe(1+).     

Size-dependent magnetism of deposited small iron clusters studied by x-ray magnetic circular dichroism

The size-dependent magnetic properties of small iron clusters deposited on ultrathin nickel films have been studied with circularly polarized synchrotron radiation. With the use of sum rules, orbital and spin magnetic moments have been extracted from x-ray magnetic circular dichroism spectra. The ratio of orbital to spin magnetic moments varies considerably with cluster size, reflecting the dependence of magnetic properties on cluster size and geometry. These variations can be explained in terms of enhanced orbital moments in small clusters.     

Magnetic moments and structure of the nuclei3H,3He and of baryons

From a solution of the problem of magnetic moments of the nuclei³H and³He, two properties are obtained: - These nuclei have mixed orbital ground states. - These states are not charge symmetric. The first property is expected to hold also for baryons in the quark model, on account of recently measured magnetic moments. Supporting evidence and implications for baryon structure are discussed.     

Baryon scattering at high energies: wave function,impact factor,and gluon radiation

The scattering of a baryon consisting of three massive quarks is investigated in the high energy limit of perturbative QCD. A model of a relativistic proton-like wave function, dependent on valence quark longitudinal and transverse momenta and on quark helicities, is proposed, and we derive the baryon impact factors for two, three and four t-channel gluons. We find that the baryonic impact factor can be written as a sum of three pieces: in the first one a subsystem consisting of two of the three quarks behaves very much like the quark–antiquark pair in γ^* scattering, whereas the third quark acts as a spectator. The second term belongs to the odderon, whereas in the third (C-even) piece all three quarks participate in the scattering. This term is new and has no analogue in γ^* scattering. We also study the small x evolution of gluon radiation for each of these three terms. The first term follows the same pattern of gluon radiation as the γ^*-initiated quark–antiquark dipole, and, in particular, it contains the BFKL evolution followed by the 2→4 transition vertex (triple pomeron vertex). The odderon term is described by the standard BKP evolution, and the baryon couples to both known odderon solutions, the Janik–Wosiek solution and the BLV solution. Finally, the t-channel evolution of the third term starts with a three-reggeized gluon state, which then, via a new 3→4 transition vertex, couples to the four-gluon (two-pomeron) state. We briefly discuss a few consequences of these findings, in particular the pattern of unitarization of high energy baryon scattering amplitudes.     

Magnetic Effect Versus Thermal Effect on Quark Matter with a Running Coupling at Finite Densities

We investigate the quark matter in a strong magnetic field in the framework of SU(2) NJL model with a magnetic-field-dependent coupling. The spin polarization, the entropy per baryon, and the energy are studied by analyzing the competition of the magnetic effect and the thermal effect. The stronger magnetic field can enhance the spin polarization, arrange quarks in a uniform spin orientation, and change the energy per baryon drastically. However,it can hardly affect the entropy per baryon, which is dominated by the temperature. As the temperature increases, more quarks will be excited from the lowest Landau level up to higher Landau levels.     

Quark recombination in deuteron fragmentation

Low-P _T production off deuteron in the fragmentation region is studied on the basis of a six-quark picture of deuteron. It is argued that all the six valence quarks participate equally in the hadronization processes associated with meson and baryon productions. Thex-distribution of the inclusive cross sections is studied in the framework of a quark recombination model. Fit with experimental data leads to a determination of the valence and sea quark distributions in deuteron. Probabilistic arguments applied to a general recombination picture are shown to lead to some experimentally verifiable constraints on baryon multiplicities. Possibilities of determining the various recombination probabilities are also discussed.     

Transition magnetic moments of J~P=3/2~+ decuplet to J~P=1/2~+ octet baryons in the chiral constituent quark model

In light of the developments of the chiral constituent quark model(χ~(CQM)) in studying low energy hadronic matrix elements of the ground-state baryons, we extend this model to investigate their transition properties.The magnetic moments of transitions from the J~P=3/2~+ decuplet to J~P=1/2~+ octet baryons are calculated with explicit valence quark spin, sea quark spin and sea quark orbital angular momentum contributions. Since the experimental data is available for only a few transitions, we compare our results with the results of other available models. The implications of other complicated effects such as chiral symmetry breaking and SU(3) symmetry breaking arising due to confinement of quarks are also discussed.     

Effects of perturbative exchanges in a QCD string model

The QCD string model for baryons derived by Yu. A. Simonov and used for the calculation of baryon magnetic moments in a previous paper is extended to include also perturbative gluon and meson exchanges. The mass spectrum of the baryon multiplet is studied. For the meson interaction, either pseudoscalar or pseudovector coupling is used. Predictions are compared with the experimental data. Besides these exchanges, the influence of excited quark orbitals on the baryon ground state are considered by performing a multichannel calculation. The nucleon-Δ splitting increases due to the mixing of higher quark states, while the baryon magnetic momenta decrease. The multichannel calculation with perturbative exchanges is shown to yield reasonable magnetic moments, while the mass spectrum is close to experiment.     

Analysis of (1/2)~+ baryon states containing fourth-family quarks from QCD sum rules

When the fourth generation of quarks have sufficiently small mixing with ordinary standard-model quarks, the hadrons made up from these quarks can be long-lived enough. We analyze the (1/2)⁺ baryon states containing fourth-generation quarks and standard-model quarks, i.e. the charm or bottom quarks, in the QCD sum rules approach. Considering the perturbative and two gluon condensate contributions in the calculation, we give the numerical results of the masses and pole residues.