Quark Degrees Of Freedom Research Articles

The dimensionally reduced effective theory for quarks in high-temperature QCD

We show that QCD undergoes a partial dimensional reduction at high temperatures also in the quark sector. In the kinematic region relevant to screening physics, where the lowest Matsubara modes are close to their “mass shells”, all static Green functions involving both quarks and gluons are reproducible in the high- T limit by a renormalizable three-dimensional Lagrangian up to order g̃ 2(T) ∼ 1/1nT. This three-dimensional theory only contains explicitly the lightest bosonic and fermionic Matsubara modes, while the heavier modes correct the tree-level couplings and generate extra local vertices. We also find that the quark degrees of freedom that have been retained in the reduced theory are non-relativistic in the high- T limit. We then improve our result to order g̃ 4(T) through an explicit non-relativistic expansion, in the spirit of the heavy-quark effective theory. This effective theory is relevant for studying QCD screening phenomena with observables made from quarks, e.g. mesonic and baryonic currents, already at temperatures not much higher than the chiral transition temperature T c .

Spatiotemporal chaos in chiral soliton models.

In this paper, we study the chiral soliton models for baryons, from the viewpoint of classical nonlinear dynamical systems. In particular, we discuss (i) the Skyrme model, an effective Lagrangian model with purely mesonic degrees of freedom based on chiral symmetry, whose topological soliton solutions are identified with baryons, as well as (ii) a quark-meson model, in which the quark degrees of freedom are explicitly present. We consider fluctuations around the B=1 soliton solutions of these models and solve the spherically symmetric, time-dependent systems. Numerical studies indicate that the phase space around the Skyrme soliton solution exhibits spatiotemporal chaos. In contrast with this, the soliton of the quark-meson model is stable even for large perturbations. \textcopyright{} 1996 The American Physical Society.

Field-theoretical construction of the nucleon-nucleon potential with quark degrees of freedom

Field-theoretical construction of the nucleon-nucleon potential with quark degrees of freedom

Measurement of the tensor analyzing power T20 for d

The tensor analyzing power ${T}_{20}$ for the reaction ${d}^{\ensuremath{\uparrow}}{+}^{12}\mathrm{C}\ensuremath{\rightarrow}p(0\ifmmode^\circ\else\textdegree\fi{})+X$ has been measured in the region of proton internal momenta $k$ in light-cone dynamics up to 1 GeV/ $c$. Measurements have been carried out at Dubna Synchrophasotron with polarized deuteron beam at deuteron momenta up to 9 GeV/ $c$. When $k$ increases the experimental values of ${T}_{20}$ have a tendency to approach the value ( $\ensuremath{-}$0.3) obtained by the calculation based on the reduced nuclear amplitude method in which the quark degrees of freedom are taken into account.

Quark degrees of freedom in nuclei

In this paper we search quark degrees of freedom in nuclei by studying electromagnetic processes and double-charge-exchange reaction at the resonance region. We propose a hybrid quark hadron (HQH) model to describe nuclear structure. The model solves many long-standing puzzles in nuclear physics. Our principal finding in this paper is that multiquark cluster effect plays an important role in nuclear phenomena at higher monumentum transfers.

S 1×S 2 as a bag membrane and its einstein-weyl geometry

In the hybrid skyrmion in which an Anti-de Sitter bag is imbedded into the skyrmion configuration a S^{1}\times S^{2} membrane is lying on the compactified spatial infinity of the bag [H. Rosu, Nuovo Cimento B 108, 313 (1993)]. The connection between the quark degrees of freedom and the mesonic ones is made through the membrane, in a way that should still be clarified from the standpoint of general relativity and topology. The S^1 \times S^2 membrane as a 3-dimensional manifold is at the same time a Weyl-Einstein space. We make here an excursion through the mathematical body of knowledge in the differential geometry and topology of these spaces which is expected to be useful for hadronic membranes

Thermodynamics of a quark-meson plasma in the Nambu-Jona-Lasinio model

We investigate the thermodynamics of a quark-meson plasma in the two-flavour Nambu-Jona-Lasinio (NJL) model. We calculate the thermodynamic functions to order 1 N c in an expansion in the inverse number of colours, which is seen to include mesonic exchange. After discussing the bound and resonant states that occur, and the associated scattering phase shifts, we compute their contribution to the behaviour of the thermodynamic potential and the bulk thermodynamic quantities that follow from this, such as entropy and energy densities. In the chiral limit, the chiral phase-transition line is calculated in the ( μ-T) plane; the transition is of first order at low temperature and of second order at high temperature, for zero chemical potential. The crossing point is at T = 78 MeV. As is expected physically, the mesonic or collective degrees of freedom play a dominant role at low temperature, while the quark degrees of freedom are most relevant at high temperature. In keeping with the fact that the NJL model is not confining, we see that a mixed phase exists. For μ = 0, it lies in the temperature range80 ⩽ T ⩽ 140MeV. Other thermodynamic quantities of interest, such as the chiral and quark susceptibilities are also calculated, but in the mean-field approximation only, since they are unaltered by the inclusion of fluctuations about the mean field, to the level of approximation made.

Dynamical regimes of the QCD string with light and heavy quarks

The generalization of the effective action [1] of the quark--antiquark system in the confining vacuum is performed for the case of arbitrary quark masses. The interaction of quarks is described by the averaged Wilson loop for which we use the minimal area law asymptotics. The system is quantized by the path integral method and the quantum Hamiltonian is obtained. It contains not only quark degrees of freedom but also the string energy density. As well as in the equal masses case [1] two dynamical regimes are found [2]: for large orbital excitations ($l \gg 1$) the system is represented as rotating string, which leads to asymptotically linear Regge trajectories, while at small $l$ one obtains a potential-like relativistic or nonrelativistic regime. In the limiting cases of light-light and heavy-light mesons a unified description is developed [2]. For the Regge trajectories one obtains nearly straight-line patterns with the slope very close to $1/2 \pi \sigma$ and $1/ \pi\sigma$ correspondingly. The upper bound on the light quark(s) masses which doesn't change considerably this property of the trajectories is also found.

A microscopic understanding of the structure functions of finite nuclei

A number of relatively successful quark model descriptions of nuclear matter have recently been developed, based on a mean-field description of non-overlapping nucleon bags bound by the self-consistent exchange of scalar (σ) and vector (ω) mesons. The meson fields modify the internal quark motion in the nucleon bag in medium and this induces a saturation mechanism for nuclear matter. The model gives a nuclear compressibility around 220 MeV and provides a rather satisfactory interpretation of nuclear matter properties. Furthermore, it has recently been shown that the twist-2 valence quark distributions of the free nucleon calculated in the MIT bag model are in reasonable agreement with world data. By using a local density approximation we combine these developments to microscopically investigate the structure functions of finite nuclei. The effect of nuclear shadowing and antishadowing is taken into account phenomenologically. The model can quantitatively reproduce the existing experimental data - the so-called European Muon Collaboration (EMC) effect. We show that the quark degrees of freedom in nuclei and the final state interaction between the debris of the struck nucleon and the residual nucleus both play important roles in the nuclear structure functions. Thus the conventional treatment based on nucleon binding within the impulse approximation is quantitatively unreliable. We also study the effects of the self-coupling terms of the σ field and nucleon-nucleon correlations on some nuclear matter properties and the nuclear structure functions.

Mass bounds in the standard model.

Nonperturbative triviality and vacuum stability mass bounds are obtained for the Higgs scalar and top quark degrees of freedom in the standard electroweak model using Wilson renormalization group techniques. Particular attention is given to the effect of the generalized top Yukawa coupling on the scalar mass upper bound.

Relativistic scattering theory of two bound quark-antiquark systems in the low- and intermediate-energy region

Relativistic three-dimensional equations for meson-meson scattering amplitudes are constructed considering the mesons as bound quark-antiquark systems. The resulting equations have a form identical with the field-theoretical Low equations for the meson-meson scattering amplitude without quark degrees of freedom. Therefore the solution of these equations and their linearized representations satisfy the unitarity condition and the low-energy theorems. In order to build the effective potential of the proposed equations one uses meson-quark-antiquark bound-state wave functions and\((q\bar q)(q\bar q) \to (q\bar q)(q\bar q)\) transition matrices. Also these quark-antiquark bound-state wave functions satisfy the relativistic three-dimensional integral equation, which contains all retardation effects.

Chapter VII. Chiral Bag Model

We review the chiral bag model for the nucleon. The model incorporates the quark and meson degrees of freedom for the description of short and long distance phenomena, respec­ tively. Using the original chiral bag model with the pion as the only mesonic degree of freedom, we study the Casimir effects and spin and isospin projection in detail. The model is then extended to include the vector mesons for a more realistic description of the nucleon. Static properties of the nucleon are studied in deta.il using this model with the emphasis on bag radius dependence of observables in relation to the question of the Cheshire cat principle and the role of the explicit quark degree of freedom in low energy properties of the nucleon.

Quark and nuclear matter with strangeness

Nuclear matter with strangeness has been extensively studied as hypernuclei in conjunction with quark degrees of freedom in the nucleus. The present status of our knowledge of hypernuclei is reviewed from an experimental point of view. Especially, the existence of the H dibaryon is related to the ground state of nuclear matter with S = -2. The recent experiments on this subject are discussed in detail. The forms of quark matter with strangeness called strangelets or nuclearites have been discussed in relation to the QCD phase transition in the early universe and in relativistic heavy ion collisions. The methods to search for these objects are discussed and the present status of the experiments is presented.

SEARCH FOR NARROW ISOVECTOR DIBARYONS CLOSE TO LOW MASSES THRESHOLD

For masses larger than 2 % + MT, narrow peaks in two proton invariant (missing) masses spectra : M (M ) are now well established. The existence of such PP x narrow peaks is not so unquestionable below 2 $ + Mr. Tensor analyzing power T20 + of the invariant mass M has been studied using the p(d,pp)~ reaction. Depending PP on the assumption done for background substraction -a hble or an oscillation mayappear at M = 1941 MeV. Additional data are under analysis and will improve the statistics. PP It is well known that narrow structures have been obsenred in two nucleon invariant masses, or missing mass experiments/l/. These structures have been tentatively attributed to isovector dibaryons although no proof has been advanced to associate them with quark degrees of freedom. This assumption relies only on the fact that such structures are not present in classical nuclear physics using meson exchanges and nucleon or delta degrees of freedom. The experimental certainty depends mainly on the quantum numbers and the mass of studied dibaryons. Up to now, none has been observed in T = 0/2/ or T = 2/31 isospin states. In case of T = 1 isospin, for the range of masses larger than 2 $ + M* (M > 2.016 GeV) there is a good agreement for narrow peaks between results obtained at Saturnel41 : measurement of d2a/dQdMx for 38e(p,d)~ reaction, and Lanpf/5/ where Ay was measured. These data correspond to the second generation of experiments, since they have used electronical detectors. ('1 Present address : Ben Gourion University of the Negev, P.O. Box 653 84105Beer Sheva, Israel. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990631 C6-372 COLLOQUE DE PHYSIQUE Below 2.016 GeV, many structures have been presented, corresponding usually -but not alwaysto the first generation of experiments studying bubble chamber pictures. Here the masses of experimentally observed structures do not concentrate around a few values, but are more or less distributed from 2 % to 2 % + M?/1/. The necessity of a precise second generation experiment close to the two nucleon mass appears then naturally. 3 Such an experiment has been carried out at SATURNE with the p(d,pp)X reaction. Tensor polarized deuteron beam of 1.722 GeV has been used and the two protons detected at 0 using the SPES3 facility. The properties of the spectrometer : pmax/pmin z 2.5, and As2 = sr allow the study of the following two proton invariant mass range : 2 % < M < 1.963 GeV, PP the counting rate decreasing regularly from small to larger masses M . This result is mainPP ly due to pp FSI giving rise to a narrow maximum a few MeV above 2 M . Due to the granulaP rity of the detection, when two trajectories intercept a same scintillator of one among the two major trigger detection plans or give rise to two mixing tracks on the focal pla~e, the event is lost. This produces a loss of efficiency in the first three MeV of the spectra.This explains that our FSI is located around 1880.7 MeV, that is ?. 3 MeV higher than the peak maximum from low energy experiments. The detection has been described elsewhere/3/. The empty target accounts for about 5 % of the counting rate. The FWHM of the experimental wcdth for M is 1.2 MeV. The data have been therefore binned into 2 MeV intervals. The M hisPP PP togram presented in Fig. 1 is quite continuous, without any discontinuity having a number of standard deviations (S .D .) larger than 2.2. id = 1.722 GeV epp =0° 0.2 188

PION ABSORPTION IN A SINGLE STEP ON THREE NUCLEONS IN He3 AND He4: A POSSIBLE SIGNAL FOR QUARK DEGREES OF FREEDOM IN NUCLEI

We discuss a new mechanism by which pions are absorbed in a single-step process on three correlated nucleons in 3He and 4He, leading to three nucleons which share the pion energy comparably. The mechanism invokes quark degrees of freedom in nuclei, with a small probability of about 6% in He, but nevertheless leads to cross-sections of several millibarns. Comparison is made to recent experiments, and to conventional mechanisms.

Few-body form factors:Mesonic or quark degrees of freedom?

Besides the standard description of the electromagnetic operator in nuclei in terms of meson-exchange currents, which we shall briefly review, three typical models of the nucleon structure lead to various contributions to the current. We shall discuss successively relativistic corrections in the constituent quark model, the definition of the electromagnetic operator for a multibag system and finally some important results from the Skyrme description of the nucleon.

Quark exchange currents within the quark cluster model

Nuclear exchange currents arising from the quark degrees of freedom are studied in the quark cluster model. A proper definition of the two-body currents to be used for the conventional nuclear wave functions is examined. The electromagnetic form factors of the deuteron, the Gamow-Teller matrix elements for A = 16±1 and A = 40±1 systems and the electro-breakup of 4He are discussed as examples of application.

Composite particle scattering in a hybrid quark-hadron model

We propose a hybrid quark-hadron model to describe high-energy composite particle scattering. Parameters for the quark sector in the model are determined by fitting pp scattering data. We apply the model without any free parameter to proton-alpha elastic scattering. Our model yields a quite large improvement over various calculations done before and provides a very good agreement with existing available data. Our results also show that the quark degrees of freedom in NN interaction play an important role in improving theoretical description of experimental data and must be included in future calculations.

The nucleon-nucleon force and the quark degrees of freedom

This review gives a detailed overview of the current status of our understanding of the nucleon-nucleon forces. The authors review the known long-range meson exchange forces and explain how these forces originate from an underlying quark model constrained by chiral symmetry, a symmetry that is very well satisfied in low-energy nuclear phenomena. These effective meson exchange forces describe the large-impact-parameter nucleon-nucleon scattering. The authors show how the small-impact-parameter nucleon-nucleon scattering can be explained by the quark structure of the nucleons and why this quark model is successful in reproducing the energy dependence of the "measured" $S$- and $P$- wave nucleon-nucleon phase shifts.

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 4 ΛHe(1 +).