The quark compound bag model: Defining the admixture of six-quark bag in the deuteron

The probability of the six-quark bag part of the deuteron is defined within recently formulated quark compound bag (QCB) model. An upper limit of about 1 % for the admixture of the confined bag in the deuteron is found for the QCB potential supplied by the long range Paris interaction. The six-quark bag corrections to the static multipole moments of the deuteron are estimated to be ?1 % forμ _d and ?6% forQ _d .     

Vacuum condensates in the bag model

The consequences of having quark and gluon condensates inside a MIT bag are investigated. We show that one naturally expects a state dependent bag constant and a colour-magnetic interaction term ～R². This gives the possibility of having a small strong coupling constant α_s inside the bag.     

Bag model of hadrons with massive quarks

We generalize the bag model of hadrons to include massive quarks and investigate in particular the properties of the l = 0 octet and decimet baryons with m_p = m_n = 0, m_λ = 270 MeV (obtained by fitting the decimet). The magnetic moments and weak axial vector/vector transition moments are calculated and compared with experimental data for cases in which data exists. We also speculate on the mechanism by which the remaining degeneracies in the baryons are lifted and on modifications which are necessary to construct a bag model of mesons.     

Excited quark-quark interactions in a bag model

We calculate the colour magnetic and electric fields,B ^a andE ^a , in the static cavity approximation of the MIT bag model. This is done rigorously for all direct and exchange four-currents of quarks in theS _1/2,P _1/2 andP _3/2 bag states. We then find the quark-quark colour interaction for direct and exchange diagrams. The one-pion exchange diagrams due to chiral symmetry are also calculated.     

Radiative transitions of baryons in the MIT bag

Resonance photoproduction is studied via the MIT bag model and comparison is made with a single quark transition SU(6)_θ analysis. Correspondence is found between the two pictures. The bag model predictions are shown to be in good qualitative agreement with experiment; however, specific numerical results are rather model dependent and in general less successful.     

P-Matrix analysis of the dibaryon1 D 2 resonance

We study the relation between intermediate energy nucleon-nucleon scattering and the eigenstates of the bag model using theP-matrix formalism. Data of existing phase-shift analyses are employed to calculate theP-matrix for the coupled¹ D ₂(pp) and⁵ S ₂(NΔ) channels in the energy region above the Δ-isobar production threshold. TheP-matrix calculated for the equivalent hadronic bag radiib=1.4?1.5 fm is shown to have a pole in the mass range 2.31–2.34 GeV in agreement with the MIT bag model prediction of theI=1,J ^P =2⁺ 6-quark state with the mass 2.34 GeV. The hadronic shift of this state is shown to be ≈200 MeV; the dibaryon pole of theS-matrix is located at the energy 2.15–2.17 GeV with the width ≈100–200 MeV.     

Multiquark clusters in nuclei. distributions ofu- andd-quarks

Effects related to the possible admixture of multiquark (mainly 12q) clusters (bags) in nuclei are discussed. In particular the differencies in theu- andd-quark distributions inN≠Znuclei are considered. The quark distribution function is assumed to consist of two components: the nucleon component and the bag one (mainly the 12q-bag). The yields ofπ-mesons and nucleons into the backward hemisphere are governed mainly by the contribution of the bag component. It comes from the available data on theπ ⁺/π ^? andn/p ratios that thed-quarks in the bag component of nuclei withN>Z are “more soft” than theu-quarks in contrast to the situation in the nucleon component. The manifestation of the latter effect in theA-dependence of the ratio of structure functionsR=F _2A(x)/F _2D(x) is also discussed.     

Meson properties from a bag model compared to lattice theory and heavy ion experiments

A bag at temperature (T) with pressureB(T)=B(0)[1?(T/T _c )⁴] is shown to be consistent with recent lattice data on the π and the ρ mesons. The limiting temperature,T _l , of the pion bag from the Bekenstein entropy bound is lower than that of other mesons. This agrees with the thermal distribution of π,K and the ρ in heavy ion collisions, which (unlike proton-nucleus or pp data) show a marked difference inT of pion and other mesons in the mid-rapidity region.     

One-gluon-exchange effects on semileptonic decay of baryon octet

We study one-gluon-exchange (OGE) effects ong _A /g _V of weak semileptonic decay processes of theSU(3) baryon octet. We especially investigate the systematic behavior of OGE effects on all the possible decay processes and search for better overall agreements between theory and experiment. We adopt the MIT bag model to describe quarks confined in a baryon. We also use a formulation of the perturbative QCD in which gluon field is also confined in the bag. Our calculations respect the boundary conditions which achieve the confinement of each field within the bag. The magnitudes of our corrections tog _A /g _V have a variation of 1 ～30% depending on the decay, processes. They have process-dependence quite different from that of the uncorrected values ofg _A /g _V primarily governed by theSU(6) symmetry. These OGE effects ong _A /g _V reflectSU(6) asymmetric spin-spin correlation between quarks due to the color magnetic interactions. Especially, the OGE corrections tog _A /g _V are very important in the processes, \(\Sigma ^ - \to n + l^ - + \bar v_l ,\Sigma ^ - \to p + l^ - + \bar v_l \) and \(\Xi ^ - \to \Xi ^0 + e^ - + \bar v_e \) . Consequently, we clarify that the OGE effects can sizably improve theoretical values ofg _A /g _V in favor of overall agreements with the present experimental data of semileptonic decays of the baryon octet, at least, in the MIT bag model.     

fπΔΔ and the cloudy bag model

The cloudy bag model predicts f_πΔΔ a factor of two greater than the experimental results deduced by Arndt et al. Possible explanations are explored.     

A new classical solution of the Yang-Mills field equations

The MIT bag theory is used to calculate the quarl anti-quark annihilation amplitude for the ??γ transition and produces m_?²/∫_? = 0.13 compared to the experimental result of 0.11.     

The proton spin in the chiral bag model: Casimir contributions and Cheshire Cat Principle

The flavor singlet axial charge has been a source of study in the last years due to its relation to the so-called Proton Spin Problem. The relevant flavor singlet axial current is anomalous, i.e., its divergence contains a piece which is the celebrated U_A(1) anomaly. This anomaly is intimately associated with the η′ meson, which gets its mass from it. When the gauge degrees of freedom of QCD are confined within a volume as is presently understood, the U_A(1) anomaly is known to induce color anomaly leading to “leakage” of the color out of the confined volume (or bag). For consistency of the theory, this anomaly should be canceled by a boundary term. This “color boundary term” inherits part or most of the dynamics of the volume (i.e., QCD). In this paper, we exploit this mapping of the volume to the surface via the color boundary condition to perform a complete analysis of the flavor singlet axial charge in the chiral bag model using the Cheshire Cat Principle. This enables us to obtain the hitherto missing piece in the axial charge associated with the gluon Casimir energies. The result is that the flavor singlet axial charge is small independent of the confinement (bag) size ranging from the skyrmion picture to the MIT bag picture, thereby confirming the (albeit approximate) Cheshire Cat phenomenon.     

Collective rearrangement of spin excitation spectrum of antiferromagnet with magnetic impurities in external magnetic field

Antiferromagnetic resonance (AFMR) in Co_1?cMn_cF₂ (c ～ 10^-4) in the external magnetic field is investigated. It was established that coherent and incoherent rearrangements (CR and ICR) of spectrum in the region of AFMR and impurity lines crossing are possible in the same sample depending on field orientation. Topology of the phase diagram of spin excitation (SE) spectrum in variables c-H is obtained.     

The ABC effect and chiral-bag-model form factors

We investigate the ABC effect in the reaction n+p → d+(ππ)⁰ using the chiral lagrangian of Weinberg in the tree approximation. Chiral-bag-model form factors are used for the vertices. We obtain cross sections as a function of energy for deuteron scattering angles θ_d = 0° and 10.5°. The results are compared with the latest experimental measurements. The fit to the absolute value of the cross section is obtained taking the bag radius R as a free parameter; the fitted value of R turns out to be that expected in the light of previous studies in the framework of chiral bag models. The ABC effect is almost completely accounted for in terms of the lowest-order diagrams.     

Large Nc in nonperturbative QCD

We comment on large N_c within the context of nonperturbative QCD. In particular, we consider the QCD-based bag model, the instanton gas approximation, and the η′ mass.     

Phenomenological sizes of confinement regions in baryons

Standard order of magnitude estimates from QCD indicate that the radius of the quarkgluon core in the nucléon is Λ _QCD ^?1 ?1 fm. However, in work with the chiral bag model, we have found that the effective confinement size for low energy reactions can be as small as ～ 1/2 fm or smaller. This shrinking of the effective confinement size has been attributed to the pressure of the pion cloud surrounding the quark core. The concept of confinement size is evidently subtle in light-quark systems, due to the chiral vacuum structure. This is indicated by the “Cheshire Cat” phenomenon, in which physical observables tend to be insensitive to the bag radiusR. In four dimensions, no exact Cheshire Cat principle has yet been established but it is likely to involve infinitely many mesons. We suggest that when strange quarks are present, a qualitative change occurs in the Cheshire Cat picture; in particular, we propose that strangeness provides an obstruction to this picture. We present a phenomenological indication that when strange quarks are present, the bag radiusR is frozen at a value substantially larger than 0.5 fm by as much as a factor of two. Roughly speaking, the Cheshire Cat picture emerges from a near cancellation between repulsive quark kinetic and attractive pion-cloud energies in the case of the nucleon. In theΛ andΣ particles, however, replacement of one up or down quark by a strange quark removes ～ 1/N_c of the attraction from the coupling of the quarks to the pion cloud. This upsets the balance needed for the Cheshire Cat phenomenon and makes larger strange baryons more favorable energetically than the 0.5 fm ones appropriate for pureu- andd-systems. Since the above argument is crude, we appeal strongly to phenomenology. We find that magnetic moments of strange baryons favor a bag radius R?1.1 fm. We find that the excited states of theΛ-hyperons favor similarly large bag radii. Somewhat less convincingly, we argue that — due to perturbative effects — the bag radius appropriate to the Δ(1232) lies intermediate between that of the nucleon and of the strange baryons.     

Infrared absorption on impurity excitations near the upper edge of spin-wave band of the antiferromagnet

The dependence of infrared (IR) light absorption on external magnetic field is considered when the impurity level approaches the upper edge of spin-wave band of antiferromagnet. It is explained a strongly nonlinear field dependence and rapid decrease of corresponding line intensity been observed experimentally in Fe_1?cCo_cF₂. Such behaviour is due to existence of some subthreshold range near the band edge and corresponds to the incoherent collective rearrangement (ICR) of the system spectrum.     

Instanton effects on the interaction potential between light quarks and the isomultiplet mass splitting of baryons

On the basis of current assumptions of instanton theory we derive strictly the explicit dependence on the masses and spins of the instanton induced potential between a pair of light quarks in baryons, namelyV ₁₂=γ+β(m _* ^1+α )(m _* ^2+α )η(1-σ ₁ησ ₁), wherem _i ^* andσ _i (i=1.2) are respectively the mass and Pauli spin of theith quark. On the additional basis of the MIT bag model, we obtain γ=c/R ³ and β=b/R ³>0, and α>0 is independent of the radiusR of the baryon. The magnitudes of the parametersb and α are also estimated. The MIT bag model is improved by taking into account this potential. Isomultiplet mass splitting formulas are derived in good agreement with experiment.