Meson-baryon Model Research Articles

Study of the [formula omitted] asymmetry in the proton

The study of s−s¯ asymmetry is essential to better understand of the structure of nucleon and also the perturbative and nonperturbative mechanisms for sea quark generation. Actually, the nature and dynamical origins of this asymmetry have always been an interesting subject to research both experimentally and theoretically. One of the most powerful models can lead to s−s¯ asymmetry is the meson–baryon model (MBM). In this work, using a simplified configuration of this model suggested by Pumplin, we calculate the s−s¯ asymmetry for different values of cutoff parameter Λ, to study the dependence of model to this parameter and also to estimate the theoretical uncertainty imposed on the results due to its uncertainty. Then, we study the evolution of distributions obtained both at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) using different evolution schemes. It is shown that the evolution of the intrinsic quark distributions from a low initial scale, as suggested by Chang and Pang, is not a good choice at NNLO using variable flavor number scheme (VFNS).

Intrinsic strange distributions in the nucleon from the light-cone models

Precise knowledge of the strange and antistrange quark distributions of the nucleon is a major step toward better understanding of the strong interaction and the nucleon structure. Moreover, the $ s-\bar s $ asymmetry in the nucleon plays an important role in some physical processes involving hadrons. The goal of this paper is the study of intrinsic strange contribution to the strange sea of the nucleon. To this aim, we calculate the intrinsic strange distributions from the various light-cone models including BHPS, scalar five-quark and meson-baryon models and then compare their results. These models can lead to the rather different distributions for the intrinsic strange that are dominated in different values of $ x $. Furthermore, the meson-baryon model leads to the $ s-\bar s $ asymmetry that can be comparable in some situations to the result obtained from the global analysis of PDFs. We also present a simple parametrization for each model prediction of intrinsic strange distribution.

Compositeness of the strange, charm, and beauty odd parityΛstates

We study the dependence on the quark mass of the compositeness of the lowest-lying odd parity hyperon states. Thus, we pay attention to $\Lambda-$like states in the strange, charm and beauty, sectors which are dynamically generated using a unitarized meson-baryon model. In the strange sector we use an SU(6) extension of the Weinberg-Tomozawa meson-baryon interaction, and we further implement the heavy-quark spin symmetry to construct the meson-baryon interaction when charmed or beauty hadrons are involved. In the three examined flavor sectors, we obtain two $J^P=1/2^-$ and one $J^P=3/2^-$ $\Lambda$ states. We find that the $\Lambda$ states which are bound states (the three $\Lambda_b$) or narrow resonances (one $\Lambda(1405)$ and one $\Lambda_c(2595)$) are well described as molecular states composed of $s$-wave meson-baryon pairs. The $\frac{1}{2}^-$ wide $\Lambda(1405)$ and $\Lambda_c(2595)$ as well as the $\frac{3}{2}^-$ $\Lambda(1520)$ and $\Lambda_c(2625)$ states display smaller compositeness and so they would require new mechanisms, such as $d$-wave interactions.

ψ(3770)resonance and its production inp¯p→DD¯

The production of a $D\bar D$ meson-pair in antiproton-proton ($\bar p p$) annihilation close to the production threshold is investigated, with special emphasis on the role played by the $\psi$(3770) resonance. The study is performed in a meson-baryon model where the elementary charm production process is described by baryon exchange. Effects of the interactions in the initial and final states are taken into account rigorously, where the latter involves also those due to the $\psi$(3770). The predictions for the $D\bar D$ production cross section are in the range of 30 -- 250 nb, the contribution from the $\psi$(3770) resonance itself amounts to roughly 20 -- 80 nb.

Phenomenology of nonperturbative charm in the nucleon

We perform a comprehensive analysis of the role of nonperturbative (or intrinsic) charm in the nucleon, generated through Fock state expansions of the nucleon wave function involving five-quark virtual states represented by charmed mesons and baryons. We consider contributions from a variety of charmed meson-baryon states and find surprisingly dominant effects from the ${\overline{D}}^{*0}{\mathrm{\ensuremath{\Lambda}}}_{c}^{+}$ configuration. Particular attention is paid to the existence and persistence of high-$x$ structure for intrinsic charm, and the $x$ dependence of the $c\ensuremath{-}\overline{c}$ asymmetry predicted in meson-baryon models. We discuss how studies of charmed baryons and mesons in hadronic reactions can be used to constrain models, and outline future measurements that could further illuminate the intrinsic charm component of the nucleon.

Nonperturbative charm content of the nucleon

We propose a new meson-baryon model for intrinsic charm in the proton. Particular attention is paid to the existence and persistence of high x behavior, especially in the structure function F c¯ c 2 (x, Q 2 ).

Polarization observables in deuteron photodisintegration below 360 MeV

High precision measurements of induced and transferred recoil proton polarization in d(γ→,p→)n have been performed for photon energies of 277–357 MeV and θcm=20°–120°. The measurements were motivated by a longstanding discrepancy between meson–baryon model calculations and data at higher energies. At the low energies of this experiment, theory continues to fail to reproduce the data, indicating that either something is missing in the calculations and/or there is a problem with the accuracy of the nucleon–nucleon potential being used.

Structure of Meson-Baryon Interaction Vertices

We present a microscopic derivation of the form factors of strong-interaction piNN and piNDelta vertices within a relativistic constituent quark model. The results are compared with form factors from phenomenological meson-baryon models and recent lattice QCD calculations. We give an analytical representation of the vertex form factors suitable for applications in further studies of hadron reactions.

Contribution of asymmetric strange–antistrange sea to the Paschos–Wolfenstein relation

The NuTeV Collaboration reported a value of sin2θw measured in neutrino–nucleon deep inelastic scattering, and found that the value is three standard deviations from the standard model prediction. This result is obtained under the assumption that the strange quark–antiquark sea of nucleons are symmetric, and that the up and down quark distributions are symmetric with the simultaneous interchange of u↔d and p↔n. We discuss the contribution of asymmetric strange–antistrange sea to the Paschos–Wolfenstein relation in the extraction of weak mixing angle sin2θw. We also point out that the contribution of asymmetric strange–antistrange sea should remove roughly 30–80% of the discrepancy between the NuTeV result and other determinations of sin2θw when using the light-cone meson–baryon model to calculate the contribution of the strange–antistrange sea.

Polarization measurements in high-energy deuteron photodisintegration.

We present measurements of the recoil proton polarization for the d(gamma-->,p-->)n reaction at straight theta(c.m.) = 90 degrees for photon energies up to 2.4 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. The induced polarization p(y) vanishes above 1 GeV, contrary to meson-baryon model expectations, in which resonances lead to large polarizations. However, the polarization transfer Cx does not vanish above 1 GeV, inconsistent with hadron helicity conservation. Thus, we show that the scaling behavior observed in the d(gamma,p)n cross sections is not a result of perturbative QCD. These data should provide important tests of new nonperturbative calculations in the intermediate energy regime.

Consistent meson-field-theoretical description ofppbremsstrahlung

A parameter-free and relativistic extension of the RuhrPot meson-baryon model is used to define the dominant isoscalar meson-exchange currents. We compute pp-bremsstrahlung observables below the \ensuremath{\pi}-production threshold using a relativistic hadronic current density that includes impulse, wave function reorthonormalization, meson-recoil, N\ifmmode\bar\else\textasciimacron\fi{}N creation and annihilation, \ensuremath{\rho}\ensuremath{\pi}\ensuremath{\gamma} + \ensuremath{\omega}\ensuremath{\pi}\ensuremath{\gamma} + \ensuremath{\rho}\ensuremath{\eta}\ensuremath{\gamma} + \ensuremath{\omega}\ensuremath{\eta}\ensuremath{\gamma} vector-meson decay, and N\ensuremath{\Delta}\ensuremath{\gamma}(\ensuremath{\pi},\ensuremath{\rho}) exchange currents. We obtain a good description of the available data. The N\ensuremath{\Delta}\ensuremath{\gamma}(\ensuremath{\pi}) current is shown to dominate the large two-body contributions and closed-form expressions for various nonrelativistic approximations are analyzed. An experimental sensitivity to the admixture of pseudoscalar and pseudovector admixture of the NN\ensuremath{\pi} interaction is demonstrated. We examine the Lorentz invariance of the NN\ensuremath{\rightleftarrows}NN t matrices and show a dominantly pseudovector NN\ensuremath{\pi} coupling renders impulse approximation calculations without boost operators to be essentially exact. Conversely, a similar analysis of the \ensuremath{\Delta}N\ensuremath{\rightleftarrows}NN transitions shows that boost operators and the two-body N\ensuremath{\Delta}\ensuremath{\gamma} wave function reorthonormalization meson-recoil currents are required in NN, \ensuremath{\Delta}N, and \ensuremath{\Delta}\ensuremath{\Delta} coupled channel t-matrix applications. The need for additional data is stressed. \textcopyright{} 1996 The American Physical Society.

Hadron production from quark-gluon plasma via mixed phase

Well-controlled hadronization of the quark-gluon plasma via a mixed phase is described by a set of rate equations supplemented with the one-dimensional scaling hydrodynamics. The reaction rates for the hadronization processes are constrained by the equilibrium condition for the static mixed phase using the fractional volume formalism. As an example, two models of hadronization of baryon-free quark-gluon plasma are constructed according to different choices of the constituents of the free hadron gas: the meson-baryon model and the pseudoscalar-vector model. Time evolution of the flavor composition and hadron multiplicity is studied in detail. It was found that (i) the abundance of strange hadrons at the final stage of the phase transition is insensitive to the strangeness abundance in the initial stage, (ii) the K π ratio in the final stage is essentially given by the equilibrium value for the hadronic phase, (iii) the baryon/meson ratio is sensitive to the transition temperature, (iv) the entropy is approximately conserved or increases slightly, and (v) the period of the mixed phase era depends on both the critical temperature and the effective number of hadronic degrees of freedom. The time-integrated ratio of the vector meson yields 〈 φ>/<ρ 0 + ω〉 is found to be twice as large as the maximum enhancement observed by the NA38 experiment on muon pair production. It is suggested that suppression of resonance production relative to direct pseudoscalar meson production may serve as a signature of thermal equilibrium in the hadronic phase.

Bootstrap Conditions from the Veneziano Representation

A set of self-consistency conditions for ratios of hadron-hadron-hadron coupling constants is derived from the Veneziano model for scattering amplitudes and some simple assumptions. This set includes the crossing-matrix condition of the static meson-baryon model and the condition that the absence of resonances in exotic states implies a cancellation from Regge trajectories of opposite signatures. For simplicity, the spins of the external mesons and baryons are neglected. The conditions require that the particles correspond to representations of a Lie group, and that only certain sets of representations are allowed. The relation to previous bootstrap models is discussed. An illustrative solution corresponding to $\mathrm{SU}(3)$ is obtained.

Duality functions in a meson-baryon model

Duality functions, i.e. constraints on the integration variables in a multichannel representation of the Veneziano form are developed for the multiparticle amplitude describing the emission (or absorption) of n elementary mesons by a baryon with an isobaric spectrum. The sequence of graphs related by duality consists of all distinct permutations of the order of absorption of the mesons by the baryon, and the resulting amplitude is expressible in terms of the resonance poles associated with any one of them. This is the generalisation of a model proposed by Burnett and Schwarz. This model has also recently been generalized in a completely different direction by Mandelstam.

SU 3 meson-baryon model with small symmetry-breaking interaction

It is shown by means of second-order calculations that the SU 3 model with small symmetry-breaking interaction leads to the difficulty of a negative intrinsic mass for baryons. It is concluded that Okubo's derivation of the mass formulae cannot be regarded as satisfactory unless it is established that the higher-order effects are able to remove this difficulty.