Showing papers on "Meson published in 1989"

The Meson Theory of Nuclear Forces and Nuclear Structure

Abstract: Nowadays it has become customary in nuclear physics to denote by “tradition” the approach that considers nucleons and mesons as the relevant degrees of freedom. It is the purpose of this chapter to review this “traditional” approach in the area of nuclear forces and their applications to nuclear structure.

The relativistic mean-field description of nuclei and nuclear dynamics

Abstract: The relativistic mean-field model of the nucleus is reviewed. It describes the nucleus as a system of Dirac nucleons which interact in a relativistic covariant manner via meson fields. The meson fields are treated as mean fields, i.e. as non-quantal c-number fields. The effect of the Dirac sea of the nucleons is neglected. The model is interpreted as a phenomenological ansatz providing a self-consistent relativistic description of nuclei and nuclear dynamics. It is viewed, so to say, as the relativistic generalisation of the Skyrme-Hartree-Fock ansatz. The capability and the limitations of the model to describe nuclear properties are discussed. Recent applications to spherical and deformed nuclei and to nuclear dynamics are presented.

Semileptonic B and D Decays in the Quark Model

Abstract: We predict the matrix elements and resulting electron spectra for semileptonic meson decays using the quark potential model. Particular attention is paid to the high-energy electron end-point region in B decay since it is crucial to a determination of the b→u weak mixing angle. It is argued that in this region the usual inclusive ("quark decay") calculations are unjustified and must be replaced by explicit sums over decays of the original meson into low-mass exclusive hadronic final states.

Experimental study of muon pairs produced by 252-GeV pions on tungsten

Abstract: We present the results of a study of muon pairs with invariant masses greater than 4.05 GeV/${c}^{2}$ produced in high-energy pion-nucleon interactions. The production cross section together with the inferred pion and nucleon structure functions are reported and compared with other experiments and with QCD predictions. The transverse-momentum distributions are also presented. Finally, the full angular distribution in cos\ensuremath{\theta} and \ensuremath{\varphi} is given as a function of mass, Feynman x, and transverse momentum. Longitudinal photon polarization is seen in the lower portion of the mass range at high ${x}_{\ensuremath{\pi}}$. This result is compared with a higher-twist model.

QCD Sum Rules in Exclusive Kinematics and Pion Wave Function

Abstract: We suggest a modification of the QCD sum rules for three-point correlation functions that relates hadron expectation values of an operator of interest to properties of the QCD vacuum in alternating external fields. A new sum rule is obtained for the nucleon magnetic moments. Relations are found between the couplings gπNN, gρωπ and the value of the pion wave function at the point with equal momentum carried by the quark and the antiquark. Our results seem to exclude the possibility of having a pronounced dip in the pion wave function in the middle point, as has been assumed on the evidence of a large value of the second moment.

Heavy quark physics

Abstract: Various aspects of the physics of heavy quarks and of CP violation are reviewed.

Exclusive Processes in Quantum Chromodynamics

Abstract: Large momentum transfer exclusive processes and the short distance structure of hadronic wave functions can be systematically analyzed within the context of perturbative QCD. We review predictions of meson form factors, two‐photon processes γγ → MM, hadronic decays of heavy quark systems, and a number of other related QCD phenomena.

Consistent three-nucleon forces in the nuclear many-body problem

Abstract: Describing an assembly of an infinite number of nucleons in interaction via a two-body potential as a nonrelativistic many-body problem in the first place, we envisage corrections to this picture due to suppressed degrees of freedom at the level of the two-body potential At variance with relativistic many-body theory, the solution of the nonrelativistic problem with a two-body potential only is sufficiently under control at present so that evaluating corrections in this framework is of particular interest These corrections come primarily from additional three-body forces either due to finite-density effects (Pauli blocking of fermions) or are of genuine origin: relativistic dynamical processes and effects from the intrinsic structure of the nucleon Recalling the successful treatment of electromagnetic interactions in nuclei in terms of meson-exchange currents, we establish novel consistency requirements between the initial two-body force and the well-identified residual three-body force In this way no new parameters enter in the three-body force, save for the controversial mass of the fictitious scalar ``\ensuremath{\sigma}'' meson We show further that the nucleon-antinucleon pair term required in the analysis of meson-exchange currents has a genuine three-body counterpart resulting from time-ordered diagrams containing a single Z branch Its contribution to the energy per particle is repulsive and varies with a high power of the density Thereby we obtain the important saturating effect present in relativistic mean-field approaches We envisage next the role of the first radial nucleon resonance ${N}^{\mathrm{*}}$((1/2,1)/2) (Roper resonance) in inducing a specific three-body force The meson-nucleon-Roper coupling constants and form factors are evaluated in a relativistic quark model Gathering all self-consistent corrections to the binding energy per particle of infinitely many nucleons, we find that the final equation of state is solely governed by the density dependence of medium corrections to the free \ensuremath{\sigma}-meson mass We discuss a first attempt to extract this density dependence from an empirical equation of state

SU(3) predictions for nonleptonic B-meson decays

Abstract: The smallness of the up-, down-, and strange-quark masses compared with the QCD scale makes SU(3) flavor an approximate symmetry of the strong interactions. The B-, B0, and Bs0 mesons form a 3¯ representation of SU(3). Using the SU(3) transformation properties of the effective Hamiltonian for weak nonleptonic B-meson decays, relations are derived between B-meson decay amplitudes. Some of these relations may provide information on the importance of various competing effects that can occur in nonleptonic B-meson decays.

CP violation in seesaw models of quark masses.

Abstract: CP phenomenology in ``seesaw'' models of quark masses is shown to parallel that of the usual left-right-symmetric models with the additional advantage that it provides a natural solution to the strong CP problem. For the case where the third-generation mixing parameter ${V}_{\mathrm{ub}}$ is extremely small, the neutral-Higgs-boson interactions lead to \ensuremath{\epsilon}'/\ensuremath{\epsilon}\ensuremath{\simeq}${10}^{\mathrm{\ensuremath{-}}3}$ and the electric dipole moment of the neutron ${d}_{n}$\ensuremath{\simeq}${10}^{\mathrm{\ensuremath{-}}25}$ e cm. Smallness of the neutrino masses is understood as a two-loop effect.

K/ pi ratios in relativistic nuclear collisions: A signature for the quark-gluon plasma?

Abstract: Collisions of Si(14.5{ital A} GeV)+Au are investigated in the relativistic-quantum-molecular-dynamics approach. The calculated pseudorapidity distributions for central collisions compare well with recent experimental data, indicating a large degree of nuclear stopping and thermalization. Nevertheless, nonequilibrium effects play an important role in such complex multihadron reactions: They lead to a strong enhancement of the total kaon production cross sections, in good agreement with the experimental data, without requiring the formation of a deconfined quark-gluon plasma.

Formfactor effects in exclusiveD andB decays

Abstract: Recent experimental results on the semileptonicD→K * transition seem to be in conflict with quark model expectations Motivated by this finding we reinvestigate the predictions for exclusiveD andB decays in the relativistic quark model approach Some of the invariant formfactors relevant for the transition matrix elementsD→K * andB→D * depend strongly on an explicit quarkmass-dependent integral over the meson wave functions The dependence of decay rates and spectra in semi-leptonicD andB transitions on this integral is analysed and discussed in detail Furthermore, we discuss how the predictions of the relativistic quark model for semi-leptonicD andB decays can be tested through measurements of the polarization of the produced vector mesonK * andD *, respectively Some remarks on exclusive nonleptonic two-body decays of the heavy mesons are also presented Finally the theoretical uncertainties for the determination of the K-M matrix element |V ub | from exclusive semi-leptonic decays are discussed

Angular correlations in the decay B' VV and CP violation

Abstract: We study the exclusive decays of B mesons into a pair of spin-1 mesons. We look at the occurring asymmetries that could signal CP violation and estimate their relative size. We find that angular asymmetries are not significantly smaller than partial-rate asymmetries, and that their study can help disentangle the complicated dynamics of these decays.

Prompt leptons in cosmic rays.

Abstract: Energy spectra and zenith-angle distributions of cosmic-ray muons, neutrinos and antineutrinos of prompt generation for energy interval (1÷106) TeV are calculated. For calculations of differential cross-sections of D±, D0,\(\bar D\)0 and Λc production inNN and πN interactions the recombination quark-parton model (RQPM) is used. Accounting of nuclear effects is done by using the additive quark model and the optical model of nucleus. Detailed comparison of results obtained in RQPM with corresponding predictions of quark-gluon string model (MQGS) is carried out. Dynamics of semi-leptonic D- and Λc and energy losses of muons in the atmosphere are taken into account. Calculations of hadronic cascades in the atmosphere are done with accounting of growth with energy of total inelastic hadron-nucleus cross-sections, steepening of primary cosmic-ray spectrum and processes of pion regeneration. The comparison of our calculations with experimental data and with calculations of other authors is given.

Relativistic Hartree calculations of odd-A nuclei

Abstract: Ground-state properties of odd-{ital A} nuclei near closed shells are calculated using self-consistent, relativistic mean-field models of baryon-meson dynamics Contributions from {sigma}, {omega}, and {rho} mesons and the photon are included Comparisons are made between linear and nonlinear mean-field models, and with calculations including vacuum polarization in a local density approximation Results are given for (intrinsic) binding energies, rms radii, magnetic and quadrupole moments, currents, and elastic magnetic scattering form factors, with comparisons to other calculations and to experiment Bulk systematics are well reproduced by the nonlinear model and, as expected, isoscalar magnetic moments in light nuclei are close to Schmidt predictions for all models At higher momentum transfer, currents are enhanced compared to nonrelativistic single-particle predictions, in disagreement with experiment Extensions to include pions and derivative corrections to the vacuum polarization current are discussed

Collective modes in hot and dense matter

Abstract: We propose a model for a relativistic many-body system at finite temperature in the framework of thermo field dynamics, which is a real-time formalism of finite-temperature field theory. Our model contains the scalar (\ensuremath{\sigma}) and the vector (\ensuremath{\omega}) mesons as well as the Dirac nucleon. The full propagator and self-energy for each particle are presented in terms of spectral representations. The Feynman rules for a perturbation expansion are shown. They are applied to the study of collective modes in hot and dense matter within the random-phase approximation. The dispersion relations of the longitudinal and transverse collective modes in the meson branch are calculated. We also estimate the effective meson mass which is defined as the energy needed to create one meson at rest in extreme matter. The effects of vacuum fluctuations are also examined. They contribute a fair amount to the collective modes through the effective nucleon mass.