Showing papers on "Pion published in 1985"

Neutron Stars Are Giant Hypernuclei

Abstract: Neutron stars are studied in the framework of Lagrangian field theory of interacting nucleons, hyperons, and mesons, which is solved in the mean field approximation The theory is constrained to account for the four bulk properties of nuclear matter; the saturation binding and density, compressibility, and charge symmetry energy The cores of the heavier neutron stars are found to be dominated by hyperons, and the total hyperon population for such stars is 15%--20%, depending on whether pions condense or not The rho-meson, which contributes to the isospin symmetry energy, has an important influence on the baryon populations Lepton populations are strongly suppressed and charge neutrality is achieved among the hadrons A possible consequence for the decay time of the magnetic field of pulsars and hence for their active lifetime is mentioned

Microscopic theory of pion production and sidewards flow in heavy-ion collisions.

Abstract: Nuclear collisions from 0.3 to 2 GeV/nucleon are studied in a microscopic theory based on Vlasov's self-consistent mean field and Uehling-Uhlenbeck's two-body collision term which respects the Pauli principle. The theory explains simultaneously the observed collective flow and the pion multiplicity and gives their dependence on the nuclear equation of state.

Ultrahigh-energy cosmic-ray spectrum

Abstract: We analyze the evolution of the ultrahigh-energy cosmic-ray spectrum upon traversing the 2.7 \ifmmode^\circ\else\textdegree\fi{}K microwave background with respect to pion photoproduction, pair-production reactions, and cosmological effects. Our approach employs exact transport equations which manifestly conserve nucleon number and embody the laboratory details of these reactions. A spectrum enhancement appears around 6\ifmmode\times\else\texttimes\fi{}${10}^{19}$ eV due to the ``pile-up'' of energy-degraded nucleons, and a ``dip'' occurs around ${10}^{19}$ eV due to combined effects. Both of these features appear in the observational spectrum. We analyze the resulting neutrino spectrum and the effects of cosmological source distributions. We present a complete model of the ultrahigh-energy spectrum and anisotropy in reasonable agreement with observation and which predicts an observable electron-neutrino spectrum.

Application of chiral perturbation theory to K→2π decays

Abstract: Chiral perturbation theory is applied to the decay K→2π. It is shown that, to quadratic order in meson masses, the amplitude for K→2π can be written in terms of the unphysical amplitudes K→π and K→0, where 0 is the vacuum. One may then hope to calculate these two simpler amplitudes with lattice Monte Carlo techniques, and thereby gain understanding of the ΔI=1/2 rule in K decay. The reason for the presence of the K→0 amplitude is explained: it serves to cancel off unwanted renormalization contributions to K→π. We make a rough test of the practicability of these ideas in Monte Carlo studies. We also describe a method for evaluating meson decay constants which does not require a determination of the quark masses.

Quark model of light mesons with dynamically broken chiral symmetry.

Abstract: We study the meson spectrum in a model with a confining Lorentz-vector: and hence chiral-invariant: interaction between massless quark fields. As shown in a previous work, chiral invariance is spontaneously broken. In the case of the harmonic oscillator, as the Fourier transform of the potential is the Laplacian of a delta function, the Bethe-Salpeter (BS) equation: a system of linear integral equations in general: splits into a system of differential equations that we solve in the broken vacuum. Without appealing to any spin-spin interaction, we find, besides the massless pseudoscalar, a vector meson at the right scale and an excited pion and two vectors in the 1--2-GeV region. Moreover, we find a large L-S splitting with the expected ordering for a vector interaction. We study in detail the BS wave function for the pion in motion, necessary to compute axial-vector-current matrix elements, and recover well known relations of current algebra. We compute f/sub ..pi../ and find on general grounds that f/sub piprime/ = 0 in the chiral limit, where ..pi..' is any radially excited pion. The pion satisfies the expected dispersion law for a Goldstone boson, ..omega..(p)..-->..cp (p..-->..0). .AE

Experimental estimation of the sum of pion electrical and magnetic polarizabilities

Abstract: The pion radiative scattering πZ→πZγ at 40 GeV was investigated. Coulomb scattering-i.e. pion Compton-effect-dominates for small fourmomentum transfers in this reaction. In our previous paper we found the pion polarizability to beβ π =(−6.8±1.4)·10−43, cm3, where we assume following the theory that the sum of electrical and magnetic polarizabilities (α π +β π )≃0. More complete analysis of our data is done in order to test this assumption. The result for (α π +β π ) is in agreement with theoretical predictions.

On one pion exchange potential with quark exchange in the resonating group method

Abstract: The effect of quark exchange between different nucleons on the one pion exchange potential is studied in the framework of the resonating group method. The calculated phase shifts including the one pion exchange potential with quark exchange in addition to the one gluon plusσ meson exchange are shown to be consistent with experiments. Especially thep-wave phase shifts are improved by taking into account the quark exchange on the one pion exchange potential.

Inclusive hadron production in upsilon decays and in nonresonant electron-positron annihilation at 10.49 GeV.

Abstract: We report measurements of single-particle inclusive spectra and two-particle correlations in decays of the UPSILON(1S) resonance and in nonresonant annihilations of electrons and positrons at center-of-mass energy 10.49 GeV, just below BB-bar threshold. These data were obtained using the CLEO detector at the Cornell Electron Storage Ring (CESR) and provide information on the production of ..pi.., K, rho, K(, phi, p, ..lambda.., and ..xi.. in quark and gluon jets. The average multiplicity of hadrons per event for upsilon decays (compared with continuum annihilations) is 11.4 (10.5) pions, 2.4 (2.2) kaons, 0.6 (0.5) rho/sup 0/, 1.2 (0.8) K(, 0.6 (0.4) protons and antiprotons, 0.15 (0.08) phi, 0.19 (0.07) ..lambda.. and Lambda-bar, and 0.016 (0.005) ..xi../sup -/ and Xi-bar /sup +/. We have also seen evidence for eta and f/sup 0/ production. The most significant differences between upsilon and continuum final states are (1) the inclusive energy spectra fall off more rapidly with increasing particle energy in upsilon decays, (2) the production of heavier particles, especially baryons, is not as strongly suppressed in upsilon decays, and (3) baryon and antibaryon are more likely to be correlated at long range in upsilon decay than in continuum events.

Dynamical model for pion photoproduction in the delta region

Abstract: A phenomenological model for pion photoproduction is constructed incorporating the dynamics of pions, nucleons, and ..delta..'s. The importance of nonresonant background interactions for the elastic ..pi..N scattering and pion photoproduction is emphasized. The photoproduction amplitudes calculated in our model satisfy two-body unitarity, so that the requirement imposed by Watson's theorem is automatically fulfilled. By fitting the amplitudes to data, M1 and E2 ..gamma..N..--> delta.. transition amplitudes are estimated, eliminating background contributions. The results are A/sub 1/2/(M1) = (-84 +- 5) x 10/sup -3/ GeV/sup -1/2/ and E2/M1 = (3.7 +- 0.4) %. Our M1 amplitude disentangled from the ..pi..N rescattering term is in good agreement with the quark model predictions. The sign and magnitude of our E2 amplitude are, however, incompatible with the existing quark models.

Bose Einstein symmetrisation, coherence and chaos; with particular application toe + e − annihilation

Abstract: The phenomenon of Bose-Einstein correlation within the debris of hadronic final states is discussed in simple terms, with particular attention to the nature of coherent and chaotic sources. The Artru-Mennessier string is taken as a model for the pion source ine+e− annihilation and the predicted Bose-Einstein correlations between like pion pairs evaluated. The results are in good accord with such data as exist.

Study of Bose-Einstein correlations in e + e - annihilation at 29 GeV

Abstract: Bose-Einstein correlations between like-sign pions have been investigated in e/sup +/e/sup -/ annihilation at ..sqrt..s = 29 GeV using the Time Projection Chamber detector at the SLAC e/sup +/e/sup -/ storage ring PEP. The production rate of like-sign pion pairs with small relative momentum is found to be increased by more than 50% over the rate expected for uncorrelated production of pions. From the correlation length, a typical source radius of 0.65 fm is derived. Data are consistent with a spherical shape of the pion source. No dependence of radius or correlation strength on the event multiplicity is observed.

Successes and difficulties of unified quark-antiquark potential models

Abstract: We study the qualitative and quantitative properties of the spectrum of a two-body hamiltonian with relativistic kinematics. We show that this kinematics leads in a natural way to the observed features of light flavour (u, d, s) spectroscopy. After having established the basic properties of the operator (p2+m2)1/2+V(r) in the cases of linear or logarithmic potentials, we show that, to first approximation, all\(q_1 \bar q_2\) meson states can be reproduced with a very simple universal flavour-independent potential whose parameters are directly related to basic physical quantities: the Regge slopes of light flavours and the quasi-logarithmic coupling strength of heavy quarks. We can derive equivalent effective non-relativistic hamiltonians which justify the successes of N.R. approaches. The main difficulties encountered, in particular in incorporating spin effects, appear to be due to the fact that, in phenomenological potential models, chiral symmetry and the ensuing Goldstone nature of the pion cannot be implemented in a natural way. Hence, such an approach can take its full predictive power only if it is based on a deeper field-theoretic level.

Nuclear effects in deep-inelastic lepton scattering

Abstract: We present a detailed study of a conventional nuclear model in which the observed differences between the deep-inelastic structure functions of nuclei and of free nucleons, ${F}_{2}$(x,${Q}^{2}$) and q\ifmmode\bar\else\textasciimacron\fi{}(x,${Q}^{2}$), are due to scattering from exchange pions in nuclei associated with the mechanisms responsible for nuclear binding. We assume that the quark and antiquark momentum distributions of these nucleons and pions are unaffected by the nuclear medium. We write the nuclear structure functions as a sum of convolutions of (measured) isolated-hadron structure functions with hadron momentum distributions derived from nuclear potential models. We show that all but one feature of deep-inelastic neutrino and charged-lepton data are reproduced by our model. The exception is the magnitude of the excess above unity of the ratio ${F}_{2}^{A}$(x)/${F}_{2}^{\mathrm{D}(\mathrm{x})}$ for xl0.2, observed only by the European Muon Collaboration (EMC) experiment. If these EMC data are reduced in normalization by 5%, consistent with experimental uncertainties, then all features of the data are reproduced. We stress the importance of new neutrino and muon measurements at small x.

Systematics of pi n Scattering in Chiral Soliton Models

Abstract: We study the spin and isospin structure of pion-nucleon scattering amplitudes in the class of models, including the Skyrme model, in which the nucleon arises as a soliton built of pion fields. We derive general energy-independent linear relations among partial-wave amplitudes for the processes ..pi..N..--> pi..N and ..pi..N..--> pi delta.. and show that these relations are in reasonably good agreement with experiment.

Multiplicity, momentum and angular characteristics of π - mesons for pC, dC, αC and CC interactions at 4.2 GeV/c per nucleon

Abstract: Light ion collisions with carbon target at 4.2 GeV/c/N are studied. Pion multiplicity distributions, momentum and angular spectra are analysed. These data are described in terms of models assuming independent interactions of nucleons from the projectile nucleus with the target.

Calculations of deeply bound pionic states in heavy and superheavy atoms

Abstract: Binding energies and widths of pions bound in the 1s state of heavy and superheavy atoms are evaluated. The repulsive strong interaction lowers the Coulomb binding energy by up to 50%. The energy width of the strongest bound state investigated becomes comparable with the binding energy. No drastic consequence for pion propagation in nuclear matter is expected.

Tests of models for quark and gluon fragmentation ine+e− annihilation at\(\sqrt s = 29 GeV\)

Abstract: Three currently used fragmentation models are tested by studying the distribution of particles between jet axes in the 3-jet events ofe+e− annihilation, using data collected by the Time Projection Chamber at PEP. These three models — the Lund String model, the Webber Cluster model and an Independent Fragmentation model (IF) — each implement different Lorentz-frame structures for the fragmentation process of quarks and gluons into hadrons. The Lund model provides a good description of the data, while the IF model does not. The Webber model, which is untuned, does not describe the absolute particle densities between jets, but correctly predicts the ratios of those densities, which are less sensitive to the tuning.