10,774 research outputs found
Potential surfaces for O atom-polymer reactions
Ab initio quantum chemistry methods are used to study the energetics of interactions of O atoms with organic compounds. Polyethylene (CH2)n has been chosen as the model system to study the interactions of O(3P) and O(1D) atoms with polymers. In particular, H abstraction is investigated and polyethylene is represented by a C3 (propane) oligomeric model. The gradient method, as implemented in the GRADSCF package of programs, is used to determine the geometries and energies of products and reactants. The saddle point, barrier geometry is determined by minimizing the squares of the gradients of the potential with respect to the internal coordinates. To correctly describe the change in bonding during the reaction at least a two configuration MCSCF (multiconfiguration self consistent field) or GVB (generalized valence bond) wave function has to be used. Basis sets include standard Pople and Dunning sets, however, increased with polarization functions and diffuse p functions on both the C and O atoms. The latter is important due to the O(-) character of the wave function at the saddle point and products. Normal modes and vibrational energy levels are given for the reactants, saddle points and products. Finally, quantitative energetics are obtained by implementing a small CAS (complete active space) approach followed by limited configuration interaction (CI) calculations. Comparisons are made with available experimental data
Novel Quark Fragmentation Functions and the Nucleon's Transversity Distribution
We define twist-two and twist-three quark fragmentation functions in Quantum
Chromodynamics (QCD) and study their physical implications. Using this
formalism we show how the nucleon's transversity distribution can be measured
in single pion inclusive electroproduction.Comment: 10 pages, uses PHYZZX macro package, 2 PostScript figures (added
using FIGURES). MIT-CTP-215
Chiral-Odd and Spin-Dependent Quark Fragmentation Functions and their Applications
We define a number of quark fragmentation functions for spin-0, -1/2 and -1
hadrons, and classify them according to their twist, spin and chirality. As an
example of their applications, we use them to analyze semi-inclusive
deep-inelastic scattering on a transversely polarized nucleon.Comment: 19 pages in Plain TeX, MIT CTP #221
Color Non-Singlet Spectroscopy
Study of the spectrum and structure of color non-singlet combinations of
quarks and antiquarks, neutralized by a non-dynamical compensating color
source, may provide an interesting way to address questions about QCD that
cannot be addressed by experiment at the present time. These states can be
simulated in lattice QCD and the results can be used to improve
phenomenological models of hadrons. Here these ideas are applied to color
triplet states of qqqq and qq bar q.Comment: References added and typos correcte
Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra
We present precise measurements of the X-ray gas mass fraction for a sample
of luminous, relatively relaxed clusters of galaxies observed with the Chandra
Observatory, for which independent confirmation of the mass results is
available from gravitational lensing studies. Parameterizing the total
(luminous plus dark matter) mass profiles using the model of Navarro, Frenk &
White (1997), we show that the X-ray gas mass fractions in the clusters
asymptote towards an approximately constant value at a radius r_2500, where the
mean interior density is 2500 times the critical density of the Universe at the
redshifts of the clusters. Combining the Chandra results on the X-ray gas mass
fraction and its apparent redshift dependence with recent measurements of the
mean baryonic matter density in the Universe and the Hubble Constant determined
from the Hubble Key Project, we obtain a tight constraint on the mean total
matter density of the Universe, Omega_m = 0.30^{+0.04}_{-0.03}, and measure a
positive cosmological constant, Omega_Lambda = 0.95^{+0.48}_{-0.72}. Our
results are in good agreement with recent, independent findings based on
analyses of anisotropies in the cosmic microwave background radiation, the
properties of distant supernovae, and the large-scale distribution of galaxies.Comment: Accepted for publication in MNRAS Letters (6 pages, 3 figures
Double-Spin Transverse Asymmetries in Drell-Yan Processes
We calculate the double-spin transverse asymmetries for the Drell-Yan lepton
pair production in p-p and p-anti p collisions. We assume the transverse and
the longitudinal polarization densities to be equal at a very small scale, as
it is suggested by confinement model results. Using a global fit for the
longitudinal distributions, we find transverse asymmetries of order of 10^-2 at
most, in the accessible kinematic regions.Comment: 8 pages, REVTeX, 6 figures included as file figures.tar.g
Interference Fragmentation Functions and the Nucleon's Transversity
We introduce twist-two quark interference fragmentation functions in helicity
density matrix formalism and study their physical implications. We show how the
nucleon's transversity distribution can be probed through the final state
interaction between two mesons (, , or ) produced
in the current fragmentation region in deep inelastic scattering on a
transversely polarized nucleon.Comment: Final version to be published in Phys. Rev. Let
Spin structure and longitudinal polarization of hyperon in e+e- annihilation at high energies
Longitudinal polarizations of different kinds of hyperons produced in e+e-
annihilation at LEP I and LEP II energies in different event samples are
calculated using two different pictures for the spin structure of hyperon: that
drawn from polarized deep inelastic lepton-nucleon scattering data or that
using SU(6) symmetric wave functions. The result shows that measurements of
such polarizations should provide useful information to the question of which
picture is more suitable in describing the spin effects in the fragmentation
processes.Comment: 26 pages with 10 figures. Submitted to Phys. Rev.
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