48 research outputs found
A Model for the Pion Structure Function
The pion structure function is investigated in a simple model, where pion and
constituent quark fields are coupled through the simplest pseudoscalar
coupling. The imaginary part of the forward gamma* pi-> gamma* pi scattering
amplitude is evaluated and related to the structure functions. It is shown that
the introduction of non-perturbative effects, linked to the size of the pion
and preserving gauge invariance, allows a connection with the quark
distribution. It is predicted that higher-twist terms become negligible for Q2
larger than about 2 GeV2 and that quarks in the pion have a momentum fraction
smaller than in the proton case.Comment: 14 pages, 6 figures, LaTeX, elsart clas
On the pion-nucleon coupling constant
In view of persisting misunderstanding about the determination of the
pion-nucleon coupling constants in the Nijmegen multienergy partial-wave
analyses of pp, np, and pbar-p scattering data, we present additional
information which may clarify several points of discussion. We comment on
several recent papers addressing the issue of the pion-nucleon coupling
constant and criticizing the Nijmegen analyses.Comment: 19 pages, Nijmegen preprint THEF-NYM-92-0
The importance of the mixed phase in hybrid stars built with the Nambu-Jona-Lasinio model
We investigate the structure of hybrid stars based on two different
constructions: one is based on the Gibbs condition for phase coexistence and
considers the existence of a mixed phase (MP), and the other is based on the
Maxwell construction and no mixed phase is obtained. The hadron phase is
described by the non-linear Walecka model (NLW) and the quark phase by the
Nambu-Jona-Lasinio model (NJL). We conclude that the masses and radii obtained
are model dependent but not significantly different for both constructions.Comment: 8 pages, 7 figures, 3 table
Consistent histories of systems and measurements in spacetime
Traditional interpretations of quantum theory in terms of wave function
collapse are particularly unappealing when considering the universe as a whole,
where there is no clean separation between classical observer and quantum
system and where the description is inherently relativistic. As an alternative,
the consistent histories approach provides an attractive "no collapse"
interpretation of quantum physics. Consistent histories can also be linked to
path-integral formulations that may be readily generalized to the relativistic
case. A previous paper described how, in such a relativistic spacetime path
formalism, the quantum history of the universe could be considered to be an
eignestate of the measurements made within it. However, two important topics
were not addressed in detail there: a model of measurement processes in the
context of quantum histories in spacetime and a justification for why the
probabilities for each possible cosmological eigenstate should follow Born's
rule. The present paper addresses these topics by showing how Zurek's concepts
of einselection and envariance can be applied in the context of relativistic
spacetime and quantum histories. The result is a model of systems and
subsystems within the universe and their interaction with each other and their
environment.Comment: RevTeX 4; 37 pages; v2 is a revision in response to reviewer
comments, connecting the discussion in the paper more closely to consistent
history concepts; v3 has minor editorial corrections; accepted for
publication in Foundations of Physics; v4 has a couple minor typographical
correction
Warm stellar matter with deconfinement: application to compact stars
We investigate the properties of mixed stars formed by hadronic and quark
matter in -equilibrium described by appropriate equations of state (EOS)
in the framework of relativistic mean-field theory. We use the non- linear
Walecka model for the hadron matter and the MIT Bag and the Nambu-Jona-Lasinio
models for the quark matter. The phase transition to a deconfined quark phase
is investigated. In particular, we study the dependence of the onset of a mixed
phase and a pure quark phase on the hyperon couplings, quark model and
properties of the hadronic model. We calculate the strangeness fraction with
baryonic density for the different EOS. With the NJL model the strangeness
content in the mixed phase decreases. The calculations were performed for T=0
and for finite temperatures in order to describe neutron and proto-neutron
stars. The star properties are discussed. Both the Bag model and the NJL model
predict a mixed phase in the interior of the star. Maximum allowed masses for
proto-neutron stars are larger for the NJL model ( M)
than for the Bag model ( M).Comment: RevTeX,14 figures, accepted to publication in Physical Review
Effect of Magnetic Impurities on Suppression of the Transition Temperature in Disordered Superconductors
We calculate the first-order perturbative correction to the transition
temperature in a superconductor with both non-magnetic and magnetic
impurities. We do this by first evaluating the correction to the effective
potential, , and then obtain the first-order correction to the
order parameter, , by finding the minimum of . Setting
finally allows to be evaluated. is now a function of
both the resistance per square, , a measure of the non-magnetic
disorder, and the spin-flip scattering rate, , a measure of the
magnetic disorder. We find that the effective pair-breaking rate per magnetic
impurity is virtually independent of the resistance per square of the film, in
agreement with an experiment of Chervenak and Valles. This conclusion is
supported by both the perturbative calculation, and by a non-perturbative
re-summation technique.Comment: 29 pages, 9 figure
Stationarity of Inflation and Predictions of Quantum Cosmology
We describe several different regimes which are possible in inflationary
cosmology. The simplest one is inflation without self-reproduction of the
universe. In this scenario the universe is not stationary. The second regime,
which exists in a broad class of inflationary models, is eternal inflation with
the self-reproduction of inflationary domains. In this regime local properties
of domains with a given density and given values of fields do not depend on the
time when these domains were produced. The probability distribution to find a
domain with given properties in a self-reproducing universe may or may not be
stationary, depending on the choice of an inflationary model. We give examples
of models where each of these possibilities can be realized, and discuss some
implications of our results for quantum cosmology. In particular, we propose a
new mechanism which may help solving the cosmological constant problem.Comment: 30 pages, Stanford preprint SU-ITP-94-24, LaTe
Chiral symmetry breaking, color superconductivity and color neutral quark matter: a variational approach
We investigate the vacuum realignment for chiral symmetry breaking and color
superconductivity at finite density in Nambu-Jona-Lasinio model in a
variational method. The treatment allows us to investigate simultaneous
formation of condensates in quark antiquark as well as in diquark channels. The
methodology involves an explicit construction of a variational ground state and
minimisation of the thermodynamic potential. Color and electric charge
neutrality conditions are imposed through introduction of appropriate chemical
potentials. Color and flavor dependent condensate functions are determined
through minimisation of the thermodynamic potential. The equation of state is
calculated. Simultaneous existence of a mass gap and superconducting gap is
seen in a small window of quark chemical potential within the model when charge
neutrality conditions are not imposed. Enforcing color and electric charge
neutrality conditions gives rise to existence of gapless superconducting modes
depending upon the magnitude of the gap and the difference of the chemical
potentials of the condensing quarks.Comment: 13 pages, 6 figures,to appear in Phys. Rev.
Neutron star properties in the quark-meson coupling model
The effects of internal quark structure of baryons on the composition and
structure of neutron star matter with hyperons are investigated in the
quark-meson coupling (QMC) model. The QMC model is based on mean-field
description of nonoverlapping spherical bags bound by self-consistent exchange
of scalar and vector mesons. The predictions of this model are compared with
quantum hadrodynamic (QHD) model calibrated to reproduce identical nuclear
matter saturation properties. By employing a density dependent bag constant
through direct coupling to the scalar field, the QMC model is found to exhibit
identical properties as QHD near saturation density. Furthermore, this modified
QMC model provides well-behaved and continuous solutions at high densities
relevant to the core of neutron stars. Two additional strange mesons are
introduced which couple only to the strange quark in the QMC model and to the
hyperons in the QHD model. The constitution and structure of stars with
hyperons in the QMC and QHD models reveal interesting differences. This
suggests the importance of quark structure effects in the baryons at high
densities.Comment: 28 pages, 10 figures, to appear in Physical Review
Coulomb Gauge QCD, Confinement, and the Constituent Representation
Quark confinement and the genesis of the constituent quark model are examined
in nonperturbative QCD in Coulomb gauge. We employ a self-consistent method to
construct a quasiparticle basis and to determine the quasiparticle interaction.
The results agree remarkably well with lattice computations. They also
illustrate the mechanism by which confinement and constituent quarks emerge,
provide support for the Gribov-Zwanziger confinement scenario, clarify several
perplexing issues in the constituent quark model, and permit the construction
of an improved model of low energy QCD.Comment: 43 pages, 14 figures, revtex, uses psfig.st