68 research outputs found
The Flavor Asymmetry of the Nucleon Sea
We re-examine the effects of anti-symmetry on the anti-quarks in the nucleon
sea arising from gluon exchange and pion exchange between confined quarks.
While the effect is primarily to suppress anti-down relative to anti-up quarks,
this is numerically insignificant for the pion terms.Comment: To appear in Phys. Rev.
Chiral Hybrid Bag Model with the Boson Field inside the Bag
The three-phase version of the hybrid chiral bag model, containing the phase
of asymptotic freedom, the hadronization phase as well as the intermediate
phase of constituent quarks, is proposed. For this model the self-consistent
solution, which takes into account the fermion vacuum polarization effects, is
found in (1+1) D. Within this solution the total energy of the bag, including
the one-loop contribution from the Dirac's sea, is studied as the function of
the bag geometry under condition of nonvanishing boson condensate density in
the interior region. The existence and uniqueness of the ground state bag
configuration, which minimizes the total energy and contains all the three
phases, are shown.Comment: 17 text pages, 3 figure
Non-perturbative Gluons and Pseudoscalar Mesons in Baryon Spectroscopy
We study baryon spectroscopy including the effects of pseudoscalar meson
exchange and one gluon exchange potentials between quarks, governed by
. The non-perturbative, hyperspherical method calculations show that
one can obtain a good description of the data by using a quark-meson coupling
constant that is compatible with the measured pion-nucleon coupling constant,
and a reasonably small value of .Comment: 12 pages; Submitted to Phys. Rev. C. Rapid Communication
Correlator mixing and mass reduction as signals of chiral symmetry restoration
Chiral symmetry restoration in a dense medium is to some extent a consequence of the nuclear pion cloud. These pions induce a mixing of the axial and vector current contributions in the axial and vector correlators. We discuss their influence on hadron masses and investigate the signal produced by the remaining contribution associated with chiral symmetry restoration. Using the quark-meson coupling model we find that the latter is associated with the reduction of hadron masses.J. Delorme, M. Ericson, P. A. M. Guichon, and A. W. Thoma
Sigma-term physics in the perturbative chiral quark model
We apply the perturbative chiral quark model (PCQM) at one loop to analyse
meson-baryon sigma-terms. Analytic expressions for these quantities are
obtained in terms of fundamental parameters of low-energy pion-nucleon physics
(weak pion decay constant, axial nucleon coupling, strong pion-nucleon form
factor) and of only one model parameter (radius of the nucleonic three-quark
core). Our result for the piN sigma term of about 45 MeV is in good agreement
with the value deduced by Gasser, Leutwyler and Sainio using
dispersion-relation techniques and exploiting the chiral symmetry constraints.Comment: 19 pages, LaTeX-file, 2 Figure
Baryon masses in a chiral expansion with meson-baryon form factors
The chiral expansion of the one-loop corrections to baryon masses is examined
in a generic meson-cloud model with meson-baryon form factors. For pion loops,
the expansion is rapidly convergent and at fourth order in accurately
reproduces the full integral. In contrast, the expansion is found to converge
very slowly for kaon loops, raising questions about the usefulness of chiral
expansions for kaon-baryon physics. Despite the importance of high-order terms,
relations like that of Gell-Mann and Okubo are well satisfied by the baryon
masses calculated with the full integral. The pion cloud cloud makes a
significant contribution to the sigma commutator, while kaon cloud
gives a very small strangeness content in the nucleon.Comment: 20 pages (RevTeX), 2 figures (attached
Infinite Nuclear Matter on the Light Front: Nucleon-Nucleon Correlations
A relativistic light front formulation of nuclear dynamics is developed and
applied to treating infinite nuclear matter in a method which includes the
correlations of pairs of nucleons: this is light front Brueckner theory. We
start with a hadronic meson-baryon Lagrangian that is consistent with chiral
symmetry. This is used to obtain a light front version of a one-boson-exchange
nucleon-nucleon potential (OBEP). The accuracy of our description of the
nucleon-nucleon (NN) data is good, and similar to that of other relativistic
OBEP models. We derive, within the light front formalism, the Hartree-Fock and
Brueckner Hartree-Fock equations. Applying our light front OBEP, the nuclear
matter saturation properties are reasonably well reproduced. We obtain a value
of the compressibility, 180 MeV, that is smaller than that of alternative
relativistic approaches to nuclear matter in which the compressibility usually
comes out too large. Because the derivation starts from a meson-baryon
Lagrangian, we are able to show that replacing the meson degrees of freedom by
a NN interaction is a consistent approximation, and the formalism allows one to
calculate corrections to this approximation in a well-organized manner. The
simplicity of the vacuum in our light front approach is an important feature in
allowing the derivations to proceed. The mesonic Fock space components of the
nuclear wave function are obtained also, and aspects of the meson and nucleon
plus-momentum distribution functions are computed. We find that there are about
0.05 excess pions per nucleon.Comment: 39 pages, RevTex, two figure
Dynamics of Light Antiquarks in the Proton
We present a comprehensive analysis of the recent data from the E866
experiment at Fermilab on Drell-Yan production in pD and pp collisions, which
indicates a non-trivial x-dependence for the asymmetry between u-bar and d-bar
quark distributions in the proton. The relatively fast decrease of the
asymmetry at large x suggests the important role played by the chiral structure
of the nucleon, in particular the pi-N and pi-Delta components of the nucleon
wave function. At small x the data require an additional non-chiral component,
which may be attributed to the Pauli exclusion principle as first suggested by
Field and Feynman.Comment: version to appear in Phys. Rev.
Prefrontal Cortex Glutamate Correlates with Mental Perspective-Taking
Background: Dysfunctions in theory of mind and empathic abilities have been suggested as core symptoms in major psychiatric disorders including schizophrenia and autism. Since self monitoring, perspective taking and empathy have been linked to prefrontal (PFC) and anterior cingulate cortex (ACC) function, neurotransmitter variations in these areas may account for normal and pathological variations of these functions. Converging evidence indicates an essential role of glutamatergic neurotransmission in psychiatric diseases with pronounced deficits in empathy. However, the role of the glutamate system for different dimensions of empathy has not been investigated so far. Methodology/Principal Findings: Absolute concentrations of cerebral glutamate in the ACC, left dorsolateral PFC and left hippocampus were determined by 3-tesla proton magnetic resonance spectroscopy (1H-MRS) in 17 healthy individuals. Three dimensions of empathy were estimated by a self-rating questionnaire, the Interpersonal Reactivity Index (IRI). Linear regression analysis showed that dorsolateral PFC glutamate concentration was predicted by IRI factor ââperspective takingââ (T = 22.710, p = 0.018; adjusted alpha-level of 0.017, Bonferroni) but not by ââempathic concernâ â or ââpersonal distressââ. No significant relationship between IRI subscores and the glutamate levels in the ACC or left hippocampus was detected. Conclusions/Significance: This is the first study to investigate the role of the glutamate system for dimensions of theory of mind and empathy. Results are in line with recent concepts that executive top-down control of behavior is mediated b
Determinants of the number of mammography units in 31 countries with significant mammography screening
In the 2000s, most of the female population of industrialised countries had access to mammography breast cancer screening, but with variable modalities among the countries. We assessed the number of mammography units (MUs) in 31 European, North American and Asian countries where significant mammography activity has existed for over 10 years, collecting data on the number of such units and of radiologists by contacting institutions in each country likely to provide the relevant information. Around 2004, there were 32â324âMU in 31 countries, the number per million women ranging from less than 25 in Turkey, Denmark, the Netherlands, the United Kingdom, Norway, Poland and Hungary to more than 80 in Cyprus, Italy, France, the United States and Austria. In a multivariate analysis, the number of MUs was positively associated with the number of radiologists (P=0.0081), the number of women (P=0.0023) and somewhat with the country surface area (P=0.077). There is considerable variation in the density of MU across countries and the number of MUs in service are often well above what would be necessary according to local screening recommendations. High number of MUs in some countries may have undesirable consequences, such as unnecessarily high screening frequency and decreased age at which screening is started
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