4,713 research outputs found
The quark condensate at finite temperature
The temperature evolution of the quark condensate is studied using three
different methods. In the spirit of a many-body approach we make an expansion
in the scalar density up to second order. Our result is consistent chiral
perturbation theory to two-loop order.Comment: Latex 14 pages + 1 figure (postscript
Chiral Lagrangians and Quark Condensate in Nuclei
We study the evolution with density of the quark condensate in the nuclear
medium with interacting nucleons and including the short range correlations. We
work with two chiral models, the linear sigma model and the non-linear one. For
the last one we use two versions, one which does not satisfy PCAC, and another
one which does. We show that the quark condensate, as other observables, is
independent on the variant selected. The application to physical pions excludes
the linear sigma model as a credible one. In the non-linear models our
conclusions are: first there is no systematic reaction imposed by chiral
symmetry against symmetry restoration, second, if one keeps only the s-wave
pion-nucleon interaction, the quark condensate evolves essentially linearly
with density, as if the nucleons were non interacting. The main correction
arises from the p-wave pion-nucleon interaction. Last, in the s-wave optical
potential, chiral symmetry tolerates but does not impose two body terms. On the
other hand the effect of correlations linked to the isospin symmetric amplitude
is negligible.Comment: 24 pages, LaTex, 7 PostScript Figures, a couple of misprints
corrected , 2 references added, a few modifications of the main text and
conclusion, to be published in Nucl. Phys.
Chiral Symmetry Restoration and Parity Mixing
We derive the expressions of the vector and axial current from a chiral
Lagrangian restricted to nucleons and pions. They display mixing terms between
the axial and vector currents. We study the modifications in the nuclear medium
of the coupling constants of the axial current, namely the pion decay constant
and the nucleonic axial one due to the requirements of chiral symmetry. We
express the renormalizations in terms of the local scalar pion density. The
latter also governs the quark condensate evolu- tion and we discuss the link
between this evolution and the renormaliza- tions. In the case of the nucleon
axial coupling constant this renormali- zation corresponds to a new type of
exchange currents, with two exchanged pions. We give an estimate for the
resulting quenching. Although moderate it helps explaining the quenching
experimentally observed.Comment: Latex, 15 pages. Several references and one figure added. New
discussion of some points has been included. Treatment of the renormali-
zation of the nucleon axial coupling constant has been develope
Neutrino and antineutrino quasielastic interactions with nuclei
We investigate the interaction of neutrinos and antineutrinos with nuclei. We
explore in particular the role played by the multinucleon excitations which can
contaminate the quasielastic cross section. For neutrinos the multinucleon term
produces a sizable increase of the quasielastic cross section. Part of the
effect arises from tensor correlations. For antineutrinos this influence is
smaller owing to the axial-vector interference which increases the relative
importance of the terms which are not affected by these multinucleon
excitations
Pion Scalar Density and Chiral Symmetry Restoration at Finite Temperature and Density
This paper is devoted to the evaluation of the pionic scalar density at
finite temperature and baryonic density. We express the latter effect in terms
of the nuclear response evaluated in the random phase approxima- tion. We
discuss the density and temperature evolution of the pionic density which
governs the quark condensate evolution. Numerical evalua- tions are performed.Comment: 13 pages, Latex File, 10 eps Figure
Axial and Vector Correlator Mixing in Hot and Dense Hadronic Matter
We study the manifestations of chiral symmetry restoration which have a
significance for the parity mixing. Restricting to pions and nucleons we
establish a formalism for the expression of the vector correlator, which
displays the mixing of the axial correlator into the vector one and unifies the
cases of the heat bath and the dense medium. We give examples of mixing
cross-sections. We also establish a link between the energy integrated mixing
cross-sections and the pion scalar density which governs the quenching factors
of coupling constants, such as the pion decay one, as well as the quark
condensate evolution.Comment: 12 pages, Latex, 4 PostScript Figure
Quark Condensate in the Deuteron
We study the changes produced by the deuteron on the QCD quark condensate by
means the Feynman-Hellmann theorem and find that the pion mass dependence of
the pion-nucleon coupling could play an important role. We also discuss the
relation between the many body effect of the condensate and the meson exchange
currents, as seen by photons and pions. For pion probes, the many-body term in
the physical amplitude differs significantly from that of soft pions, the one
linked to the condensate. Thus no information about the many-body term of the
condensate can be extracted from the pion-deuteron scattering length. On the
other hand, in the Compton amplitude, the relationship with the condensate is a
more direct one.Comment: to appear in Physics Review C (19 pages, 3 figures
The Pion-Nucleon coupling constant from np charge exchange scattering
A novel extrapolation method has been used to deduce the charged Pion-Nucleon
coupling constant from backward differential scattering cross sections. We
applied it to new measurements performed at 162 MeV at the The Svedberg
Laboratory in Uppsala. In the angular range , the
carefully normalized data are steeper than those of most previous measurements.
The extracted value, , in good agreement with
the classical value, is higher than those determined in recent nucleon-nucleon
partial-wave analyses.Comment: 6 pages, 3 encapsulated figures, epsfig, menu97.cls (included
Dilepton production in proton-proton and quasifree proton-neutron reactions at 1.25 GeV
We investigate the pp --> pp e+e- and quasifree pn --> pn e+e- reactions
within an effective Lagrangian model at the laboratory kinetic energy of 1.25
GeV for which experimental data have recently been reported by the HADES
Collaboration. The model uses a meson-exchange approximation to describe the
initial nucleon-nucleon (NN) scattering. Contributions to the reaction
amplitudes are included from the NN bremsstrahlung as well as from the
excitation, propagation and radiative decay of the Delta(1230) isobar state. It
is found that the HADES data on the e+e- invariant mass distribution in the pp
--> pp e+e- reaction are reproduced excellently by our model where the Delta
isobar term dominates the spectrum. However, the quasifree pn --> pn e+e- cross
sections are underpredicted in the invariant mass region of 0.40 - 0.55 GeV/c^2
even after including contributions of the eta Dalitz decay and the subthreshold
production and decay of the rho^0 meson via the baryonic resonance N*(1520). In
the case of the quasifree pn --> pn e+e- reaction, a strong sensitivity to the
pion electromagnetic form factor is observed which helps to bring the
calculated cross sections closer to the data in the higher dilepton mass
region.Comment: 12 pages 3, figures, version to appear in Phys. Rev. C (Rapid
Communications
Do Distance Learning Students Taking Business and Technology Courses at Olympic College Achieve the Same Grade Point Averages as On-Campus Students in the Same Courses
To guide this problem, the following hypothesis was established: 1. There will be no significant difference in the GPAs of online and on-campus students in the same Business and Technology courses at Olympic College
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