14,273 research outputs found
What happened to the Cosmological QCD Phase Transition?
The scenario that some first-order phase transitions may have taken place in
the early Universe offers us one of the most intriguing and fascinating
questions in cosmology. Indeed, the role played by the latent "heat" or energy
released in the phase transition is highly nontrivial and may lead to some
surprising, important results. In this paper, we take the wisdom that the
cosmological QCD phase transition, which happened at a time between 10^(-5) sec
and 10^(-4) sec or at the temperature of about 150 MeV and accounts for
confinement of quarks and gluons to within hadrons, would be of first order. To
get the essence out of the scenario, it is sufficient to approximate the true
QCD vacuum as one of degenerate theta-vacua and when necessary we try to model
it effectively via a complex scalar field with spontaneous symmetry breaking.
We examine how and when "pasted" or "patched" domain walls are formed, how long
such walls evolve in the long run, and we believe that the significant portion
of dark matter could be accounted for in terms of such domain-wall structure
and its remnants. Of course, the cosmological QCD phase transition happened in
the way such that the false vacua associated with baryons and many other
color-singlet objects did not disappear (that is, using the bag-model language,
there are bags of radius 1.0 fermi for the baryons) - but the amount of the
energy remained in the false vacua is negligible. The latent energy released
due to the conversion of the false vacua to the true vacua, in the form of
"pasted" or "patched" domain walls in the short run and their numerous evolved
objects, should make the concept of the "radiation-dominated" epoch, or of the
"matter-dominated" epoch to be re-examined.Comment: 16 pages, 1 figur
Temperature Dependence of Gluon and Quark Condensates as from Linear Confinement
The gluon and quark condensates and their temperature dependence are
investigated within QCD premises. The input for the former is a gauge invariant
kernel made up of the direct (D), exchange (X) and contact(C) QCD
interactions in the lowest order, but with the perturbative propagator
replaced by a `non-perturbative form obtained via two
differentiations: , ( a scale
parameter), and then setting , to simulate linear confinement. Similarly
for the input kernel the gluon propagator is replaced by the above
form. With these `linear' simulations, the respective condensates are
obtained by `looping' up the gluon and quark lines in the standard manner.
Using Dimensional regularization (DR), the necessary integrals yield the
condensates plus temperature corrections, with a common scale parameter
for both. For gluons the exact result is . Evaluation
of the quark condensate is preceded by an approximate solution of the SDE for
the mass function , giving a recursive formula, with convergence achieved
at the third iteration. Setting the scale parameter equal to the
universal Regge slope , the gluon and quark condensates at T=0 are
found to be and respectively, in fair accord
with QCD sum rule values. Next, the temperature corrections (of order
for both condensates) is determined via finite-temperature field theory a la
Matsubara. Keywords: Gluon Condensate, mass tensor, gauge invariance, linear
confinement, finite-temperature, contour-closing. PACS: 11.15.Tk ; 12.38.Lg ;
13.20.CzComment: 13 pages (LaTeX) including 2 figure
Parity-Violating Nuclear Force as derived from QCD Sum Rules
Parity-violating nuclear force, as may be accessed from parity violation
studies in nuclear systems, represents an area of nonleptonic weak interactions
which has been the subject of experimental investigations for several decades.
In the simple meson-exchange picture, parity-violating nuclear force may be
parameterized as arising from exchange of \pi, \rho, \omega, or other meson(s)
with strong meson-nucleon coupling at one vertex and weak parity-violating
meson-nucleon coupling at the other vertex. The QCD sum rule method allows for
a fairly complicated, but nevertheless straightforward, leading-order
loop-contribution determination of the various parity-violating MNN couplings
starting from QCD (with the nontrivial vacuum) and Glashow-Salam-Weinberg
electroweak theory. We continue our earlier investigation of parity-violating
\pi NN coupling (by Henley, Hwang, and Kisslinger) to other parity-violating
couplings. Our predictions are in reasonable overall agreement with the results
estimated on phenomenological grounds, such as in the now classic paper of
Desplanques, Donoghue, and Holstein (DDH), in the global experimental fit of
Adelberger and Haxton (AH), or the effective field theory (EFT) thinking of
Ramsey-Musolf and Page (RP).Comment: 17 pages, 5 figure
QCD Sum Rules and Chiral Symmetry Breaking
By matching the world of hadrons to that of some effective chiral quark
theory, we determine the induced condensates for QCD sum rules in the presence
of an external pion field. The observed values of the strong and weak
coupling constants are understood.Comment: 20 pages, plain Te
Electromagnetic decays of vector mesons as derived from QCD sum rules
We apply the method of QCD sum rules in the presence of external
electromagnetic fields to the problem of the electromagnetic
decays of various vector mesons, such as , and . The induced condensates obtained previously
from the study of baryon magnetic moments are adopted, thereby ensuring the
parameter-free nature of the present calculation. Further consistency is
reinforced by invoking various QCD sum rules for the meson masses. The
numerical results on the various radiative decays agree very well with the
experimental data.Comment: To appear in Phys. Lett.
Parity violation in deuteron photo-disintegration
We analyze the energy dependence for two types of parity-non-conserving
(PNC) asymmetries in the reaction in the near-threshold
region. The first one is the asymmetry in reaction with circularly polarized
photon beam and unpolarized deuteron target. The second one corresponds to
those with an unpolarized photon beam and polarized target. We find that the
two asymmetries have quite different energy dependence, and their shapes are
sensitive to the PNC-meson exchange coupling constants.
The predictions for the future possible experiments to provide definite
constraints for the PNC-coupling constants are discussed.Comment: 22 pages, 12 figures. Submitted to Phys.Rev.C 10Oct.0
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