96 research outputs found
Low-momentum Pion Enhancement Induced by Chiral Symmetry Restoration
The thermal and nonthermal pion production by sigma decay and its relation
with chiral symmetry restoration in a hot and dense matter are investigated.
The nonthermal decay into pions of sigma mesons which are popularly produced in
chiral symmetric phase leads to a low-momentum pion enhancement as a possible
signature of chiral phase transition at finite temperature and density.Comment: 3 pages, 2 figure
In-medium pi-pi Correlation Induced by Partial Restoration of Chiral Symmetry
We show that both the linear and the non-linear chiral models give an
enhancement of the pi-pi cross section near the 2pi threshold in the
scalar-iso-scalar (I=J=0) channel in nuclear matter. The reduction of the
chiral condensate, i.e., the partial chiral restoration in nuclear matter, is
responsible for the enhancement in both cases. We extract an effective
4pi-nucleon vertex which is responsible for the enhancement but has not been
considered in the non-liear models for in-medium pi-pi interaction. Relation of
this vertex and a next-to-leading order terms in the heavy-baryon chiral
lagrangian, L_piN^(2), is also discussed.Comment: 5 pages, 5 eps figure, REVTe
The Fokker-Planck equation for bistable potential in the optimized expansion
The optimized expansion is used to formulate a systematic approximation
scheme to the probability distribution of a stochastic system. The first order
approximation for the one-dimensional system driven by noise in an anharmonic
potential is shown to agree well with the exact solution of the Fokker-Planck
equation. Even for a bistable system the whole period of evolution to
equilibrium is correctly described at various noise intensities.Comment: 12 pages, LATEX, 3 Postscript figures compressed an
Spectral functions in the sigma-channel near the critical end point
Spectral functions in the -channel are investigated near the chiral
critical end point (CEP), that is, the point where the chiral phase transition
ceases to be first-ordered in the -plane of the QCD phase diagram. At
that point the meson becomes massless in spite of explicit breaking of
the chiral symmetry. It is expected that experimental signatures peculiar to
CEP can be observed through spectral changes in the presence of abnormally
light mesons. As a candidate, the invariant-mass spectrum for diphoton
emission is estimated with the chiral quark model incorporated. The results
show the characteristic shape with a peak in the low energy region, which may
serve as a signal for CEP. However, we find that the diphoton multiplicity is
highly suppressed by infrared behaviors of the meson. Experimentally,
in such a low energy region below the threshold of two pions, photons from
are major sources of the background for the signal.Comment: 12 pages, 8 figures, 1 figure replaced, minor modification
Thermal and Nonthermal Pion Enhancements with Chiral Symmetry Restoration
The pion production by sigma decay and its relation with chiral symmetry
restoration in a hot and dense matter are investigated in the framework of the
Nambu-Jona-Lasinio model. The decay rate for the process sigma -> 2pion to the
lowest order in a 1/N_c expansion is calculated as a function of temperature T
and chemical potential mu. The thermal and nonthermal enhancements of pions
generated by the decay before and after the freeze-out present only in the
crossover region of the chiral symmetry transition. The strongest nonthermal
enhancement is located in the vicinity of the endpoint of the first-order
transition.Comment: Latex2e, 12 pages, 8 Postscript figures, submitted to Phys. Rev.
Linear sigma model and chiral symmetry at finite temperature
The chiral phase transition is investigated within the framework of the
linear sigma model at finite temperature. We concentrate on the meson sector of
the model and calculate the finite temperature effective potential in the
Hartree approximation by using the Cornwall-Jackiw-Tomboulis formalism of
composite operators. The effective potential is calculated for N=4 involving
the usual sigma and three pions and in the large N approximation involving N-1
pion fields. In the N=4 case we have examined the theory both in the chiral
limit and with the presence of a symmetry breaking term which generates the
pion masses. In both cases we have solved the system of the resulting gap
equations for the thermal effective masses of the particles numerically and we
have investigated the evolution of the effective potential. In the N=4 case
there is indication of a first order phase transition and the Goldstone theorem
is not satisfied. The situation is different in the general case using the
large approximation, the Goldstone theorem is satisfied and the phase
transition is of the second order. For this analysis we have ignored quantum
effects and we used the imaginary time formalism for calculations.Comment: 14 pages, 5 eps figures, RevTex, axodraw.st
A Precursor of Chiral Symmetry Restoration in the Nuclear Medium
Spectral enhancement near the 2m_{\pi} threshold in the I=J=0 channel in
nuclei is shown to be a distinct signal of the partial restoration of chiral
symmetry. The relevance of this phenomenon with the possible detection of
2\pi^{0} and 2\gamma in hadron-nucleus and photo-nucleus reactions is
discussed.Comment: Revtex, 4 pages, 3 eps figures, title and introduction changed, to
appear in Phys. Rev. Let
Out of equilibrium O (N) linear-sigma system - Construction of perturbation theory with gap- and Boltzmann-equations
We establish from first principles a perturbative framework that allows us to
compute reaction rates for processes taking place in nonequilibrium
linear-sigma systems in broken phase. The system of our concern is quasiuniform
system near equilibrium or nonequilibrium quasistationary system. We employ the
closed-time-path formalism and use the so-called gradient approximation. No
further approximation is introduced. In the course of construction of the
framework, we obtain the gap equation that determines the effective masses of
and of , and the generalized Boltzmann equation that describes
the evolution of the number-density functions of and of .Comment: 18 page
The Chiral Fermion Meson Model at Finite Temperature
We study the chiral fermion meson model which is the well known linear sigma
model of Gell-Mann-and-Levy at finite temperature.A modified self-consistent
resummation (MSCR) which resums higher order terms in the perturbative
expansion is proposed. It is shown that with the MSCR the problem of tachyonic
masses is solved, the renormalization of the gap equations is carried out and
the Goldstone's theorem is verified. We also apply the method to investigate
another known case at high temperature and compare with results found in the
literature.Comment: 31 pages, 9 EPS figures. Final version with extended Concluding
Remarks section, accepted for publication in Phys. Rev.
Screened Perturbation Theory to Three Loops
The thermal physics of a massless scalar field with a phi^4 interaction is
studied within screened perturbation theory (SPT). In this method the
perturbative expansion is reorganized by adding and subtracting a mass term in
the lagrangian. We consider several different mass prescriptions that
generalize the one-loop gap equation to two-loop order. We calculate the
pressure and entropy to three-loop order and the screening mass to two-loop
order. In contrast to the weak-coupling expansion, the SPT-improved
approximations appear to converge even for rather large values of the coupling
constant.Comment: 30 pages, 10 figure
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