92 research outputs found
Chiral symmetry breaking in hot matter
This series of three lectures covers (a) a basic introduction to symmetry
breaking in general and chiral symmetry breaking in QCD, (b) an overview of the
present status of lattice data and the knowlegde that we have at finite
temperature from chiral perturbation theory. (c) Results obtained from the
Nambu--Jona-Lasinio model describing static mesonic properties are discussed as
well as the bulk thermodynamic quantities. Divergences that are observed in the
elastic quark-antiquark scattering cross-section, reminiscent of the phenomenon
of critical opalescence in light scattering, is also discussed. (d) Finally, we
deal with the realm of systems out of equilibrium, and examine the effects of a
medium dependent condensate in a system of interacting quarks.Comment: 62 LaTex pages, incorporating 23 figures. Lectures given at the
eleventh Chris-Engelbrecht Summer School in Theoretical Physics, 4-13
February, 1998, to be published by Springer Verla
Transition rates for q q bar -> pi pi pi in a chiral model
We investigate the nature of transition rates for the hadronization process
of q q bar -> Pi Pi Pi as opposed to the transition rates for q q bar -> Pi Pi,
within the Nambu--Jona-Lasinio model that has manifest chiral symmetry. Feynman
diagrams appropriate to this process can be classified according to the
expansion in the inverse number of colors 1/N_c. Two of these types of graphs
are seen to be either 's-like' or 't-like' in nature. A further graph that
contains both s-like and t-like elements, and which is denoted as st-like, is
also present. To describe such a process with two incoming and three outgoing
particles, it is necessary to extend the number of Mandelstam invariants. It is
convenient to introduce seven such variables, of which only five are
independent. The cross section for two incoming particles to three outgoing
particles is then reexpressed in integral form in terms of these invariants. As
a function of sqrt s, the final expression is reduced to an integral over the
four remaining invariants. The limits of integration, which are now
non-trivial, are also discussed. The transition rate for the explicit case of u
u bar -> Pi^+ Pi^- Pi^0, is evaluated numerically, assuming non-chiral pions,
m_Pi = 135MeV. The rate for three pion production is found to be of the same
order of magnitude as for two pion production, making this a non-negligible
contribution to hadronization.Comment: 33 LaTex pages plus 1 postscript file containing 12 figures. To be
published in Nuclear Physics
Confinement in the Coulomb Gauge Model
The Coulomb gauge model of QCD is studied with the introduction of a
confining potential into the scalar part of the vector potential. Using a Green
function formalism, we derive the self-energy for this model, which has both
scalar and vector parts, and . A rotation of these
variables leads to the so-called gap and energy equations. We then analyse the
divergence structure of these equations. As this depends explicitly on the form
of potential, we give as examples both the linear plus Coulomb and
quadratically confining potentials. The nature of the confining single particle
Green function is investigated, and shown to be divergent due to the infrared
singularities caused by the confining potential. Solutions to the gap equation
for the simpler case of quadratic confinement are found both semi-analytically
and numerically. At finite temperatures, the coupled set of equations are
solved numerically in two decoupling approximations. Although chiral symmetry
is found only to be exactly restored as , the chiral condensate
displays a steep drop over a somewhat small temperature range.Comment: 31 pages Revtex, 2 PS files containing 11 figures, accepted for
publication in Annals Of Physics (NY
Comment on ``Relativistic kinetic equations for electromagnetic, scalar and pseudoscalar interactions''
It is found that the extra quantum constraints to the spinor components of
the equal-time Wigner function given in a recent paper by Zhuang and Heinz
should vanish identically. We point out here the origin of the error and give
an interpretation of the result. However, the principal idea of obtaining a
complete equal-time transport theory by energy averaging the covariant theory
remains valid. The classical transport equation for the spin density is also
found to be incorrect. We give here the correct form of that equation and
discuss briefly its structure.Comment: 5 pages LaTe
Mass Generation in the Supersymmetric Nambu--Jona--Lasinio Model in an External Magnetic Field
The mass generation in the (3+1)-dimensional supersymmetric
Nambu-Jona-Lasinio model in a constant magnetic field is studied. It is shown
that the external magnetic field catalyzes chiral symmetry breaking.Comment: LaTeX file, 6 pages. Talk given at the International Seminar
dedicated to the memory of Dmitrij Volkov "Supersymmetry and Quantum Field
Theory", Kharkov, Ukraine, January 5-7, 199
Derivation of transport equations for a strongly interacting Lagrangian in powers of and
Transport theory for an interacting fermionic system is reviewed and applied
to the chiral Lagrangian of the Nambu-Jona-Lasinio model. Two expansions must
be applied: an expansion in the inverse number of colors, , due to the
nature of the strong coupling theory, and a semiclassical expansion, in powers
of . The quasiparticle approximation is implemented at an early stage,
and spin effects are omitted. The self-energy is evaluated, self-consistently
only in the Hartree approximation, and semi-perturbatively in the collision
integral. In the Hartree approximation, , the Vlasov equation is
recovered to , together with an on-mass shell constraint equation,
that is automatically fulfilled by the quasiparticle ansatz. The expressions
for the self-energy to order lead to the collision term. Here one
sees explicitly that particle-antiparticle creation and annihilation processes
are suppressed that would otherwise be present, should an off-shell energy
spectral function be admitted. A clear identification of the , and
channel scattering processes in connection with the self-energy graphs is made
and the origin of the mixed terms is made evident. Finally, after ordering
according to powers in , a Boltzmann-like form for the collision
integral is obtained
Responses of quark condensates to the chemical potential
The responses of quark condensates to the chemical potential, as a function
of temperature T and chemical potential \mu, are calculated within the
Nambu--Jona-Lasinio (NJL) model. We compare our results with those from the
recent lattice QCD simulations [QCD-TARO Collaboration, Nucl. Phys. B (Proc.
Suppl.) 106, 462 (2002)]. The NJL model and lattice calculations show
qualitatively similar behavior, and they will be complimentary ways to study
hadrons at finite density. The behavior above T_c requires more elaborated
analyses.Comment: 3 pages, 2 figs, based on a contribution to the Prof. Osamu Miyamura
memorial symposium, Hiroshima University, Nov. 16-17, 2001; slightly revised,
accepted for publication in Physical Review
A Study of the Nambu--Jona-Lasinio Model on the Lattice
We present our full analysis of the two flavor Nambu--Jona-Lasinio model with
chiral symmetry on the four--dimensional hypercubic
lattice with naive and Wilson fermions. We find that this model is an excellent
toy field theory to investigate issues related to lattice QCD. We use the large
approximation to leading order in to obtain non perturbative
analytical results over almost the whole parameter range. By using numerical
simulations we estimate that the size of the corrections for most of the
quantities we consider are small and in this way we strengthen the validity of
the leading order large calculations. We obtain results regarding the
approach to the continuum chiral limit, the effects of the zero momentum
fermionic modes on finite lattices and the scalar and pseudoscalar spectrum.
Note: The full ps file of this preprint is also available via anonymous ftp to
ftp.scri.fsu.edu. To get the ps file, ftp to this address and use for username
"anonymous" and for password your complete E-mail address. The file is in the
directory pub/vranas (to go to that directory type: cd pub/vranas) and is
called NJL_long.ps (to get it type: get NJL_long.ps)Comment: 35 pages, LaTex file. (Added section with title: "The zero pion mass
line on a finite lattice at large ".
Density Effect on Hadronization of a Quark Plasma
The hadronization cross section in a quark plasma at finite temperature and
density is calculated in the framework of Nambu--Jona-lasinio model with
explicit chiral symmetry breaking. In apposition to the familiar temperature
effect, the quark plasma at high density begins to hadronize suddenly. It leads
to a sudden and strong increase of final state pions in relativistic heavy ion
collisions which may be considered as a clear signature of chiral symmetry
restoration.Comment: Latex2e, 11 pages, 7 Postscript figures, submitted to Phys. Rev.
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