762 research outputs found
Flow Equations for N Point Functions and Bound States
We discuss the exact renormalization group or flow equation for the effective
action and its decomposition into one particle irreducible N point functions.
With the help of a truncated flow equation for the four point function we study
the bound state problem for scalar fields. A combination of analytic and
numerical methods is proposed, which is applied to the Wick-Cutkosky model and
a QCD-motivated interaction. We present results for the bound state masses and
the Bethe-Salpeter wave function. (Figs. 1-4 attached as separate uuencoded
post-script files.)Comment: 17 pages, HD-THEP-93-3
Quantum fermions and quantum field theory from classical statistics
An Ising-type classical statistical ensemble can describe the quantum physics
of fermions if one chooses a particular law for the time evolution of the
probability distribution. It accounts for the time evolution of a quantum field
theory for Dirac particles in an external electromagnetic field. This yields in
the non-relativistic one-particle limit the Schr\"odinger equation for a
quantum particle in a potential. Interference or tunneling arise from classical
probabilities.Comment: 15 pages, proceedings Emergent Quantum Mechanics, Heinz von Foerster
conference, Vienn
Effective Nonlocal Euclidean Gravity
A nonlocal form of the effective gravitational action could cure the
unboundedness of euclidean gravity with Einstein action. On sub-horizon length
scales the modified gravitational field equations seem compatible with all
present tests of general relativity and post-Newtonian gravity. They induce a
difference in the effective Newton's constant between regions of space with
vanishing or nonvanishing curvature scalar (or Ricci tensor). In cosmology they
may lead to a value for the critical density after inflation. The
simplest model considered here appears to be in conflict with nucleosynthesis,
but generalizations consistent with all cosmological observations seem
conceivable.Comment: 12 pages, LaTe
Spontaneous symmetry breaking in the colored Hubbard model
The Hubbard model is reformulated in terms of different ``colored'' fermion
species for the electrons or holes at different lattice sites.
Antiferromagnetic ordering or d-wave superconductivity can then be described in
terms of translationally invariant expectation values for colored composite
scalar fields. A suitable mean field approximation for the two dimensional
colored Hubbard model shows indeed phases with antiferromagnetic ordering or
d-wave superconductivity at low temperature. At low enough temperature the
transition to the antiferromagnetic phase is of first order. The present
formulation also allows an easy extension to more complicated microscopic
interactions.Comment: 19 pages, 5 figure
The beta functions of a scalar theory coupled to gravity
We study a scalar field theory coupled to gravity on a flat background, below
Planck's energy. Einstein's theory is treated as an effective field theory.
Within the context of Wilson's renormalization group, we compute gravitational
corrections to the beta functions and the anomalous dimension of the scalar
field, taking into account threshold effects.Comment: 13 pages, plainTe
Dynamics of dark energy with a coupling to dark matter
Dark energy and dark matter are the dominant sources in the evolution of the
late universe. They are currently only indirectly detected via their
gravitational effects, and there could be a coupling between them without
violating observational constraints. We investigate the background dynamics
when dark energy is modelled as exponential quintessence, and is coupled to
dark matter via simple models of energy exchange. We introduce a new form of
dark sector coupling, which leads to a more complicated dynamical phase space
and has a better physical motivation than previous mathematically similar
couplings.Comment: 11 pages, 4 figures, revtex, references adde
Non-linear Matter Spectra in Coupled Quintessence
We consider cosmologies in which a dark-energy scalar field interacts with
cold dark matter. The growth of perturbations is followed beyond the linear
level by means of the time-renormalization-group method, which is extended to
describe a multi-component matter sector. Even in the absence of the extra
interaction, a scale-dependent bias is generated as a consequence of the
different initial conditions for baryons and dark matter after decoupling. The
effect is enhanced significantly by the extra coupling and can be at the 2-3
percent level in the range of scales of baryonic acoustic oscillations. We
compare our results with N-body simulations, finding very good agreement.Comment: 20 pages, 6 figures, typo correcte
Quintessence with quadratic coupling to dark matter
We introduce a new form of coupling between dark energy and dark matter that
is quadratic in their energy densities. Then we investigate the background
dynamics when dark energy is in the form of exponential quintessence. The three
types of quadratic coupling all admit late-time accelerating critical points,
but these are not scaling solutions. We also show that two types of coupling
allow for a suitable matter era at early times and acceleration at late times,
while the third type of coupling does not admit a suitable matter era.Comment: 11 pages, 8 figures, revte
Effective Action for the Quark-Meson Model
The scale dependence of an effective average action for mesons and quarks is
described by a nonperturbative flow equation. The running couplings lead to
spontaneous chiral symmetry breaking. We argue that for strong Yukawa coupling
between quarks and mesons the low momentum physics is essentially determined by
infrared fixed points. This allows us to establish relations between various
parameters related to the meson potential. The results for and
\VEV{\olpsi\psi} are not very sensitive to the poorly known details of the
quark--meson effective action at scales where the mesonic bound states form.
For realistic constituent quark masses we find around 100\MeV.Comment: 56 pages (including 10 figures and 1 table), uses epsf.st
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