429 research outputs found
Non-perturbative calculations for the effective potential of the symmetric and non-Hermitian field theoretic model
We investigate the effective potential of the symmetric
field theory, perturbatively as well as non-perturbatively. For the
perturbative calculations, we first use normal ordering to obtain the first
order effective potential from which the predicted vacuum condensate vanishes
exponentially as in agreement with previous calculations. For the
higher orders, we employed the invariance of the bare parameters under the
change of the mass scale to fix the transformed form totally equivalent to
the original theory. The form so obtained up to is new and shows that all
the 1PI amplitudes are perurbative for both and regions. For
the intermediate region, we modified the fractal self-similar resummation
method to have a unique resummation formula for all values. This unique
formula is necessary because the effective potential is the generating
functional for all the 1PI amplitudes which can be obtained via and thus we can obtain an analytic calculation for the 1PI
amplitudes. Again, the resummed from of the effective potential is new and
interpolates the effective potential between the perturbative regions.
Moreover, the resummed effective potential agrees in spirit of previous
calculation concerning bound states.Comment: 20 page
The Non-Trivial Effective Potential of the `Trivial' lambda Phi^4 Theory: A Lattice Test
The strong evidence for the `triviality' of (lambda Phi^4)_4 theory is not
incompatible with spontaneous symmetry breaking. Indeed, for a `trivial' theory
the effective potential should be given exactly by the classical potential plus
the free-field zero-point energy of the shifted field; i.e., by the one-loop
effective potential. When this is renormalized in a simple, but nonperturbative
way, one finds, self-consistently, that the shifted field does become
non-interacting in the continuum limit. For a classically scale-invariant (CSI)
lambda Phi^4 theory one finds m_h^2 = 8 pi^2 v^2, predicting a 2.2 TeV Higgs
boson. Here we extend our earlier work in three ways: (i) we discuss the
analogy with the hard-sphere Bose gas; (ii) we extend the analysis from the CSI
case to the general case; and (iii) we propose a test of the predicted shape of
the effective potential that could be tested in a lattice simulation.Comment: 22 pages, LaTeX, DE-FG05-92ER40717-
The Path-Integral Approach to the N=2 Linear Sigma Model
In QFT the effective potential is an important tool to study symmetry
breaking phenomena. It is known that, in some theories, the canonical approach
and the path-integral approach yield different effective potentials. In this
paper we investigate this for the Euclidean N=2 linear sigma model. Both the
Green's functions and the effective potential will be computed in three
different ways. The relative merits of the various approaches are discussed.Comment: 2 figure
Dark matter halos and the anisotropy of ultra-high energy cosmic rays
Several explanations for the existence of Ultra High Energy Cosmic Rays
invoke the idea that they originate from the decay of massive particles created
in the reheating following inflation. It has been suggested that the decay
products can explain the observed isotropic flux of cosmic rays. We have
calculated the anisotropy expected for various models of the dark matter
distribution and find that at present data are too sparse above eV to discriminate between different models. However we show that with
data from three years of operation of the southern section of the Pierre Auger
Observatory significant progress in testing the proposals will be made.Comment: 21 pages, 6 figures (ps), Astroparticle Physics (accepted for
publication
One- and many-body effects on mirages in quantum corrals
Recent interesting experiments used scanning tunneling microscopy to study
systems involving Kondo impurities in quantum corrals assembled on Cu or noble
metal surfaces. The solution of the two-dimensional one-particle Schrodinger
equation in a hard wall corral without impurity is useful to predict the
conditions under which the Kondo effect can be projected to a remote location
(the quantum mirage). To model a soft circular corral, we solve this equation
under the potential W*delta(r-r0), where r is the distance to the center of the
corral and r0 its radius. We expand the Green's function of electron surface
states Gs0 for r<r0 as a discrete sum of contributions from single poles at
energies epsilon_i-I*delta_i. The imaginary part delta_i is the half-width of
the resonance produced by the soft confining potential, and turns out to be a
simple increasing function of epsilon_i. In presence of an impurity, we solve
the Anderson model at arbitrary temperatures using the resulting expression for
Gs0 and perturbation theory up to second order in the Coulomb repulsion U. We
calculate the resulting change in the differential conductance Delta dI/dV as a
function of voltage and space, in circular and elliptical corrals, for
different conditions, including those corresponding to recent experiments. The
main features are reproduced. The role of the direct hybridization between
impurity and bulk, the confinement potential, the size of the corral and
temperature on the intensity of the mirage are analyzed. We also calculate
spin-spin correlation functions.Comment: 13 pages, 12 figures, accepted for publication in Phys. Rev. B.
Calculations of spin correlations within an additional approximation adde
Density Matrix Renormalisation Group Approach to the Massive Schwinger Model
The massive Schwinger model is studied, using a density matrix
renormalisation group approach to the staggered lattice Hamiltonian version of
the model. Lattice sizes up to 256 sites are calculated, and the estimates in
the continuum limit are almost two orders of magnitude more accurate than
previous calculations. Coleman's picture of `half-asymptotic' particles at
background field theta = pi is confirmed. The predicted phase transition at
finite fermion mass (m/g) is accurately located, and demonstrated to belong in
the 2D Ising universality class.Comment: 38 pages, 18 figures, submitted to PR
Overexpression of miRNA-25-3p inhibits Notch1 signaling and TGF-ÎČ-induced collagen expression in hepatic stellate cells
During chronic liver injury hepatic stellate cells (HSCs), the principal source of extracellular matrix in the fibrotic liver, transdifferentiate into pro-fibrotic myofibroblast-like cells - a process potentially regulated by microRNAs (miRNAs). Recently, we found serum miRNA-25-3p (miR-25) levels were upregulated in children with Cystic Fibrosis (CF) without liver disease, compared to children with CF-associated liver disease and healthy individuals. Here we examine the role of miR-25 in HSC biology. MiR-25 was detected in the human HSC cell line LX-2 and in primary murine HSCs, and increased with culture-induced activation. Transient overexpression of miR-25 inhibited TGF-ÎČ and its type 1 receptor (TGFBR1) mRNA expression, TGF-ÎČ-induced Smad2 phosphorylation and subsequent collagen1α1 induction in LX-2 cells. Pull-down experiments with biotinylated miR-25 revealed Notch signaling (co-)activators ADAM-17 and FKBP14 as miR-25 targets in HSCs. NanoString analysis confirmed miR-25 regulation of Notch- and Wnt-signaling pathways. Expression of Notch signaling pathway components and endogenous Notch1 signaling was downregulated in miR-25 overexpressing LX-2 cells, as were components of Wnt signaling such as Wnt5a. We propose that miR-25 acts as a negative feedback anti-fibrotic control during HSC activation by reducing the reactivity of HSCs to TGF-ÎČ-induced collagen expression and modulating the cross-talk between Notch, Wnt and TGF-ÎČ signaling
Quantum Mechanics of the Vacuum State in Two-Dimensional QCD with Adjoint Fermions
A study of two-dimensional QCD on a spatial circle with Majorana fermions in
the adjoint representation of the gauge groups SU(2) and SU(3) has been
performed. The main emphasis is put on the symmetry properties related to the
homotopically non-trivial gauge transformations and the discrete axial symmetry
of this model. Within a gauge fixed canonical framework, the delicate interplay
of topology on the one hand and Jacobians and boundary conditions arising in
the course of resolving Gauss's law on the other hand is exhibited. As a
result, a consistent description of the residual gauge symmetry (for
SU(N)) and the ``axial anomaly" emerges. For illustrative purposes, the vacuum
of the model is determined analytically in the limit of a small circle. There,
the Born-Oppenheimer approximation is justified and reduces the vacuum problem
to simple quantum mechanics. The issue of fermion condensates is addressed and
residual discrepancies with other approaches are pointed out.Comment: 44 pages; for hardcopies of figures, contact
[email protected]
A numerical study of multi-soliton configurations in a doped antiferromagnetic Mott insulator
We evaluate from first principles the self-consistent Hartree-Fock energies
for multi-soliton configurations in a doped, spin-1/2, antiferromagnetic Mott
insulator on a two-dimensional square lattice. We find that nearest-neighbor
Coulomb repulsion stabilizes a regime of charged meron-antimeron vortex soliton
pairs over a region of doping from 0.05 to 0.4 holes per site for intermediate
coupling 3 < U/t <8. This stabilization is mediated through the generation of
``spin-flux'' in the mean-field antiferromagnetic (AFM) background. Holes
cloaked by a meron-vortex in the spin-flux AFM background are charged bosons.
Our static Hartree-Fock calculations provide an upper bound on the energy of a
finite density of charged vortices. This upper bound is lower than the energy
of the corresponding charged stripe configurations. A finite density of charge
carrying vortices is shown to produce a large number of unoccupied electronic
levels in the Mott-Hubbard charge transfer gap. These levels lead to
significant band tailing and a broad mid-infrared band in the optical
absorption spectrum as observed experimentally. At very low doping (below 0.05)
the doping charges create extremely tightly bound meron-antimeron pairs or even
isolated conventional spin-polarons, whereas for very high doping (above 0.4)
the spin background itself becomes unstable to formation of a conventional
Fermi liquid and the spin-flux mean-field is energetically unfavorable. Our
results point to the predominance of a quantum liquid of charged, bosonic,
vortex solitons at intermediate coupling and intermediate doping
concentrations.Comment: 12 pages, 25 figures; added references, modified/eliminated some
figure
Extracting Br(omega->pi^+ pi^-) from the Time-like Pion Form-factor
We extract the G-parity-violating branching ratio Br(omega->pi^+ pi^-) from
the effective rho-omega mixing matrix element Pi_{rho omega}(s), determined
from e^+e^- -> pi^+ pi^- data. The omega->pi^+ pi^- partial width can be
determined either from the time-like pion form factor or through the constraint
that the mixed physical propagator D_{rho omega}^{mu nu}(s) possesses no poles.
The two procedures are inequivalent in practice, and we show why the first is
preferred, to find finally Br(omega->pi^+ pi^-) = 1.9 +/- 0.3%.Comment: 12 pages (published version
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