1,650 research outputs found
Zeros of tree amplitudes at rest and symmetries of mechanical systems
We consider the tree amplitudes of production of scalar particles by
particles of another kind, where both initial and final particles are at
rest and on mass shell, in a model of two scalar fields with symmetric
interaction and unequal masses. We find that these amplitudes are zero except
for the lowest possible and , and that the cancellation of the
corresponding Feynman graphs occurs due to a special symmetry of the classical
mechanical counterpart of this theory. This feature is rather general and is
inherent in various other scalar field theories.Comment: 11 pages Preprint INR-830/93, August 199
Overview of the SME: Implications and Phenomenology of Lorentz Violation
The Standard Model Extension (SME) provides the most general
observer-independent field theoretical framework for investigations of Lorentz
violation. The SME lagrangian by definition contains all Lorentz-violating
interaction terms that can be written as observer scalars and that involve
particle fields in the Standard Model and gravitational fields in a generalized
theory of gravity. This includes all possible terms that could arise from a
process of spontaneous Lorentz violation in the context of a more fundamental
theory, as well as terms that explicitly break Lorentz symmetry. An overview of
the SME is presented, including its motivations and construction. Some of the
theoretical issues arising in the case of spontaneous Lorentz violation are
discussed, including the question of what happens to the Nambu-Goldstone modes
when Lorentz symmetry is spontaneously violated and whether a Higgs mechanism
can occur. A minimal version of the SME in flat Minkowski spacetime that
maintains gauge invariance and power-counting renormalizability is used to
search for leading-order signals of Lorentz violation. Recent Lorentz tests in
QED systems are examined, including experiments with photons, particle and
atomic experiments, proposed experiments in space and experiments with a
spin-polarized torsion pendulum.Comment: 40 pages, Talk presented at Special Relativity: Will it Survive the
Next 100 Years? Potsdam, Germany, February, 200
Electromagnetic transitions between giant resonances within a continuum-RPA approach
A general continuum-RPA approach is developed to describe electromagnetic
transitions between giant resonances. Using a diagrammatic representation for
the three-point Green's function, an expression for the transition amplitude is
derived which allows one to incorporate effects of mixing of single and double
giant resonances as well as to take the entire basis of particle-hole states
into consideration. The radiative widths for E1 transition between the
charge-exchange spin-dipole giant resonance and Gamow-Teller states are
calculated for ^{90}Nb and ^{208}Bi nuclei. The importance of the mixing is
stressed.Comment: 10 pages, 2 figures, uses elsart.st
Dynamical stabilization of runaway potentials at finite density
We study four dimensional non-abelian gauge theories with classical moduli.
Introducing a chemical potential for a flavor charge causes moduli to become
unstable and start condensing. We show that the moduli condensation in the
presence of a chemical potential generates nonabelian field strength
condensates. These condensates are homogeneous but non-isotropic. The end point
of the condensation process is a stable homogeneous, but non-isotropic, vacuum
in which both gauge and flavor symmetries and the rotational invariance are
spontaneously broken. Possible applications of this phenomenon for the gauge
theory/string theory correspondence and in cosmology are briefly discussed.Comment: revtex4, 4 pages; v.2: journal versio
Gluonic phase in neutral two-flavor dense QCD
In the Ginzburg-Landau approach, we describe a new phase in neutral
two-flavor quark matter in which gluonic degrees of freedom play a crucial
role. We call it a gluonic phase. In this phase gluonic dynamics cure a
chromomagnetic instability in the 2SC solution and lead to spontaneous
breakdown of the color gauge symmetry, the electromagnetic U(1), and the
rotational SO(3). In other words, the gluonic phase describes an anisotropic
medium in which the color and electric superconductivities coexist. Because
most of the initial symmetries in this system are spontaneously broken, its
dynamics is very rich.Comment: Revtex4, 11 pages, clarifications and new section added, references
added, to appear in Phys. Lett.
Preeclampsia is associated with compromized maternal synthesis of long chain polyunsaturated fatty acids leading to offspring deficiency
Obesity and excessive lipolysis are implicated in preeclampsia (PE). Intrauterine growth restriction is associated with low maternal body mass index and decreased lipolysis. Our aim was to assess how maternal and offspring fatty acid metabolism is altered in mothers in the third trimester of pregnancy with PE (n=62) or intrauterine growth restriction (n=23) compared with healthy pregnancies (n=164). Markers of lipid metabolism and erythrocyte fatty acid concentrations were measured. Maternal adipose tissue fatty acid composition and mRNA expression of adipose tissue fatty acid–metabolizing enzymes and placental fatty acid transporters were compared. Mothers with PE had higher plasma triglyceride (21%, P<0.001) and nonesterified fatty acid (50%, P<0.001) concentrations than controls. Concentrations of major n−6 and n−3 long-chain polyunsaturated fatty acids in erythrocytes were 23% to 60% lower (all P<0.005) in PE and intrauterine growth restriction mothers and offspring compared with controls. Subcutaneous adipose tissue Δ−5 and Δ−6 desaturase and very long-chain fatty acid elongase mRNA expression was lower in PE than controls (respectively, mean [SD] control 3.38 [2.96] versus PE 1.83 [1.91], P=0.030; 3.33 [2.25] versus 1.03 [0.96], P<0.001; 0.40 [0.81] versus 0.00 [0.00], P=0.038 expression relative to control gene [square root]). Low maternal and fetal long-chain polyunsaturated fatty acid concentrations in PE may be the result of decreased maternal synthesis
Tree amplitudes at multiparticle threshold in a model with softly broken symmetry
Tree amplitudes of the production of two kinds of scalar particles at
threshold from one virtual particle are calculated in a model of two scalar
fields with symmetric quartic interaction and unequal masses. These
amplitudes exhibit interesting factorial and exponential behaviour at large
multiplicities. As a by-product we observe that the kinematically allowed decay
of one real particle into real particles of another kind, all at rest, has
zero tree amplitude in this model for .Comment: 17 pages. Preprint INR-823/9
Semimicroscopical description of the simplest photonuclear reactions accompanied by excitation of the giant dipole resonance in medium-heavy mass nuclei
A semimicroscopical approach is applied to describe photoabsorption and
partial photonucleon reactions accompanied by the excitation of the giant
dipole resonance (GDR). The approach is based on the continuum-RPA (CRPA) with
a phenomenological description for the spreading effect. The phenomenological
isoscalar part of the nuclear mean field, momentum-independent Landau-Migdal
particle-hole interaction, and separable momentum-dependent forces are used as
input quantities for the CRPA calculations. The experimental photoabsorption
and partial -reaction cross sections in the vicinity of the GDR are
satisfactorily described for Y, Ce and Pb target nuclei.
The total direct-neutron-decay branching ratio for the GDR in Ca and
Pb is also evaluated.Comment: 19 pages, 5 eps figure
Meson masses in large Nf QCD from the Bethe-Salpeter equation
We solve the homogeneous Bethe-Salpeter (HBS) equation for the scalar,
pseudoscalar, vector, and axial-vector bound states of quark and anti-quark in
large Nf QCD with the improved ladder approximation in the Landau gauge. The
quark mass function in the HBS equation is obtained from the Schwinger-Dyson
(SD) equation in the same approximation for consistency with the chiral
symmetry. Amazingly, due to the fact that the two-loop running coupling of
large Nf QCD is explicitly written in terms of an analytic function, large Nf
QCD turns out to be the first example in which the SD equation can be solved in
the complex plane and hence the HBS equation directly in the time-like region.
We find that approaching the chiral phase transition point from the broken
phase, the scalar, vector, and axial-vector meson masses vanish to zero with
the same scaling behavior, all degenerate with the massless pseudoscalar meson.
This may suggest a new type of manifestation of the chiral symmetry restoration
in large Nf QCD.Comment: 33 pages, 16 figures. Typos are corrected. Minor corrections and
references are added. Version to appear in Phys. Rev.
Charge Distribution in Soft Chiral Pion Bremsstrahlung
Bremsstrahlung of soft chiral pions in high-energy processes is considered.
The distribution over number of neutral and charged pions is shown to obey the
inverse square root law. This law is argued to be generic for multiple coherent
production of soft chiral pions.Comment: 10 page
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