Zeros of tree amplitudes at rest and symmetries of mechanical systems

We consider the tree amplitudes of production of $n_2$ scalar particles by $n_1$ 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 $O(2)$ symmetric interaction and unequal masses. We find that these amplitudes are zero except for the lowest possible $n_1$ and $n_2$, 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 O(2)O(2) 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 $O(2)$ 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 $n$ real particles of another kind, all at rest, has zero tree amplitude in this model for $n>2$.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 $(n,\gamma)$-reaction cross sections in the vicinity of the GDR are satisfactorily described for $^{89}$Y, $^{140}$Ce and $^{208}$Pb target nuclei. The total direct-neutron-decay branching ratio for the GDR in $^{48}$Ca and $^{208}$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