442 research outputs found
Pseudovector vs. pseudoscalar coupling in one-boson exchange NN potentials
We examine the effects of pseudoscalar and pseudovector coupling of the pi
and eta mesons in one-boson exchange models of the NN interaction using two
approaches: time-ordered perturbation theory unitarized with the relativistic
Lippmann-Schwinger equation, and a reduced Bethe-Salpeter equation approach
using the Thompson equation. Contact terms in the one-boson exchange amplitudes
in time-ordered perturbation theory lead naturally to the introduction of
s-channel nucleonic cutoffs for the interaction, which strongly suppresses the
far off-shell behavior of the amplitudes in both approaches. Differences
between the resulting NN predictions of the various models are found to be
small, and particularly so when coupling constants of the other mesons are
readjusted within reasonable limits.Comment: 24 pages, 4 figure
Threshold meson production and cosmic ray transport
An interesting accident of nature is that the peak of the cosmic ray
spectrum, for both protons and heavier nuclei, occurs near the pion production
threshold. The Boltzmann transport equation contains a term which is the cosmic
ray flux multiplied by the cross section. Therefore when considering pion and
kaon production from proton-proton reactions, small cross sections at low
energy can be as important as larger cross sections at higher energy. This is
also true for subthreshold kaon production in nuclear collisions, but not for
subthreshold pion production.Comment: 9 pages, 1 figur
Dissipation, noise and DCC domain formation
We investigate the effect of friction on domain formation in disoriented
chiral condensate. We solve the equation of motion of the linear sigma model,
in the Hartree approximation, including a friction and a white noise term. For
quenched initial condition, we find that even in presence of noise and
dissipation domain like structure emerges after a few fermi of evolution.
Domain size as large as 5 fm can be formed.Comment: 7 pages, 3 figure
Effect of friction on disoriented chiral condensate formation
We have investigated the effect of friction on the DCC domain formation. We
solve the Newton equation of motion for the O(4) fields, with quenched initial
condition. The initial fields are randomly distributed in a Gaussian form. In
one dimensional expansion, on the average, large DCC domains can not be formed.
However, in some particular orbits, large instabilities may occur. This
possibility also greatly diminishes with the introduction of friction. But, if
the friction is large, the system may be overdamped and then, there is a
possibility of large DCC domain formation in some events.Comment: 9 pages, including figure
Alignment in Gamma-Hadron Families of Cosmic Rays
Alignment of main fluxes of energy in a target plane is found in families of
cosmic ray particles detected in deep lead X-ray chambers. The fraction of
events with alignment is unexpectedly large for families with high energy and
large number of hadrons. This can be considered as evidence for the existence
of coplanar scattering of secondary particles in interaction of particles with
superhigh energy, eV. Data analysis suggests that production of
most aligned groups occurs low above the chamber and is characterized by a
coplanar scattering and quasiscaling spectrum of secondaries in the
fragmentation region. The most elaborated hypothesis for explanation of
alignment is related to the quark-gluon string rupture. However, the problem of
theoretical interpretation of our results still remains open.Comment: 15 pages, 2 tables, 6 figures (not included), Stanford University
preprint SU-ITP-94-2
Multiplicity Distributions of Squeezed Isospin States
Multiplicity distributions of neutral and charged particles arising from
squeezed coherent states are investigated. Projections onto global isospin
states are considered. We show how a small amount of squeezing can
significantly change the multiplicity distributions. The formalism is proposed
to describe the phenomenological properties of neutral and charged particles
anomalously produced in hadronic and nuclear collisions at very high energies.Comment: 17 pages, 6 figures sent upon request ([email protected]
Thermoacoustic tomography with an arbitrary elliptic operator
Thermoacoustic tomography is a term for the inverse problem of determining of
one of initial conditions of a hyperbolic equation from boundary measurements.
In the past publications both stability estimates and convergent numerical
methods for this problem were obtained only under some restrictive conditions
imposed on the principal part of the elliptic operator. In this paper
logarithmic stability estimates are obatined for an arbitrary variable
principal part of that operator. Convergence of the Quasi-Reversibility Method
to the exact solution is also established for this case. Both complete and
incomplete data collection cases are considered.Comment: 16 page
Can Disordered Chiral Condensates Form? A Dynamical Perspective
We address the issue of whether a region of disordered chiral condensate
(DCC), in which the chiral condensate has components along the pion
directions, can form. We consider a system going through the chiral phase
transition either via a quench, or via relaxation of the high temperature phase
to the low temperature one within a given time scale (of order ).
We use a density matrix based formalism that takes both thermal and quantum
fluctuations into account non-perturbatively to argue that if the linear
sigma model is the correct way to model the situation in QCD, then it is very
unlikely at least in the Hartree approximation, that a large ()
DCC region will form. Typical sizes of such regions are
and the density of pions in such regions is at most of order . We end with some speculations on how large DCC regions may be
formed.Comment: 21 pages LATEX, 12 figures available upon request via regular mail,
PITT-94-0
Strangelet spectra from type II supernovae
We study in this work the fate of strangelets injected as a contamination in
the tail of a "strange matter-driven" supernova shock. A simple model for the
fragmentation and braking of the strangelets when they pass through the
expanding oxygen shell is presented and solved to understand the reprocessing
of this component. We find that the escaping spectrum is a scaled-down version
of the one injected at the base of the oxygen shell. The supernova source is
likely to produce low-energy particles of quite independently
of the initial conditions. However, it is difficult that ultrarrelativistic
strangelets (such as the hypothetical Centauro primaries) can have an origin in
those explosive events.Comment: RevTex file, 5 pp., no figure
VIH/SIDA: COMPORTAMIENTO EPIDEMIOLÓGICO DE LA TRANSMISIÓN VERTICAL EN EL CONTEXTO GENERAL DE LA INFECCIÓN EN CHILE
Antecedentes: En embarazadas seropositivas sin profilaxis antirretroviral la transmisión vertical (TV) del VIH es de 30%, cifra que disminuye bajo al 2% con un manejo integral de prevención. Objetivo: Conocer el comportamiento epidemiológico de la TV en Chile desde la creación del programa nacional de prevención. Método: Datos aportados por el Departamento de Epidemiología del MINSAL y CONASIDA (1984-2006). Estudió de situación VIH/SIDA por año, regiones, grupo etario y sexo, con especial énfasis en TV. Resultados: Se notificaron 9.317 casos de VIH y 7.886 casos de SIDA (1984-2006), prevalencia en aumento hasta el 2003 con tendencia descendente posteriormente. La razón actual de VIH entre hombres y mujeres es de 4:1, con clara tendencia al aumento de notificación en mujeres. La principal vía de exposición sigue siendo la vía sexual (93,4%). El grupo más afectado está entre los 20-39 años (73,1% para VIH y 63,1% para SIDA), cabe destacar que entre 0-9 años se encuentra el 1,1% de los afectados por VIH y el 1,4% por SIDA, todos infectados por TV. Conclusiones: La transmisión vertical de VIH en Chile es responsable de un bajo porcentaje de las personas notificadas de VIH/SIDA, pero es la causa de la totalidad de los niños afectados por la infección en nuestro país. Se observa una importante reducción de la TV, llegando a niveles muy cercanos a los objetivos ministeriales
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