690 research outputs found
Amplitude Zeros in Production
We demonstrate that the Standard Model amplitude for at the Born-level exhibits an approximate zero located at
at
high energies, where the () are the left-handed couplings
of the -boson to fermions and is the center of mass scattering
angle of the -boson. The approximate zero is the combined result of an exact
zero in the dominant helicity amplitudes and strong gauge
cancelations in the remaining amplitudes. For non-standard couplings
these cancelations no longer occur and the approximate amplitude zero is
eliminated.Comment: 11 pages, 4 figures submitted separately as uuencoded tar-ed
postscript files, FSU-HEP-940307, UCD-94-
Plasticity and memory effects in the vortex solid phase of twinned YBa2Cu3O7 single crystals
We report on marked memory effects in the vortex system of twinned YBa2Cu3O7
single crystals observed in ac susceptibility measurements. We show that the
vortex system can be trapped in different metastable states with variable
degree of order arising in response to different system histories. The pressure
exerted by the oscillating ac field assists the vortex system in ordering,
locally reducing the critical current density in the penetrated outer zone of
the sample. The robustness of the ordered and disordered states together with
the spatial profile of the critical current density lead to the observed memory
effects
Propiedades químicas, color y humectabilidad de partículas de "Laureliopsis philippiana" (tepa) con y sin tratamiento térmico
The purpose of this work was to determine the effect of a thermal treatment in a vapor water saturated atmosphere,
on wood particles of Laureliopsis philippiana. The particles in autoclave at 150 ºC for 90 minutes and with a pressure
of 430 kPa were thermally treated, the pressure generated by the evaporation of water in the sealed autoclave.
The amount of extractives in soda, ethanol-toluene, cold and hot water, as well as, the percentages of cellulose,
holocellulose and lignin, the pH value, the percentages of volatiles and washables acids, and buffer capacity, for both
particles without and with treatment, were determined. In thermally treated particles an increase in the percentage of
extractives, cellulose and volatiles and water soluble acids was found, as well as, in thermally treated particles an
increment of acidity and a higher buffer capacity was found, in comparison with the particles without treatment. The
thermal treatment produced a color change in the particles and a reduction in their wettability. Due to the chemical
changes found in the treated particles they could present advantages in the particleboard production, allowing a
better ureaformaldehyde polymerization.El propósito de este trabajo fue determinar el efecto de un tratamiento térmico, en una atmósfera saturada de vapor
de agua, en partículas de madera de la especie Laureliopsis philippiana. Las partículas fueron tratadas en autoclave
a 150 ºC por 90 minutos y a una presión de 430 kPa, la cual se generó durante el proceso por la evaporación del agua
en el autoclave herméticamente cerrado. Se determinaron la cantidad de extraíbles en soda, etanol tolueno, agua
caliente y fría, así también, los porcentaje de celulosa, holocelulosa y lignina, el valor pH, los porcentajes de ácidos
volátiles y solubles en agua, y la capacidad tampón, tanto para partículas sin tratamiento como con tratamiento.
Para las partículas con tratamiento térmico, se encontró un aumento en el porcentaje de extraíbles, de celulosa, y
de ácidos volátiles y lavables, así como también, un aumento de la acidez y de una mayor capacidad tampón, en
comparación con las partículas sin tratamiento. El tratamiento térmico produjo un cambio de color en las partículas
y una reducción de la humectabilidad. Debido a los cambios químicos encontrados en las partículas tratadas, estas
presentarían ventajas en la fabricación de tableros de partículas, favoreciendo el fraguado de la ureaformaldehído
Propiedades químicas, color y humectabilidad de partículas de Laureliopsis philippiana (TEPA) con y sin tratamiento térmico
El propósito de este trabajo fue determinar el efecto de un tratamiento térmico, en una atmósfera saturada de vapor de agua, en partículas de madera de la especie Laureliopsis philippiana. Las partículas fueron tratadas en autoclave a 150 ºC por 90 minutos y a una presión de 430 kPa, la cual se generó durante el proceso por la evaporación del agua en el autoclave herméticamente cerrado. Se determinaron la cantidad de extraíbles en soda, etanol tolueno, agua caliente y fría, así también, los porcentaje de celulosa, holocelulosa y lignina, el valor pH, los porcentajes de ácidos volátiles y solubles en agua, y la capacidad tampón, tanto para partículas sin tratamiento como con tratamiento. Para las partículas con tratamiento térmico, se encontró un aumento en el porcentaje de extraíbles, de celulosa, y de ácidos volátiles y lavables, así como también, un aumento de la acidez y de una mayor capacidad tampón, en comparación con las partículas sin tratamiento. El tratamiento térmico produjo un cambio de color en las partículas y una reducción de la humectabilidad. Debido a los cambios químicos encontrados en las partículas tratadas, estas presentarían ventajas en la fabricación de tableros de partículas, favoreciendo el fraguado de la ureaformaldehído. AbstractThe purpose of this work was to determine the effect of a thermal treatment in a vapor water saturated atmosphere, on wood particles of Laureliopsis philippiana. The particles in autoclave at 150 ºC for 90 minutes and with a pressure of 430 kPa were thermally treated, the pressure generated by the evaporation of water in the sealed autoclave. The amount of extractives in soda, ethanol-toluene, cold and hot water, as well as, the percentages of cellulose, holocellulose and lignin, the pH value, the percentages of volatiles and washables acids, and buffer capacity, for both particles without and with treatment, were determined. In thermally treated particles an increase in the percentage of extractives, cellulose and volatiles and water soluble acids was found, as well as, in thermally treated particles an increment of acidity and a higher buffer capacity was found, in comparison with the particles without treatment. The thermal treatment produced a color change in the particles and a reduction in their wettability. Due to the chemical changes found in the treated particles they could present advantages in the particleboard production, allowing a better ureaformaldehyde polymerization
Equilibration and Dynamic Phase Transitions of a Driven Vortex Lattice
We report on the observation of two types of current driven transitions in
metastable vortex lattices. The metastable states, which are missed in usual
slow transport measurements, are detected with a fast transport technique in
the vortex lattice of undoped
2H-NbSe. The transitions are seen by following the evolution of these
states when driven by a current. At low currents we observe an equilibration
transition from a metastable to a stable state, followed by a dynamic
crystallization transition at high currents.Comment: 5 pages, 4 figure
Interaction-induced Fermi surface deformations in quasi one-dimensional electronic systems
We consider serious conceptual problems with the application of standard
perturbation theory, in its zero temperature version, to the computation of the
dressed Fermi surface for an interacting electronic system. In order to
overcome these difficulties, we set up a variational approach which is shown to
be equivalent to the renormalized perturbation theory where the dressed Fermi
surface is fixed by recursively computed counterterms. The physical picture
that emerges is that couplings that are irrelevant tend to deform the Fermi
surface in order to become more relevant (irrelevant couplings being those that
do not exist at vanishing excitation energy because of kinematical constraints
attached to the Fermi surface). These insights are incorporated in a
renormalization group approach, which allows for a simple approximate
computation of Fermi surface deformation in quasi one-dimensional electronic
conductors. We also analyze flow equations for the effective couplings and
quasiparticle weights. For systems away from half-filling, the flows show three
regimes corresponding to a Luttinger liquid at high energies, a Fermi liquid,
and a low-energy incommensurate spin-density wave. At half-filling Umklapp
processes allow for a Mott insulator regime where the dressed Fermi surface is
flat, implying a confined phase with vanishing effective transverse
single-particle coherence. The boundary between the confined and Fermi liquid
phases is found to occur for a bare transverse hopping amplitude of the order
of the Mott charge gap of a single chain.Comment: 38 pages, 39 figures. Accepted for publication in Phys. Rev.
Cosmological neutrinos
The current status of neutrino cosmology is reviewed, from the question of
neutrino decoupling and the presence of sterile neutrinos to the effects of
neutrinos on the cosmic microwave background and large scale structure.
Particular emphasis is put on cosmological neutrino mass measurements.Comment: 21 pages, 4 figures, review for NJP focus issue on neutrino
Stability of metallic stripes in the extended one-band Hubbard model
Based on an unrestricted Gutzwiller approximation (GA) we investigate the
stripe orientation and periodicity in an extended one-band Hubbard model. A
negative ratio between next-nearest and nearest neighbor hopping t'/t, as
appropriate for cuprates, favors partially filled (metallic) stripes for both
vertical and diagonal configurations. At around optimal doping diagonal
stripes, site centered (SC) and bond centered (BC) vertical stripes become
degenerate suggesting strong lateral and orientational fluctuations. We find
that within the GA the resulting phase diagram is in agreement with experiment
whereas it is not in the Hartree-Fock approximation due to a strong
overestimation of the stripe filling. Results are in agreement with previous
calculations within the three-band Hubbard model but with the role of SC and BC
stripes interchanged.Comment: 10 pages, 8 figure
Production at the Fermilab Tevatron Collider: Gauge Invariance and Radiation Amplitude Zero
The electroweak process is calculated
at tree level, including finite W width effects. In order to obtain a gauge
invariant amplitude, the imaginary parts of triangle graphs and
box diagrams have to be included, in addition to resumming the
imaginary contributions to the W polarization. We demonstrate the existence of
a radiation amplitude zero in , and discuss how it may be observed in correlations of
the and lepton rapidities at the Fermilab Tevatron.Comment: 37 pages,12 figures, postscript file also available via anonymous ftp
at ftp://ucdhep.ucdavis.edu/han/vvv/Wgmgm.p
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