61 research outputs found
Does the Vacuum Gravitate on Microscopic Scales? Rydberg Atoms Indicate Probably Not
The cosmological constant presents one of the most fascinating and
confounding problems in physics. A straightforward, seemingly robust prediction
of quantum mechanics and general relativity is that the vacuum energy
gravitates. Therefore, the cosmological constant should be enormous. It is
minuscule. Since there is no understanding of why the cosmological constant is
so small, it is important to test this idea in many different situations. In
particular, given the span of distances in astronomy and particle physics, it
is vital to test the gravitation of vacuum energy on as many distance scales as
we can. Rydberg atoms open up a new set of distances for exploration. It is
satisfying to measure the cosmological constant with an atom, but its main
significance is extending measurements to microscopic distances. Here, too,
there is no evidence of the gravitation of the vacuum. At scales of a micron
and less, we place a limit of GeV on the scale of gravitating vacuum
energy, well below the scale of GeV of the SM of particle physics.Comment: Accepted in PR
Second Stage String Fragmentation Model
A string model, advocated by Bowler, provides a physical and intuitive
picture of heavy quark fragmentation. When supplemented by an ad hoc factor of
(1-z), to suppress fragmentation near z=1, it supplies an excellent fit to the
data. We extend Bowler's model by accounting for the further decay of the
massive mesonic states produced by the initial string breaking. We find that
each subsequent string break and cascade decay beyond the first, introduces a
factor of (1-z). Furthermore we find that including a finite mass for the
quarks, which pop out of the vacuum and split the string, forces the first
string breaking to produce massive states requiring further decay. This
sequence terminates at the second stage of fragmentation where only relatively
"light" heavy meson systems are formed. Thus we naturally account for the
phenomenologically required factor of (1-z). We also predict that the ratio of
(primary) fragments-vector/(vector plus scalar) should be .61. Our second stage
string fragmentation model provides an appealing picture of heavy quark
fragmentation.Comment: 15 page
Stability of the Black Hole Horizon and the Landau Ghost
The stability of the black hole horizon is demanded by both cosmic censorship
and the generalized second law of thermodynamics. We test the consistency of
these principles by attempting to exceed the black hole extremality condition
in various process in which a U(1) charge is added to a nearly extreme
Reissner--Nordstr\"om black hole charged with a {\it different\/} type of U(1)
charge. For an infalling spherical charged shell the attempt is foiled by the
self--Coulomb repulsion of the shell. For an infalling classical charge it
fails because the required classical charge radius exceeds the size of the
black hole. For a quantum charge the horizon is saved because in order to avoid
the Landau ghost, the effective coupling constant cannot be large enough to
accomplish the removal.Comment: 12 pages, RevTe
Intervenciones psicológicas basadas en mindfulness y sus beneficios: estado actual de la cuestión
Las intervenciones basadas en mindfulness (IBM) se fundamentan en antiguas prácticas del budismo, actualizadas y adaptadas al contexto occidental. En la presente revisión se describirán las técnicas más usadas, para a continuación analizar los mecanismos de acción que el mindfulness (i.e., atención plena) parece activar como intervención psicológica. En la segunda parte se revisarán los estudios de tipo aplicado. Se comenzará analizando los ensayos clínicos y metaanálisis relativos a las intervenciones sobre problemas mentales. En un segundo momento se analizarán los estudios de eficacia y metaanálisis relativos a los problemas de salud física. En la parte final se tratan algunos temas importantes, como los posibles efectos adversos o los perfiles más adecuados a este tipo de intervención, y se revisan los temas que requieren de más investigación, como la eficacia comparada con otras técnicas de intervención ya validadas. La conclusión global en función de la investigación revisada es que las IBM son técnicas de gran versatilidad, ideales para contextos de atención primaria o similares y que logran sus mayores tasas de eficacia en pacientes con sintomatología de estrés, ansiedad o depresión
Interaction of Crohn's Disease Susceptibility Genes in an Australian Paediatric Cohort
Genetic susceptibility is an important contributor to the pathogenesis of Crohn's disease (CD). We investigated multiple CD susceptibility genes in an Australian paediatric onset CD cohort. Newly diagnosed paediatric onset CD patients (n = 72) and controls (n = 98) were genotyped for 34 single nucleotide polymorphisms (SNPs) in 18 genetic loci. Gene-gene interaction analysis, gene-disease phenotype analysis and genetic risk profiling were performed for all SNPs and all genes. Of the 34 SNPs analysed, four polymorphisms on three genes (NOD2, IL23R, and region 3p21) were significantly associated with CD status (p<0.05). All three CD specific paediatric polymorphisms on PSMG1 and TNFRSF6B showed a trend of association with p<0.1. An additive gene-gene interaction involving TLR4, PSMG1, TNFRSF6B and IRGM was identified with CD. Genes involved in microbial processing (TLR4, PSMG1, NOD2) were significantly associated either at the individual level or in gene-gene interactive roles. Colonic disease was significantly associated with disease SNP rs7517847 (IL23R) (p<0.05) and colonic and ileal/colonic disease was significantly associated with disease SNP rs125221868 (IBD5) and SLC22A4 & SLC22A4/5 variants (p<0.05). We were able to demonstrate genetic association of several genes to CD in a paediatric onset cohort. Several of the observed associations have not been reported previously in association with paediatric CD patients. Our findings demonstrate that CD genetic susceptibility in paediatric patients presents as a complex interaction between numerous genes
Electron bunch generation from a plasma photocathode
Plasma waves generated in the wake of intense, relativistic laser or particle
beams can accelerate electron bunches to giga-electronvolt (GeV) energies in
centimetre-scale distances. This allows the realization of compact accelerators
having emerging applications, ranging from modern light sources such as the
free-electron laser (FEL) to energy frontier lepton colliders. In a plasma
wakefield accelerator, such multi-gigavolt-per-metre (GV m) wakefields
can accelerate witness electron bunches that are either externally injected or
captured from the background plasma. Here we demonstrate optically triggered
injection and acceleration of electron bunches, generated in a multi-component
hydrogen and helium plasma employing a spatially aligned and synchronized laser
pulse. This ''plasma photocathode'' decouples injection from wake excitation by
liberating tunnel-ionized helium electrons directly inside the plasma cavity,
where these cold electrons are then rapidly boosted to relativistic velocities.
The injection regime can be accessed via optical density down-ramp injection,
is highly tunable and paves the way to generation of electron beams with
unprecedented low transverse emittance, high current and 6D-brightness. This
experimental path opens numerous prospects for transformative plasma wakefield
accelerator applications based on ultra-high brightness beams
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