17,834 research outputs found
Constraints on the Wtb vertex from early LHC data
We use the recent measurements of top quark decay asymmetries in ATLAS and
the t-channel single top cross section in CMS to set the first combined LHC
limits on the Wtb vertex. This combination allows to obtain much better limits
than the separate measurements. The resulting constraints are comparable,
although still weaker, than the ones obtained using Tevatron data with much
more statistics.Comment: RevTeX 4 page
Breaking the Rayleigh-Plateau instability limit using thermocavitation within a droplet
We report on the generation of liquid columns that extend far beyond the traditional Rayleigh-Plateau instability onset. The columns are driven by the acoustic pressure wave emitted after bubble collapse. A high-speed video imaging device, which records images at a rate of up to 105 fps, was employed to follow their dynamics. These bubbles, commonly termed thermocavitation bubbles, are generated by focusing a midpower (275 mW) continuous wavelength laser into a highly absorbing liquid droplet. A simple model of the propagation of the pressure wavefront emitted after the bubble collapse shows that focusing the pressure wave at the liquid-air interface drives the evolution of the liquid columns. Control over the aspect ratio of the liquid column is realized by adjusting the cavitation bubble's size, beam focus position, and droplet volume. © 2013 by Begell House, Inc
FCNCs in supersymmetric multi-Higgs doublet models
We conduct a general discussion of supersymmetric models with three families
in the Higgs sector. We analyse the scalar potential, and investigate the
minima conditions, deriving the mass matrices for the scalar, pseudoscalar and
charged states. Depending on the Yukawa couplings and the Higgs spectrum, the
model might allow the occurrence of potentially dangerous flavour changing
neutral currents at the tree-level. We compute model-independent contributions
for several observables, and as an example we apply this general analysis to a
specific model of quark-Higgs interactions, discussing how compatibility with
current experimental data constrains the Higgs sector.Comment: 30 pages, 9 figures. Comments and references added. Final version
published in Physical Review
Smoothed Particle Hydrodynamics simulations of the core-degenerate scenario for Type Ia supernovae
The core-degenerate (CD) scenario for type Ia supernovae (SN Ia) involves the
merger of the hot core of an asymptotic giant branch (AGB) star and a white
dwarf, and might contribute a non-negligible fraction of all thermonuclear
supernovae. Despite its potential interest, very few studies, and based on only
crude simplifications, have been devoted to investigate this possible scenario,
compared with the large efforts invested to study some other scenarios. Here we
perform the first three-dimensional simulations of the merger phase, and find
that this process can lead to the formation of a massive white dwarf, as
required by this scenario. We consider two situations, according to the mass of
the circumbinary disk formed around the system during the final stages of the
common envelope phase. If the disk is massive enough, the stars merge on a
highly eccentric orbit. Otherwise, the merger occurs after the circumbinary
disk has been ejected and gravitational wave radiation has brought the stars
close to the Roche lobe radius on a nearly circular orbit. Not surprisingly,
the overall characteristics of the merger remnants are similar to those found
for the double-degenerate (DD) scenario, independently of the very different
core temperature and of the orbits of the merging stars. They consist of a
central massive white dwarf, surrounded by a hot, rapidly rotating corona and a
thick debris region.Comment: 17 pages, 10 figures. Accepted for publication in MNRA
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High repetition rate femtosecond laser heat accumulation and ablation thresholds in cobalt-binder and binderless tungsten carbides
Femtosecond (fs) laser ablation has been studied for the potential of fast, high precision machining of difficult-to-machine materials like binderless tungsten carbide. Obstacles that have limited its efficiency include melting from heat accumulation (HA), particle shielding, and plasma shielding. To address HA without shielding effects, high repetition rate (57.4 MHz), ultra-low fluence fs laser irradiation is performed to study the incubation effect and subsequent HA-ablation threshold of fine-grained tungsten carbides. Exposure times on the order of 100 ms were conducted in air with fluences (1.82 to 9.09 mJ/cm2) two orders of magnitude below the single fs pulse ablation thresholds reported in literature (0.4 J/cm2). Heat accumulation at high repetition rate explains the ultra-low fluence melt threshold behavior resulting in melt crowns around ablated holes and grooves. The results of this study aid in predicting heat buildup in high repetition rate laser irradiation for applications that wish to achieve high ablation rates of difficult-to-machine, ultrahard materials and help enable shaping of binderless tungsten carbide for use in applications too extreme for bindered tungsten carbide
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