209 research outputs found
Evolution of an elliptical bubble in an accelerating extensional flow
Mathematical models that describe the dynamical behavior of a thin gas bubble embedded in a glass fiber during a fiber drawing process have been discussed and analyzed.
The starting point for the mathematical modeling was the equations presented in [1] for a glass fiber with a hole undergoing extensional flow. These equations were reconsidered here with the additional reduction that the hole, i.e. the gas bubble, was thin as compared to the radius of the fiber and of finite extent. The primary model considered was one in which the mass of the gas inside the bubble was fixed. This fixed-mass model involved equations for the axial velocity and fiber radius, and equations for the radius of the bubble and the gas pressure inside the bubble. The model equations assumed that the temperature of the furnace of the drawing tower was known.
The governing equations of the bubble are hyperbolic and predict that the bubble cannot extend beyond the limiting characteristics specified by the ends of the initial bubble shape. An analysis of pinch-off was performed, and it was found that pinch-off can occur, depending on the parameters of the model, due to surface tension when the bubble radius is small.
In order to determine the evolution of a bubble, a numerical method of solution was presented. The method was used to study the evolution of two different initial bubble shapes, one convex and the other non-convex. Both initial bubble shapes had fore-aft symmetry, and it was found that the bubbles stretched and elongated severely during the drawing process. For the convex shape, fore-aft symmetry was lost in the middle of the drawing process, but the symmetry was re-gained by the end of the drawing tower. A small amount of pinch-off was observed at each end for this case, so that the final bubble length was slightly shorter than its theoretical maximum length. For the non-convex initial shape, pinch-off occurred in the middle of the bubble resulting in two bubbles by the end of the fiber draw.
The two bubbles had different final pressures and did not have fore-aft symmetry.
An extension of the fixed-mass model was considered in which the gas in the bubble was allowed to diffuse into the surrounding glass. The governing equations for this leaky-mass model were developed and manipulated into a form suitable for a numerical treatment
Measurement of the neutron-neutron effective range in neutron-deuteron breakup
We report the most precise determination of the neutron-neutron effective range parameter from neutron-neutron quasifree scattering in neutron-deuteron breakup. The experiment setup utilized a collimated beam of 15.5 MeV neutrons and an array of eight neutron detectors positioned at angles sensitive to several quasifree scattering kinematic configurations. The two neutrons emitted from the breakup reaction were detected in coincidence and time-of-flight techniques were used to determine their energies. The beam-target luminosity was measured in-situ with the yields from neutron-deuteron elastic scattering. Rigorous Faddeev-type calculations using the CD Bonn nucleon-nucleon potential were fit to our cross-section data to determine the value of . The analysis was repeated using a semilocal momentum-space regularized chiral interaction potential. We obtained values of fm and fm using the CD Bonn and potentials, respectively. Our results are consistent with charge symmetry and previously reported values of
Z(2)-Singlino Dark Matter in a Portal-Like Extension of the Minimal Supersymmetric Standard Model.
We propose a Z2-stabilized singlino () as a dark matter candidate in extended and R-parity violating versions of the supersymmetric standard model. interacts with visible matter via a heavy messenger field S, which results in a supersymmetric version of the Higgs portal interaction. The relic abundance of can account for cold dark matter if the messenger mass satisfies GeV. Our model can be implemented in many realistic supersymmetric models such as the next-to-minimal supersymmetric (SUSY) standard model and nearly minimal SUSY standard model
Pulsars as the Source of the WMAP Haze
The WMAP haze is an excess in the 22 to 93 GHz frequency bands of WMAP
extending about 10 degrees from the galactic center. We show that synchrotron
emission from electron-positron pairs injected into the interstellar medium by
the galactic population of pulsars with energies in the 1 to 100 GeV range can
explain the frequency spectrum of the WMAP haze and the drop in the average
haze power with latitude. The same spectrum of high energy electron-positron
pairs from pulsars, which gives rise to the haze, may also generate the
observed excesses in AMS, HEAT and PAMELA. We discuss the spatial morphology of
the pulsar synchrotron signal and its deviation from spherical symmetry, which
may provide an avenue to determine the pulsar contribution to the haze.Comment: 18 pages, 4 figures. Corrected errors in fig 1-3 and added discussion
of the detailed spatial morphology of the haze signa
Cosmic Ray Anomalies from the MSSM?
The recent positron excess in cosmic rays (CR) observed by the PAMELA
satellite may be a signal for dark matter (DM) annihilation. When these
measurements are combined with those from FERMI on the total () flux
and from PAMELA itself on the ratio, these and other results are
difficult to reconcile with traditional models of DM, including the
conventional mSUGRA version of Supersymmetry even if boosts as large as
are allowed. In this paper, we combine the results of a previously
obtained scan over a more general 19-parameter subspace of the MSSM with a
corresponding scan over astrophysical parameters that describe the propagation
of CR. We then ascertain whether or not a good fit to this CR data can be
obtained with relatively small boost factors while simultaneously satisfying
the additional constraints arising from gamma ray data. We find that a specific
subclass of MSSM models where the LSP is mostly pure bino and annihilates
almost exclusively into pairs comes very close to satisfying these
requirements. The lightest in this set of models is found to be
relatively close in mass to the LSP and is in some cases the nLSP. These models
lead to a significant improvement in the overall fit to the data by an amount
dof in comparison to the best fit without Supersymmetry
while employing boosts . The implications of these models for future
experiments are discussed.Comment: 57 pages, 31 figures, references adde
Dark Matter Searches: The Nightmare Scenario
The unfortunate case where the Large Hadron Collider (LHC) fails to discover
physics Beyond the Standard Model (BSM) is sometimes referred to as the
"Nightmare scenario" of particle physics. We study the consequences of this
hypothetical scenario for Dark Matter (DM), in the framework of the constrained
Minimal Supersymmetric Standard Model (cMSSM). We evaluate the surviving
regions of the cMSSM parameter space after null searches at the LHC, using
several different LHC configurations, and study the consequences for DM
searches with ton-scale direct detectors and the IceCube neutrino telescope. We
demonstrate that ton-scale direct detection experiments will be able to
conclusively probe the cMSSM parameter space that would survive null searches
at the LHC with 100fb of integrated luminosity at 14TeV. We also
demonstrate that IceCube (80 strings plus DeepCore) will be able to probe as
much as 17% of the currently favoured parameter space after 5 years of
observation.Comment: V2: 24 pages, 6 figures, 4 tables. Replaced to match version
published in JCAP. Minor revisions made to address referee's comment
Direct detection of dark matter in models with a light Z'
We discuss the direct detection signatures of dark matter interacting with
nuclei via a Z' mediator, focussing on the case where both the dark matter and
the have mass of a few GeV. Isospin violation (i.e. different couplings to
protons and neutrons) arises naturally in this scenario. In particular it is
possible to reconcile the preferred parameter regions inferred from the
observed DAMA and CoGeNT modulations with the bounds from XENON100, which
requires f_n/f_p = -0.7. Moreover, the Z' mediator can also yield a large
spin-dependent cross-section which could contribute to the DAMA signal, while
the spin-independent cross-section is adequate to explain the CoGeNT signal.Comment: 20 pages, 3 figures. v2: matches published versio
Probing new physics with long-lived charged particles produced by atmospheric and astrophysical neutrinos
As suggested by some extensions of the Standard Model of particle physics,
dark matter may be a super-weakly interacting lightest stable particle, while
the next-to-lightest particle (NLP) is charged and meta-stable. One could test
such a possibility with neutrino telescopes, by detecting the charged NLPs
produced in high-energy neutrino collisions with Earth matter. We study the
production of charged NLPs by both atmospheric and astrophysical neutrinos;
only the latter, which is largely uncertain and has not been detected yet, was
the focus of previous studies. We compute the resulting fluxes of the charged
NLPs, compare those of different origins, and analyze the dependence on the
underlying particle physics setup. We point out that even if the astrophysical
neutrino flux is very small, atmospheric neutrinos, especially those from the
prompt decay of charmed mesons, may provide a detectable flux of NLP pairs at
neutrino telescopes such as IceCube. We also comment on the flux of charged
NLPs expected from proton-nucleon collisions, and show that, for theoretically
motivated and phenomenologically viable models, it is typically sub-dominant
and below detectable rates.Comment: 27 pages, 6 figures; accepted for publication in JCA
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