13,242 research outputs found
Viscous dissipative effects in isotropic brane cosmology
We consider the dynamics of a viscous cosmological fluid in the generalized
Randall-Sundrum model for an isotropic brane. To describe the dissipative
effects we use the Israel-Hiscock-Stewart full causal thermodynamic theory. In
the limiting case of a stiff cosmological fluid with pressure equal to the
energy density, the general solution of the field equations can be obtained in
an exact parametric form for a cosmological fluid with constant bulk viscosity
and with a bulk viscosity coefficient proportional to the square root of the
energy density, respectively. The obtained solutions describe generally
non-inflationary brane worlds, starting from a singular state. During this
phase of evolution the comoving entropy of the Universe is an increasing
function of time, and thus a large amount of entropy is created in the brane
world due to viscous dissipative processes.Comment: 15 pages, 11 figure
Effects of vibration and shock on the performance of gas-bearing space-power Brayton cycle turbomachinery. Part 4: Suppression of rotor-bearing system vibrations through flexible bearing support damping
A bearing damper, operating on the support flexure of a pivoted pad in a tilting-pad type gas-lubricated journal bearing, has been designed, built, and tested under externally-applied random vibrations. The NASA Brayton Rotating Unit (BRU), a 36,000 rpm, 10-Kwe turbogenerator had previously been subjected in the MTI Vibration Test Laboratory to external random vibrations, and vibration response data had been recorded and analyzed for amplitude distribution and frequency content at a number of locations in the machine. Based on data from that evaluation, a piston-type damper was designed and developed for each of the two flexibly-supported journal bearing pads (one in each of the two three-pad bearings). A modified BRU, with dampers installed, has been re-tested under random vibration conditions. Root-mean-square vibration amplitudes were determined from the test data, and displacement power spectral density analyses have been performed. Results of these data reduction efforts have been compared with vibration tolerance limits. Results of the tests indicate significant reductions in vibration levels in the bearing gas-lubricant films, particularly in the rigidly-mounted pads. The utility of the gas-lubricated damper for limiting rotor-bearing system vibrations in high-speed turbomachinery has thus been demonstrated
Rotational Perturbations of Friedmann-Robertson-Walker Type Brane-World Cosmological Models
First order rotational perturbations of the Friedmann-Robertson-Walker metric
are considered in the framework of the brane-world cosmological models. A
rotation equation, relating the perturbations of the metric tensor to the
angular velocity of the matter on the brane is derived under the assumption of
slow rotation. The mathematical structure of the rotation equation imposes
strong restrictions on the temporal and spatial dependence of the brane matter
angular velocity. The study of the integrable cases of the rotation equation
leads to three distinct models, which are considered in detail. As a general
result we find that, similarly to the general relativistic case, the rotational
perturbations decay due to the expansion of the matter on the brane. One of the
obtained consistency conditions leads to a particular, purely inflationary
brane-world cosmological model, with the cosmological fluid obeying a
non-linear barotropic equation of state.Comment: 14 pages, 5 figures, REVTEX
Search for doubly charged Higgs bosons using the same-sign diboson mode at the LHC
Doubly charged Higgs bosons are predicted in many new physics models with an
extended Higgs sector that contains a Higgs triplet field. Current experimental
searches have been focusing mainly on the scenario in which the same-sign
dilepton decay modes are the dominant ones. We study the scenario where the
vacuum expectation value of the triplet field is sufficiently large so that the
associated charged Higgs bosons decay dominantly to a pair of weak gauge bosons
instead. A detailed simulation of the signal and the backgrounds is performed
for the CERN Large Hadron Collider at the collision energy of 8 TeV and 14 TeV.
We find that different cuts should be imposed for the events, depending on
whether the doubly charged Higgs boson mass is greater than about 200 GeV. In
the higher mass region, the forward jet tagging proves to be useful in
enhancing the signal significance. We show the discovery reach of the LHC
running at 8 and 14 TeV, with two benchmark triplet vacuum expectation values.
With an integrated luminosity of 10 fb at 8 TeV, the doubly charged
Higgs boson with a mass of GeV can be tested at level in
such a scenario.Comment: 18 pages, 10 figures; published in Physical Review
Spin Information from Vector-Meson Decay in Photoproduction
For the photoproduction of vector mesons, all single and double spin
observables involving vector meson two-body decays are defined consistently in
the center of mass. These definitions yield a procedure for
extracting physically meaningful single and double spin observables that are
subject to known rules concerning their angle and energy evolution. As part of
this analysis, we show that measuring the two-meson decay of a photoproduced
or does not determine the vector meson's vector polarization, but
only its tensor polarization. The vector meson decay into lepton pairs is also
insensitive to the vector meson's vector polarization, unless one measures the
spin of one of the leptons. Similar results are found for all double spin
observables which involve observation of vector meson decay. To access the
vector meson's vector polarization, one therefore needs to either measure the
spin of the decay leptons, make an analysis of the background interference
effects or relate the vector meson's vector polarization to other accessible
spin observables.Comment: 22 pages, 3 figure
Zero differential resistance in two-dimensional electron systems at large filling factors
We report on a state characterized by a zero differential resistance observed
in very high Landau levels of a high-mobility two-dimensional electron system.
Emerging from a minimum of Hall field-induced resistance oscillations at low
temperatures, this state exists over a continuous range of magnetic fields
extending well below the onset of the Shubnikov-de Haas effect. The minimum
current required to support this state is largely independent on the magnetic
field, while the maximum current increases with the magnetic field tracing the
onset of inter-Landau level scattering
Remarks on Form Factor Bounds
Improved model independent upper bounds on the weak transition form factors
are derived using inclusive sum rules. Comparison of the new bounds with the
old ones is made for the form factors h_{A_1} and h_V in B -> D* decays.Comment: 8 pages, 2 figures, title changed and typos corrected for journal
publicatio
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