2,828 research outputs found
Recycling universe
If the effective cosmological constant is non-zero, our observable universe
may enter a stage of exponential expansion. In such case, regions of it may
tunnel back to the false vacuum of an inflaton scalar field, and inflation with
a high expansion rate may resume in those regions. An ``ideal'' eternal
observer would then witness an infinite succession of cycles from false vacuum
to true, and back. Within each cycle, the entire history of a hot universe
would be replayed. If there were several minima of the inflaton potential, our
ideal observer would visit each one of these minima with a frequency which
depends on the shape of the potential. We generalize the formalism of
stochastic inflation to analyze the global structure of the universe when this
`recycling' process is taken into account.Comment: 43 pages, 10 figure
Recommended from our members
Electrophysiological Guidance of Epidural Electrode Array Implantation over the Human Lumbosacral Spinal Cord to Enable Motor Function after Chronic Paralysis.
Epidural electrical stimulation (EES) of the spinal cord has been shown to restore function after spinal cord injury (SCI). Characterization of EES-evoked motor responses has provided a basic understanding of spinal sensorimotor network activity related to EES-enabled motor activity of the lower extremities. However, the use of EES-evoked motor responses to guide EES system implantation over the spinal cord and their relation to post-operative EES-enabled function in humans with chronic paralysis attributed to SCI has yet to be described. Herein, we describe the surgical and intraoperative electrophysiological approach used, followed by initial EES-enabled results observed in 2 human subjects with motor complete paralysis who were enrolled in a clinical trial investigating the use of EES to enable motor functions after SCI. The 16-contact electrode array was initially positioned under fluoroscopic guidance. Then, EES-evoked motor responses were recorded from select leg muscles and displayed in real time to determine electrode array proximity to spinal cord regions associated with motor activity of the lower extremities. Acceptable array positioning was determined based on achievement of selective proximal or distal leg muscle activity, as well as bilateral muscle activation. Motor response latencies were not significantly different between intraoperative recordings and post-operative recordings, indicating that array positioning remained stable. Additionally, EES enabled intentional control of step-like activity in both subjects within the first 5 days of testing. These results suggest that the use of EES-evoked motor responses may guide intraoperative positioning of epidural electrodes to target spinal cord circuitry to enable motor functions after SCI
KLN theorem, magnetic mass, and thermal photon production
We study the infrared singularities associated to ultra-soft transverse
gluons in the calculation of photon production by a quark-gluon plasma. Despite
the fact that the KLN theorem works in this context and provides cancellations
of infrared singularities, it does not prevent the production rate of low
invariant mass dileptons to be sensitive to the magnetic mass of gluons and
therefore the rate to be non perturbative.Comment: 9 pages Latex document, 5 postscript figures, modified figure 5 and
slightly updated section
Inflation on the Brane with Vanishing Gravity
Many existing models of brane inflation suffer from a steep irreducible
gravitational potential between the branes that causes inflation to end too
early. Inspired by the fact that point masses in 2+1 D exert no gravitational
force, we propose a novel unwarped and non-supersymmetric setup for inflation,
consisting of 3-branes in two extra dimensions compactified on a sphere. The
size of the sphere is stabilized by a combination of a bulk cosmological
constant and a magnetic flux. Computing the 4D effective potential between
probe branes in this background, we find a non-zero contribution only from
exchange of level-1 KK modes of the graviton and radion. Identifying antipodal
points on the 2-sphere projects out these modes, eliminating entirely the
troublesome gravitational contribution to the inflationary potential.Comment: 19 pages, 11 figures, JHEP forma
Two problems in thermal field theory
In this talk, I review recent progress made in two areas of thermal field
theory. In particular, I discuss various approaches for the calculation of the
quark gluon plasma thermodynamical properties, and the problem of its photon
production rate.Comment: 10 pages Latex document, 15 postscript figures. Invited talk given at
the 6th Workshop on High Energy Particle Physics, Chennai, India, 3-15 Jan
200
Galactic rotation curves and brane world models
In the present investigation flat rotational curves of the galaxies are
considered under the framework of brane-world models where the 4d effective
Einstein equation has extra terms which arise from the embedding of the 3-brane
in the bulk. It has been shown here that these long range bulk
gravitational degrees of freedom can act as a mechanism to yield the observed
galactic rotation curves without the need for dark matter. The present model
has the advantage that the observed rotation curves result solely from
well-established non-local effects of gravitation, such as dark radiation and
dark pressure under a direct use of the condition of flat rotation curves and
does not invoke any exotic matter field.Comment: 7 Latex pages, 7 figures; Edited substantially with new figures and
references; Accepted for Mon. Not. R. Astron. So
Search for the Standard Model Higgs boson in the decay channel H→ZZ(*)→4ℓ with the ATLAS detector
A search for the Standard Model Higgs boson in the decay channel H→ZZ(*)→ℓ+ℓ−ℓ′+ℓ′−, where ℓ=e,μ, is presented. Proton-proton collision data at √s = 7 TeV recorded with the ATLAS detector and corresponding to an average integrated luminosity of 2.1 fb−1 are compared to the Standard Model expectations. Upper limits on the production cross section of a Standard Model Higgs boson with a mass between 110 and 600 GeV are derived. The observed (expected) 95% confidence level upper limit on the production cross section for a Higgs boson with a mass of 194 GeV, the region with the best expected sensitivity for this search, is 0.99 (1.01) times the Standard Model prediction. The Standard Model Higgs boson is excluded at 95% confidence level in the mass ranges 191-197, 199-200 and 214-224 GeV
Soft Yukawa couplings in supersymmetric theories
The possibility of radiatively generated fermion masses arising from chiral
flavor violation in soft supersymmetry-breaking terms is explored. Vacuum
stability constraints are considered in various classes of models, and allow in
principle all of the first- and second-generation quarks and leptons and the
-quark to obtain masses radiatively. Radiatively induced Higgs-fermion
couplings have non-trivial momentum-dependent form factors, which at low
momentum are enhanced with respect to the case of tree-level Yukawa couplings.
These form factors may be probed by various sum rules and relations among Higgs
boson decay widths and branching ratios to fermion final states. An apparent,
large, hard violation of supersymmetry also results for Higgsino couplings.
Mixing between left- and right-handed scalar superpartners is enhanced. A
radiative muon mass is shown to lead to a relatively large and potentially
measurable contribution to the muon anomalous magnetic moment. If the
light-quark masses arise radiatively, the neutron electric dipole moment is
suppressed by a natural phase alignment between the masses and dipole moment,
and is below the current experimental bound. The possibility of neutrino masses
arising from softly broken lepton number, and concomitant enhanced
sneutrino-antisneutrino oscillations, is briefly discussed.Comment: 66 pages. LaTex + RevTex. 16 figures (included). Published version
(minor changes and typos corrected
- …