4,138 research outputs found
Effect of negative ions on the diffusion of charged particles in the lower ionosphere
Effect of negative ions on diffusion of charged particles in lower ionospher
Super-Earth Atmospheres: Self-Consistent Gas Accretion and Retention
Some recently discovered short-period Earth to Neptune sized exoplanets
(super Earths) have low observed mean densities which can only be explained by
voluminous gaseous atmospheres. Here, we study the conditions allowing the
accretion and retention of such atmospheres. We self-consistently couple the
nebular gas accretion onto rocky cores and the subsequent evolution of gas
envelopes following the dispersal of the protoplanetary disk. Specifically, we
address mass-loss due to both photo-evaporation and cooling of the planet. We
find that planets shed their outer layers (dozens of percents in mass)
following the disk's dispersal (even without photo-evaporation), and their
atmospheres shrink in a few Myr to a thickness comparable to the radius of the
underlying rocky core. At this stage, atmospheres containing less particles
than the core (equivalently, lighter than a few % of the planet's mass) can be
blown away by heat coming from the cooling core, while heavier atmospheres cool
and contract on a timescale of Gyr at most. By relating the mass-loss timescale
to the accretion time, we analytically identify a Goldilocks region in the
mass-temperature plane in which low-density super Earths can be found: planets
have to be massive and cold enough to accrete and retain their atmospheres,
while not too massive or cold, such that they do not enter runaway accretion
and become gas giants (Jupiters). We compare our results to the observed
super-Earth population and find that low-density planets are indeed
concentrated in the theoretically allowed region. Our analytical and intuitive
model can be used to investigate possible super-Earth formation scenarios.Comment: Updated (refereed) versio
Determination of the Higgs-boson couplings and H-A mixing in the generalized SM-like Two Higgs Doublet Model
The feasibility of measuring the Higgs-boson properties at the Photon
Collider at TESLA has been studied in detail for masses between 200 and 350
GeV, using realistic luminosity spectra and detector simulation. We consider
the Two Higgs Doublet Model (II) with SM-like Yukawa couplings for h,
parametrized by only one parameter (tan(beta)). The combined measurement of the
invariant-mass distributions in the ZZ and W+W- decay-channels is sensitive to
both the two-photon width Gamma_{gamma gamma} and phase Phi_{gamma gamma}. From
the analysis including systematic uncertainties we found out that after one
year of Photon Collider running with nominal luminosity the expected precision
in the measurement of tan(beta) is of the order of 10%, for both light (h) and
heavy (H) scalar Higgs bosons. The H-A mixing angle Phi_{HA}, characterizing a
weak CP violation in the model with two Higgs doublets, can be determined to
about 100 mrad, for low tan(beta).Comment: 17 pages, 9 figures; published versio
The role of energy-momentum conservation in emission of Cherenkov gluons
The famous formula for the emission angle of Cherenkov radiation should be
modified when applied to hadronic reactions because of recoil effects. They
impose the upper limit on the energy of the gluon emitted at a given angle.
Also, it leads to essential corrections to the nuclear refractive index value
as determined from the angular position of Cherenkov rings.Comment: 6
Detecting photon-photon scattering in vacuum at exawatt lasers
In a recent paper, we have shown that the QED nonlinear corrections imply a
phase correction to the linear evolution of crossing electromagnetic waves in
vacuum. Here, we provide a more complete analysis, including a full numerical
solution of the QED nonlinear wave equations for short-distance propagation in
a symmetric configuration. The excellent agreement of such a solution with the
result that we obtain using our perturbatively-motivated Variational Approach
is then used to justify an analytical approximation that can be applied in a
more general case. This allows us to find the most promising configuration for
the search of photon-photon scattering in optics experiments. In particular, we
show that our previous requirement of phase coherence between the two crossing
beams can be released. We then propose a very simple experiment that can be
performed at future exawatt laser facilities, such as ELI, by bombarding a low
power laser beam with the exawatt bump.Comment: 8 pages, 6 figure
Thinning of superfluid films below the critical point
Experiments on He films reveal an attractive Casimir-like force at the
bulk -point, and in the superfluid regime. Previous work has explained
the magnitude of this force at the transition and deep in the
superfluid region but not the substantial attractive force immediately below
the -point. Utilizing a simple mean-field calculation renormalized by
critical fluctuations we obtain an effective Casimir force that is
qualitatively consistent with the scaling function obtained by
collapse of experimental data.Comment: 4 page
Combinatorics and formal geometry of the master equation
We give a general treatment of the master equation in homotopy algebras and
describe the operads and formal differential geometric objects governing the
corresponding algebraic structures. We show that the notion of Maurer-Cartan
twisting is encoded in certain automorphisms of these universal objects
Economic impact of large public programs: The NASA experience
The economic impact of NASA programs on weather forecasting and the computer and semiconductor industries is discussed. Contributions to the advancement of the science of astronomy are also considered
- …