190,743 research outputs found
Mechanism of Gravity Impulse
It is well-known that energy-momentum is the source of gravitational field.
For a long time, it is generally believed that only stars with huge masses can
generate strong gravitational field. Based on the unified theory of
gravitational interactions and electromagnetic interactions, a new mechanism of
the generation of gravitational field is studied. According to this mechanism,
in some special conditions, electromagnetic energy can be directly converted
into gravitational energy, and strong gravitational field can be generated
without massive stars. Gravity impulse found in experiments is generated by
this mechanism.Comment: 10 page
Cavity flow past a slender pointed hydrofoil
A slender-body theory for the flow past a slender, pointed hydrofoil held at a small angle of Attack to the flow, with a cavity on the upper surface, has been worked out. The approximate solution valid near the body is seen to be the sum of two components. The first consists of a distribution of two-dimensional sources located along the centroid line of the cavity to represent the variation of the cross-sectional area of the cavity. The second component represents the crossflow perpendicular to the centroid line. It is found that over the cavity boundary which envelops a constant pressure region, the magnitude of the cross-flow
velocity is not constant, but varies to a moderate extent. With this variation neglected only in the neighbourhood of the hydrofoil, the cross-flow is solved by adopting the Riabouchinsky model for the two-dimensional flow. The lift is then calculated by integrating the pressure along the chord; the dependence of the lift on cavitation number and angle of attack is shown for a specific case of the triangular plan form
Searching for the decay of a charged Higgs boson
We study the prospects for charged Higgs boson searches in the
decay channel. This loop-induced decay channel can be important if the charged
Higgs is fermiophobic, particularly when its mass is below the threshold.
We identify useful kinematic observables and evaluate the future Large Hadron
Collider sensitivity to this channel using the custodial-fiveplet charged Higgs
in the Georgi-Machacek model as a fermiophobic benchmark. We show that the LHC
with 300~fb of data at 14~TeV will be able to exclude charged Higgs
masses below about 130~GeV for almost any value of the SU(2)-triplet vacuum
expectation value in the model, and masses up to 200~GeV and beyond when the
triplet vacuum expectation value is very small. We describe the signal
simulation tools created for this analysis, which have been made publicly
available.Comment: 32 pages, 12 figures and 4 tables; v2: references added, typo fixed,
match the published versio
No functions continuous only at points in a countable dense set
We give a short proof that if a function is continuous on a countable dense
set, then it is continuous on an uncountable set. This is done for functions
defined on nonempty complete metric spaces without isolated points, and the
argument only uses that Cauchy sequences converge. We discuss how this theorem
is a direct consequence of the Baire category theorem, and also discuss
Volterra's theorem and the history of this problem. We give a simple example,
for each complete metric space without isolated points and each countable
subset, of a real-valued function that is discontinuous only on that subset.Comment: Expanded the result and added historical references and discussio
The evaporation valley in the Kepler planets
A new piece of evidence supporting the photoevaporation-driven evolution
model for low-mass, close-in exoplanets was recently presented by the
California-Kepler-Survey. The radius distribution of the Kepler planets is
shown to be bimodal, with a ``valley' separating two peaks at 1.3 and 2.6
Rearth. Such an ``evaporation-valley' had been predicted by numerical models
previously. Here, we develop a minimal model to demonstrate that this valley
results from the following fact: the timescale for envelope erosion is the
longest for those planets with hydrogen/helium-rich envelopes that, while only
a few percent in weight, double its radius. The timescale falls for envelopes
lighter than this because the planet's radius remains largely constant for
tenuous envelopes. The timescale also drops for heavier envelopes because the
planet swells up faster than the addition of envelope mass. Photoevaporation,
therefore, herds planets into either bare cores ~1.3 Rearth, or those with
double the core's radius (~2.6 Rearth). This process mostly occurs during the
first 100 Myrs when the stars' high energy flux are high and nearly constant.
The observed radius distribution further requires that the Kepler planets are
clustered around 3 Mearth in mass, are born with H/He envelopes more than a few
percent in mass, and that their cores are similar to the Earth in composition.
Such envelopes must have been accreted before the dispersal of the gas disks,
while the core composition indicates formation inside the ice-line. Lastly, the
photoevaporation model fails to account for bare planets beyond ~30-60 days, if
these planets are abundant, they may point to a significant second channel for
planet formation, resembling the Solar-System terrestrial planets.Comment: 15 pages, published in Ap
Bulk effects on topological conduction on the surface of 3-D topological insulators
The surface states of a topological insulator in a fine-tuned magnetic field
are ideal candidates for realizing a topological metal which is protected
against disorder. Its signatures are (1) a conductance plateau in long wires in
a finely tuned longitudinal magnetic field and (2) a conductivity which always
increases with sample size, and both are independent of disorder strength. We
numerically study how these experimental transport signatures are affected by
bulk physics in the interior of the topological insulator sample. We show that
both signatures of the topological metal are robust against bulk effects.
However the bulk does substantially accelerate the metal's decay in a magnetic
field and alter its response to surface disorder. When the disorder strength is
tuned to resonance with the bulk band the conductivity follows the predictions
of scaling theory, indicating that conduction is diffusive. At other disorder
strengths the bulk reduces the effects of surface disorder and scaling theory
is systematically violated, signaling that conduction is not fully diffusive.
These effects will change the magnitude of the surface conductivity and the
magnetoconductivity
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