1,311 research outputs found
Pulsed extraction of ionization from helium buffer gas
The migration of intense ionization created in helium buffer gas under the
influence of applied electric fields is considered. First the chemical
evolution of the ionization created by fast heavy-ion beams is described.
Straight forward estimates of the lifetimes for charge exchange indicate a
clear suppression of charge exchange during ion migration in low pressure
helium. Then self-consistent calculations of the migration of the ions in the
electric field of a gas-filled cell at the National Superconducting Cyclotron
Laboratory (NSCL) using a Particle-In-Cell computer code are presented. The
results of the calculations are compared to measurements of the extracted ion
current caused by beam pulses injected into the NSCL gas cell.Comment: Accepted for pubilication in Nucl. Instrum. Meth. B, 14 pages, 8
figure
Decay Modes of Unstable Strings in Plane-Wave String Field Theory
The cubic interaction vertex of light-cone string field theory in the
plane-wave background has a simple effective form when considering states with
only bosonic excitations. This simple effective interaction vertex is used in
this paper to calculate the three string interaction matrix elements for states
of arbitrary bosonic excitation and these results are used to examine certain
decay modes on the mass-shell. It is shown that the matrix elements of one
string to two string decays involving only bosonic excitations will vanish to
all orders in 1/mu on the mass-shell when the number of excitations on the
initial string is less than or equal to two, but in general will not vanish
when the number of excitations is greater than two. Also, a truncated
calculation of the mass-shell matrix elements for one string to three string
decays of two excitation states is performed and suggests that these matrix
elements do not vanish on the mass-shell. There is, however, a quantitative
discrepancy between this last result and its (also non-vanishing) gauge theory
prediction from the BMN correspondence.Comment: 11 pages; v2: references added; v3: normalization of interaction
vertex and corresponding amplitudes changed by a factor of mu to reflect SFT
normalization (must now divide by mu to compare with BMN dual gauge theory),
and minor errors correcte
Decay of Counterflow Quantum Turbulence in Superfluid ^4He
We have simulated the decay of thermal counterflow quantum turbulence from a
statistically steady state at T=1.9[K], with the assumption that the normal
fluid is at rest during the decay. The results are consistent with the
predictions of the Vinen equation (in essence the vortex line density (VLD)
decays as t^{-1}). For the statistically steady state, we determine the
parameter c_2, which connects the curvature of the vortex lines and the mean
separation of vortices. A formula connecting the parameter \chi_2 of the Vinen
equation with c_2 is shown to agree with the results of the simulations.
Disagreement with experiment is discussed.Comment: 7 pages, 7 figure
Strong Evidence In Favor OF The Existence Of S-Matrix For Strings In Plane Waves
Field theories on the plane wave background are considered. We discuss that
for such field theories one can only form 1+1 dimensional freely propagating
wave packets. We analyze tree level four point functions of scalar field theory
as well as scalars coupled to gauge fields in detail and show that these four
point functions are well-behaved so that they can be interpreted as S-matrix
elements for 2 particle 2 particle scattering amplitudes. Therefore, at
least classically, field theories on the plane wave background have S-matrix
formulation.Comment: Latex file, 26 pages, 4 eps figures. v3: In the end of paper there is
a "Note Added" as an update of the result
Exotic torus manifolds and equivariant smooth structures on quasitoric manifolds
In 2006 Masuda and Suh asked if two compact non-singular toric varieties
having isomorphic cohomology rings are homeomorphic. In the first part of this
paper we discuss this question for topological generalizations of toric
varieties, so-called torus manifolds. For example we show that there are
homotopy equivalent torus manifolds which are not homeomorphic. Moreover, we
characterize those groups which appear as the fundamental groups of locally
standard torus manifolds.
In the second part we give a classification of quasitoric manifolds and
certain six-dimensional torus manifolds up to equivariant diffeomorphism.
In the third part we enumerate the number of conjugacy classes of tori in the
diffeomorphism group of torus manifolds. For torus manifolds of dimension
greater than six there are always infinitely many conjugacy classes. We give
examples which show that this does not hold for six-dimensional torus
manifolds.Comment: 21 pages, 2 figures, results about quasitoric manifolds adde
The M Theory Five-Brane and the Heterotic String
Brane actions with chiral bosons present special challenges. Recent progress
in the description of the two main examples -- the M theory five-brane and the
heterotic string -- is described. Also, double dimensional reduction of the M
theory five-brane on K3 is shown to give the heterotic string.Comment: 13 pages, latex, no figures; ICTP Conference Proceeding
Turbulent superfluid profiles in a counterflow channel
We have developed a two-dimensional model of quantised vortices in helium II
moving under the influence of applied normal fluid and superfluid in a
counterflow channel. We predict superfluid and vortex-line density profiles
which could be experimentally tested using recently developed visualization
techniques.Comment: 3 double figures, 9 page
Optical properties of (AlxGa1-x)(0.52)In0.48P at the crossover from a direct-gap to an indirect-gap semiconductor
The optical properties and the dynamics of excitons and the electron-hole plasma have been studied in disordered (AlxGa1−x)0.52In0.48P near to the direct-to-indirect band gap crossover. In particular we have investigated three epitaxial layers grown by solid-source molecular beam epitaxy with varying Al content x. Two of them have compositions in the immediate vicinity of the crossover point, the other is assigned to the indirect-gap regime. Both direct and indirect recombination processes contribute to the photon emission from the material. Since the relative importance of the different recombination processes depends strongly on temperature, excitation intensity, and excitation pulse duration, the processes can be identified by changing these parameters. As a result, we can determine the relative alignment of the conduction band minima and the distribution of the electrons among them. At high excitation levels the two crossover samples show stimulated emission at a photon energy of ∼2.29 eV, i.e., in the green spectral range. Using the variable stripe length method, we find an optical gain of up to ∼600 cm−1 at excitation levels of ∼350 kW/cm2.Stimulated emission involves direct recombination. This conclusion is reached from the experiments and from line-shape modeling, including a self-consistent treatment of populations and renormalization of the conduction band minima
Model- and calibration-independent test of cosmic acceleration
We present a calibration-independent test of the accelerated expansion of the
universe using supernova type Ia data. The test is also model-independent in
the sense that no assumptions about the content of the universe or about the
parameterization of the deceleration parameter are made and that it does not
assume any dynamical equations of motion. Yet, the test assumes the universe
and the distribution of supernovae to be statistically homogeneous and
isotropic. A significant reduction of systematic effects, as compared to our
previous, calibration-dependent test, is achieved. Accelerated expansion is
detected at significant level (4.3 sigma in the 2007 Gold sample, 7.2 sigma in
the 2008 Union sample) if the universe is spatially flat. This result depends,
however, crucially on supernovae with a redshift smaller than 0.1, for which
the assumption of statistical isotropy and homogeneity is less well
established.Comment: 13 pages, 2 figures, major change
Evidence for a Black Hole Remnant in the Type IIL Supernova 1979C
We present an analysis of archival X-ray observations of the Type IIL
supernova SN 1979C. We find that its X-ray luminosity is remarkably constant at
(6.5+/-0.1) x 10^38 erg/s over a period of 12 years between 1995 and 2007. The
high and steady luminosity is considered as possible evidence for a
stellar-mass (~ 5-10Msun) black hole accreting material from either a supernova
fallback disk or from a binary companion, or possibly from emission from a
central pulsar wind nebula. We find that the bright and steady X-ray light
curve is not consistent with either a model for a supernova powered by magnetic
braking of a rapidly rotating magnetar, or a model where the blast wave is
expanding into a dense circumstellar wind.Comment: 11 pages, 2 figures, to appear in New Astronom
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