481 research outputs found
Small Angle Scattering by Fractal Aggregates: A Numerical Investigation of the Crossover Between the Fractal Regime and the Porod Regime
Fractal aggregates are built on a computer using off-lattice cluster-cluster
aggregation models. The aggregates are made of spherical particles of different
sizes distributed according to a Gaussian-like distribution characterised by a
mean and a standard deviation . The wave vector dependent
scattered intensity is computed in order to study the influence of the
particle polydispersity on the crossover between the fractal regime and the
Porod regime. It is shown that, given , the location of the
crossover decreases as increases. The dependence of on
can be understood from the evolution of the shape of the center-to-center
interparticle-distance distribution function.Comment: RevTex, 4 pages + 6 postscript figures, compressed using "uufiles",
published in Phys. Rev. B 50, 1305 (1994
The spectrum of the recycled PSR J0437-4715 and its white dwarf companion
We present extensive spectral and photometric observations of the recycled
pulsar/white-dwarf binary containing PSR J0437-4715, which we analyzed together
with archival X-ray and gamma-ray data, to obtain the complete mid-infrared to
gamma-ray spectrum. We first fit each part of the spectrum separately, and then
the whole multi-wavelength spectrum. We find that the optical-infrared part of
the spectrum is well fit by a cool white dwarf atmosphere model with pure
hydrogen composition. The model atmosphere (Teff = 3950pm150K, log
g=6.98pm0.15, R_WD=(1.9pm0.2)e9 cm) fits our spectral data remarkably well for
the known mass and distance (M=0.25pm0.02Msun, d=156.3pm1.3pc), yielding the
white dwarf age (tau=6.0pm0.5Gyr). In the UV, we find a spectral shape
consistent with thermal emission from the bulk of the neutron star surface,
with surface temperature between 1.25e5 and 3.5e5K. The temperature of the
thermal spectrum suggests that some heating mechanism operates throughout the
life of the neutron star. The temperature distribution on the neutron star
surface is non-uniform. In the X-rays, we confirm the presence of a high-energy
tail which is consistent with a continuation of the cut-off power-law component
(Gamma=1.56pm0.01, Ecut=1.1pm0.2GeV) that is seen in gamma-rays and perhaps
even extends to the near-UV.Comment: 23 pages. To appear in Ap
Modeling Single Electron Transfer in Si:P Double Quantum Dots
Solid-state systems such as P donors in Si have considerable potential for
realization of scalable quantum computation. Recent experimental work in this
area has focused on implanted Si:P double quantum dots (DQDs) that represent a
preliminary step towards the realization of single donor charge-based qubits.
This paper focuses on the techniques involved in analyzing the charge transfer
within such DQD devices and understanding the impact of fabrication parameters
on this process. We show that misalignment between the buried dots and surface
gates affects the charge transfer behavior and identify some of the challenges
posed by reducing the size of the metallic dot to the few donor regime.Comment: 11 pages, 7 figures, submitted to Nanotechnolog
An All-Sky Search for Three Flavors of Neutrinos from Gamma-Ray Bursts with the IceCube Neutrino Observatory
We present the results and methodology of a search for neutrinos produced in
the decay of charged pions created in interactions between protons and
gamma-rays during the prompt emission of 807 gamma-ray bursts (GRBs) over the
entire sky. This three-year search is the first in IceCube for shower-like
Cherenkov light patterns from electron, muon, and tau neutrinos correlated with
GRBs. We detect five low-significance events correlated with five GRBs. These
events are consistent with the background expectation from atmospheric muons
and neutrinos. The results of this search in combination with those of
IceCube's four years of searches for track-like Cherenkov light patterns from
muon neutrinos correlated with Northern-Hemisphere GRBs produce limits that
tightly constrain current models of neutrino and ultra high energy cosmic ray
production in GRB fireballs.Comment: 33 pages, 14 figures; minor changes made to match published version
in the Astrophysical Journal, 2016 June 2
Improved limits on dark matter annihilation in the Sun with the 79-string IceCube detector and implications for supersymmetry
We present an improved event-level likelihood formalism for including
neutrino telescope data in global fits to new physics. We derive limits on
spin-dependent dark matter-proton scattering by employing the new formalism in
a re-analysis of data from the 79-string IceCube search for dark matter
annihilation in the Sun, including explicit energy information for each event.
The new analysis excludes a number of models in the weak-scale minimal
supersymmetric standard model (MSSM) for the first time. This work is
accompanied by the public release of the 79-string IceCube data, as well as an
associated computer code for applying the new likelihood to arbitrary dark
matter models.Comment: 24 pages, 8 figs, 1 table. Contact authors: Pat Scott & Matthias
Danninger. Likelihood tool available at http://nulike.hepforge.org. v2: small
updates to address JCAP referee repor
Lowering IceCubeâs energy threshold for point source searches in the southern sky
Observation of a point source of astrophysical neutrinos would be a "smoking gun" signature of a cosmic-ray accelerator. While IceCube has recently discovered a diffuse flux of astrophysical neutrinos, no localized point source has been observed. Previous IceCube searches for point sources in the southern sky were restricted by either an energy threshold above a few hundred TeV or poor neutrino angular resolution. Here we present a search for southern sky point sources with greatly improved sensitivities to neutrinos with energies below 100 TeV. By selecting charged-current nu(mu) interacting inside the detector, we reduce the atmospheric background while retaining efficiency for astrophysical neutrino-induced events reconstructed with sub-degree angular resolution. The new event sample covers three years of detector data and leads to a factor of 10 improvement in sensitivity to point sources emitting below 100 TeV in the southern sky. No statistically significant evidence of point sources was found, and upper limits are set on neutrino emission from individual sources. A posteriori analysis of the highest-energy (similar to 100 TeV) starting event in the sample found that this event alone represents a 2.8 sigma deviation from the hypothesis that the data consists only of atmospheric background
Search for non-relativistic Magnetic Monopoles with IceCube
The IceCube Neutrino Observatory is a large Cherenkov detector instrumenting
of Antarctic ice. The detector can be used to search for
signatures of particle physics beyond the Standard Model. Here, we describe the
search for non-relativistic, magnetic monopoles as remnants of the GUT (Grand
Unified Theory) era shortly after the Big Bang. These monopoles may catalyze
the decay of nucleons via the Rubakov-Callan effect with a cross section
suggested to be in the range of to
. In IceCube, the Cherenkov light from nucleon decays
along the monopole trajectory would produce a characteristic hit pattern. This
paper presents the results of an analysis of first data taken from May 2011
until May 2012 with a dedicated slow-particle trigger for DeepCore, a
subdetector of IceCube. A second analysis provides better sensitivity for the
brightest non-relativistic monopoles using data taken from May 2009 until May
2010. In both analyses no monopole signal was observed. For catalysis cross
sections of the flux of non-relativistic
GUT monopoles is constrained up to a level of at a 90% confidence level,
which is three orders of magnitude below the Parker bound. The limits assume a
dominant decay of the proton into a positron and a neutral pion. These results
improve the current best experimental limits by one to two orders of magnitude,
for a wide range of assumed speeds and catalysis cross sections.Comment: 20 pages, 20 figure
The IceCube Neutrino Observatory - Contributions to ICRC 2015 Part II: Atmospheric and Astrophysical Diffuse Neutrino Searches of All Flavors
Papers on atmospheric and astrophysical diffuse neutrino searches of all
flavors submitted to the 34th International Cosmic Ray Conference (ICRC 2015,
The Hague) by the IceCube Collaboration.Comment: 66 pages, 36 figures, Papers submitted to the 34th International
Cosmic Ray Conference, The Hague 2015, v2 has a corrected author lis
Search for Dark Matter Annihilation in the Galactic Center with IceCube-79
The Milky Way is expected to be embedded in a halo of dark matter particles,
with the highest density in the central region, and decreasing density with the
halo-centric radius. Dark matter might be indirectly detectable at Earth
through a flux of stable particles generated in dark matter annihilations and
peaked in the direction of the Galactic Center. We present a search for an
excess flux of muon (anti-) neutrinos from dark matter annihilation in the
Galactic Center using the cubic-kilometer-sized IceCube neutrino detector at
the South Pole. There, the Galactic Center is always seen above the horizon.
Thus, new and dedicated veto techniques against atmospheric muons are required
to make the southern hemisphere accessible for IceCube. We used 319.7 live-days
of data from IceCube operating in its 79-string configuration during 2010 and
2011. No neutrino excess was found and the final result is compatible with the
background. We present upper limits on the self-annihilation cross-section,
\left, for WIMP masses ranging from 30 GeV up to
10 TeV, assuming cuspy (NFW) and flat-cored (Burkert) dark matter halo
profiles, reaching down to cm s, and
cm s for the
channel, respectively.Comment: 14 pages, 9 figures, Submitted to EPJ-C, added references, extended
limit overvie
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