4,973 research outputs found

### A quantification of hydrodynamical effects on protoplanetary dust growth

Context. The growth process of dust particles in protoplanetary disks can be
modeled via numerical dust coagulation codes. In this approach, physical
effects that dominate the dust growth process often must be implemented in a
parameterized form. Due to a lack of these parameterizations, existing studies
of dust coagulation have ignored the effects a hydrodynamical gas flow can have
on grain growth, even though it is often argued that the flow could
significantly contribute either positively or negatively to the growth process.
Aims. We intend to provide a quantification of hydrodynamical effects on the
growth of dust particles, such that these effects can be parameterized and
implemented in a dust coagulation code.
Methods. We numerically integrate the trajectories of small dust particles in
the flow of disk gas around a proto-planetesimal, sampling a large parameter
space in proto-planetesimal radii, headwind velocities, and dust stopping
times.
Results. The gas flow deflects most particles away from the
proto-planetesimal, such that its effective collisional cross section, and
therefore the mass accretion rate, is reduced. The gas flow however also
reduces the impact velocity of small dust particles onto a proto-planetesimal.
This can be beneficial for its growth, since large impact velocities are known
to lead to erosion. We also demonstrate why such a gas flow does not return
collisional debris to the surface of a proto-planetesimal.
Conclusions. We predict that a laminar hydrodynamical flow around a
proto-planetesimal will have a significant effect on its growth. However, we
cannot easily predict which result, the reduction of the impact velocity or the
sweep-up cross section, will be more important. Therefore, we provide
parameterizations ready for implementation into a dust coagulation code.Comment: 9 pages, 6 figures; accepted for publication in A&A; v2 matches the
manuscript sent to the publisher (very minor changes

### Two-Source Dispersers for Polylogarithmic Entropy and Improved Ramsey Graphs

In his 1947 paper that inaugurated the probabilistic method, Erd\H{o}s proved
the existence of $2\log{n}$-Ramsey graphs on $n$ vertices. Matching Erd\H{o}s'
result with a constructive proof is a central problem in combinatorics, that
has gained a significant attention in the literature. The state of the art
result was obtained in the celebrated paper by Barak, Rao, Shaltiel and
Wigderson [Ann. Math'12], who constructed a
$2^{2^{(\log\log{n})^{1-\alpha}}}$-Ramsey graph, for some small universal
constant $\alpha > 0$.
In this work, we significantly improve the result of Barak~\etal and
construct $2^{(\log\log{n})^c}$-Ramsey graphs, for some universal constant $c$.
In the language of theoretical computer science, our work resolves the problem
of explicitly constructing two-source dispersers for polylogarithmic entropy

### Radiocarbon dates from the Oxford AMS system: archaeometry datelist 35

This is the 35th list of AMS radiocarbon determinations measured at the Oxford Radiocarbon Accelerator Unit (ORAU). Amongst some of the sites included here are the latest series of determinations from the key sites of Abydos, El Mirón, Ban Chiang, Grotte de Pigeons (Taforalt), Alepotrypa and Oberkassel, as well as others dating to the Palaeolithic, Mesolithic and later periods. Comments on the significance of the results are provided by the submitters of the material

### Magnetic Moments of Dirac Neutrinos

The existence of a neutrino magnetic moment implies contributions to the
neutrino mass via radiative corrections. We derive model-independent
"naturalness" upper bounds on the magnetic moments of Dirac neutrinos,
generated by physics above the electroweak scale. The neutrino mass receives a
contribution from higher order operators, which are renormalized by operators
responsible for the neutrino magnetic moment. This contribution can be
calculated in a model independent way. In the absence of fine-tuning, we find
that current neutrino mass limits imply that $|\mu_\nu| < 10^{-14}$ Bohr
magnetons. This bound is several orders of magnitude stronger than those
obtained from solar and reactor neutrino data and astrophysical observations.Comment: 3 pages. Talk given at PANIC'0

### The Vector Analyzing Power in Elastic Electron-Proton Scattering

We compute the vector analyzing power (VAP) for the elastic scattering of
transversely polarized electrons from protons at low energies using an
effective theory of electrons, protons, and photons. We study all contributions
through second order in $E/M$, where $E$ and $M$ are the electron energy and
nucleon mass, respectively. The leading order VAP arises from the imaginary
part of the interference of one- and two-photon exchange amplitudes.
Sub-leading contributions are generated by the nucleon magnetic moment and
charge radius as well as recoil corrections to the leading-order amplitude.
Working to ${\cal O}(E/M)^2$, we obtain a prediction for $A_n$ that is free of
unknown parameters and that agrees with the recent measurement of the VAP in
backward angle $ep$ scattering.Comment: 24 pages, 11 figures. Typos fixe

### Constraints on T-Odd, P-Even Interactions from Electric Dipole Moments

We construct the relationship between nonrenormalizable,effective,
time-reversal violating (TV) parity-conserving (PC) interactions of quarks and
gauge bosons and various low-energy TVPC and TV parity-violating (PV)
observables. Using effective field theory methods, we delineate the scenarious
under which experimental limits on permanent electric dipole moments (EDM's) of
the electron, neutron, and neutral atoms as well as limits on TVPC observables
provide the most stringent bounds on new TVPC interactions. Under scenarios in
which parity invariance is restored at short distances, the one-loop EDM of
elementary fermions generate the most severe constraints. The limits derived
from the atomic EDM of $^{199}$Hg are considerably weaker. When parity symmetry
remains broken at short distances, direct TVPC search limits provide the least
ambiguous bounds. The direct limits follow from TVPC interactions between two
quarks.Comment: 43 pages, 9 figure

### Non-relativistic limit in the 2+1 Dirac Oscillator: A Ramsey Interferometry Effect

We study the non-relativistic limit of a paradigmatic model in Relativistic
Quantum Mechanics, the two-dimensional Dirac oscillator. Remarkably, we find a
novel kind of Zitterbewegung which persists in this non-relativistic regime,
and leads to an observable deformation of the particle orbit. This effect can
be interpreted in terms of a Ramsey Interferometric phenomenon, allowing an
insightful connection between Relativistic Quantum Mechanics and Quantum
Optics. Furthermore, subsequent corrections to the non-relativistic limit,
which account for the usual spin-orbit Zitterbewegung, can be neatly understood
in terms of a Mach-Zehnder interferometer.Comment: RevTex4 file, color figures, submitted for publicatio

### 2S hyperfine structure of atomic deuterium

We have measured the frequency splitting between the $(2S, F=1/2)$ and $(2S,
F=3/2)$ hyperfine sublevels in atomic deuterium by an optical differential
method based on two-photon Doppler-free spectroscopy on a cold atomic beam. The
result $f_{\rm HFS}^{(D)}(2S)= 40 924 454(7)$ Hz is the most precise value for
this interval to date. In comparison to the previous radio-frequency
measurement we have improved the accuracy by the factor of three.
The specific combination of hyperfine frequency intervals for metastable- and
ground states in deuterium atom $D_{21}=8f_{\rm HFS}^{(D)}(2S)-f_{\rm
HFS}^{(D)}(1S)$ derived from our measurement is in a good agreement with
$D_{21}$ calculated from quantum-electrodynamics theory.Comment: 7 pages, 7 figure

### The SXI telescope on board EXIST: scientific performances

The SXI telescope is one of the three instruments on board EXIST, a
multiwavelength observatory in charge of performing a global survey of the sky
in hard X-rays searching for Supermassive Black Holes. One of the primary
objectives of EXIST is also to study with unprecedented sensitivity the most
unknown high energy sources in the Universe, like high redshift GRBs, which
will be pointed promptly by the Spacecraft by autonomous trigger based on hard
X-ray localization on board. The recent addition of a soft X-ray telescope to
the EXIST payload complement, with an effective area of ~950 cm2 in the energy
band 0.2-3 keV and extended response up to 10 keV will allow to make broadband
studies from 0.1 to 600 keV. In particular, investigations of the spectra
components and states of AGNs and monitoring of variability of sources, study
of the prompt and afterglow emission of GRBs since the early phases, which will
help to constrain the emission models and finally, help the identification of
sources in the EXIST hard X-ray survey and the characterization of the
transient events detected. SXI will also perform surveys: a scanning survey
with sky coverage of about 2pi and limiting flux of 5x10^{-14}cgs plus other
serendipitous. We give an overview of the SXI scientific performance and also
describe the status of its design emphasizing how it has been derived by the
scientific requirements.Comment: 9 pages, 6 figures, to be published in Proc. of SPIE, vol 7435-11,
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