2,736 research outputs found
Two-Loop Planar Corrections to Heavy-Quark Pair Production in the Quark-Antiquark Channel
We evaluate the planar two-loop QCD diagrams contributing to the leading
color coefficient of the heavy-quark pair production cross section, in the
quark-antiquark annihilation channel. We obtain the leading color coefficient
in an analytic form, in terms of one- and two-dimensional harmonic
polylogarithms of maximal weight 4. The result is valid for arbitrary values of
the Mandelstam invariants s and t, and of the heavy-quark mass m. Our findings
agree with previous analytic results in the small-mass limit and numerical
results for the exact amplitude.Comment: 30 pages, 5 figures. Version accepted by JHE
Antenna subtraction with massive fermions at NNLO: Double real initial-final configurations
We derive the integrated forms of specific initial-final tree-level
four-parton antenna functions involving a massless initial-state parton and a
massive final-state fermion as hard radiators. These antennae are needed in the
subtraction terms required to evaluate the double real corrections to
hadronic production at the NNLO level stemming from the partonic
processes and .Comment: 24 pages, 1 figure, 1 Mathematica file attache
The connection between stellar granulation and oscillation as seen by the Kepler mission
The long and almost continuous observations by Kepler show clear evidence of
a granulation background signal in a large sample of stars, which is
interpreted as the surface manifestation of convection. It has been shown that
its characteristic timescale and rms intensity fluctuation scale with the peak
frequency (\nu_{max}) of the solar-like oscillations. Various attempts have
been made to quantify the observed signal, to determine scaling relations, and
to compare them to theoretical predictions. We use a probabilistic method to
compare different approaches to extracting the granulation signal. We fit the
power density spectra of a large set of Kepler targets, determine the
granulation and global oscillation parameter, and quantify scaling relations
between them. We establish that a depression in power at about \nu_{max}/2,
known from the Sun and a few other main-sequence stars, is also statistically
significant in red giants and that a super-Lorentzian function with two
components is best suited to reproducing the granulation signal in the broader
vicinity of the pulsation power excess. We also establish that the specific
choice of the background model can affect the determination of \nu_{max},
introducing systematic uncertainties that can significantly exceed the random
uncertainties. We find the characteristic background frequency and amplitude to
tightly scale with \nu_{max} for a wide variety of stars, and quantify a mass
dependency of the latter. To enable comparison with theoretical predictions, we
computed effective timescales and intensity fluctuations and found them to
approximately scale as \tau_{eff} \propto g^{-0.85}\,T^{-0.4} and A_{gran}
\propto (g^2M)^{-1/4}, respectively. Similarly, the bolometric pulsation
amplitude scales approximately as A_{puls} \propto (g^2M)^{-1/3}, which
implicitly verifies a separate mass and luminosity dependence of A_{puls}.Comment: 18 pages, 12 figures, accepted for A&
A photometric mode identification method, including an improved non-adiabatic treatment of the atmosphere
We present an improved version of the method of photometric mode
identification of Heynderickx et al. (1994). Our new version is based on the
inclusion of precise non-adiabatic eigenfunctions determined in the outer
stellar atmosphere according to the formalism recently proposed by Dupret et
al.(2002). Our improved photometric mode identification technique is therefore
no longer dependent on ad hoc parameters for the non-adiabatic effects. It
contains the complete physical conditions of the outer atmosphere of the star,
provided that rotation does not play a key role. We apply our improved method
to the two slowly pulsating B stars HD 74560 and HD 138764 and to the beta
Cephei star EN (16) Lac. Besides identifying the degree l of the pulsating
stars, our method is also a tool for improving the knowledge of stellar
interiors and atmospheres, by imposing constraints on parameters such as the
metallicity and the mixing-length parameter alpha (a procedure we label
non-adiabatic asteroseismology).Comment: 10 pages, 9 figures Accepted for publication in Astronomy and
Astrophysic
Integrated AWG spectrometer for on-chip optical coherence tomography and Raman spectroscopy
Silicon oxynitride-based arrayed waveguide grating (AWG) spectrometers were designed for on-chip spectral-domain optical coherence tomography (OCT) systems and Raman spectroscopy of the skin. A novel geometrical layout for Raman spectroscopy was introduced to reduce loss. Measurements show that integrated optics has a good potential for miniaturizing current OCT systems
Oscillating red giants in the CoRoT exo-field: Asteroseismic mass and radius determination
Context. Observations and analysis of solar-type oscillations in red-giant
stars is an emerging aspect of asteroseismic analysis with a number of open
questions yet to be explored. Although stochastic oscillations have previously
been detected in red giants from both radial velocity and photometric
measurements, those data were either too short or had sampling that was not
complete enough to perform a detailed data analysis of the variability. The
quality and quantity of photometric data as provided by the CoRoT satellite is
necessary to provide a breakthrough in observing p-mode oscillations in red
giants. We have analyzed continuous photometric time-series of about 11 400
relatively faint stars obtained in the exofield of CoRoT during the first 150
days long-run campaign from May to October 2007. We find several hundred stars
showing a clear power excess in a frequency and amplitude range expected for
red-giant pulsators. In this paper we present first results on a sub-sample of
these stars. Aims. Knowing reliable fundamental parameters like mass and radius
is essential for detailed asteroseismic studies of red-giant stars. As the
CoRoT exofield targets are relatively faint (11-16 mag) there are no (or only
weak) constraints on the star's location in the H-R diagram. We therefore aim
to extract information about such fundamental parameters solely from the
available time series. Methods. We model the convective background noise and
the power excess hump due to pulsation with a global model fit and deduce
reliable estimates for the stellar mass and radius from scaling relations for
the frequency of maximum oscillation power and the characteristic frequency
separation.Comment: 10 pages, 7 figures, accepted for publication in A&
Photon - Jet Correlations and Constraints on Fragmentation Functions
We study the production of a large-pT photon in association with a jet in
proton-proton collisions. We examine the sensitivity of the jet rapidity
distribution to the gluon distribution function in the proton. We then assess
the sensitivity of various photon + jet correlation observables to the photon
fragmentation functions. We argue that RHIC data on photon-jet correlations can
be used to constrain the photon fragmentation functions in a region which was
barely accessible in LEP experiments.Comment: 23 pages, 9 figure
Olfactomedin 4 Serves as a Marker for Disease Severity in Pediatric Respiratory Syncytial Virus (RSV) Infection
Funding: Statement of financial support: The study was financially supported by the VIRGO consortium, an Innovative Cluster approved by the Netherlands Genomics Initiative and partially funded by the Dutch Government (BSIK 03012). The authors have indicated they have no personal financial relationships relevant to this article to disclose. Data Availability Statement: The data is accessible at http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE69606.Peer reviewedPublisher PD
Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars
Context. The recent detection of warm HO vapor emission from the outflows
of carbon-rich asymptotic giant branch (AGB) stars challenges the current
understanding of circumstellar chemistry. Two mechanisms have been invoked to
explain warm HO vapor formation. In the first, periodic shocks passing
through the medium immediately above the stellar surface lead to HO
formation. In the second, penetration of ultraviolet interstellar radiation
through a clumpy circumstellar medium leads to the formation of HO
molecules in the intermediate wind.
Aims. We aim to determine the properties of HO emission for a sample of
18 carbon-rich AGB stars and subsequently constrain which of the above
mechanisms provides the most likely warm HO formation pathway.
Methods, Results, and Conclusions. See paper
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