5,299 research outputs found
Infrared structure of jets at NNLO: QED-type contributions
The NNLO QCD corrections to the jets can be decomposed
according to their colour factors. Out of the seven colour factors, three are
of QED-type: , and . We use the antenna subtraction
method to compute these contributions, providing complete expressions for the
subtraction terms in and .Comment: Talk presented at Loops and Legs 2006, Eisenac
Measuring the Photon Fragmentation Function at HERA
The production of final state photons in deep inelastic scattering originates
from photon radiation off leptons or quarks involved in the scattering process.
Photon radiation off quarks involves a contribution from the quark-to-photon
fragmentation function, corresponding to the non-perturbative transition of a
hadronic jet into a single, highly energetic photon accompanied by some limited
hadronic activity. Up to now, this fragmentation function was measured only in
electron-positron annihilation at LEP. We demonstrate by a dedicated
parton-level calculation that a competitive measurement of the quark-to-photon
fragmentation function can be obtained in deep inelastic scattering at HERA.
Such a measurement can be obtained by studying the photon energy spectra in
-jet events, where denotes a hadronic jet containing a
highly energetic photon (the photon jet). Isolated photons are then defined
from the photon jet by imposing a minimal photon energy fraction. For this
so-called democratic clustering approach, we study the cross sections for
isolated -jet and -jet production as well as
for the inclusive isolated photon production in deep inelastic scattering.Comment: 23 pages, LaTe
Semiparametric Cross Entropy for rare-event simulation
The Cross Entropy method is a well-known adaptive importance sampling method
for rare-event probability estimation, which requires estimating an optimal
importance sampling density within a parametric class. In this article we
estimate an optimal importance sampling density within a wider semiparametric
class of distributions. We show that this semiparametric version of the Cross
Entropy method frequently yields efficient estimators. We illustrate the
excellent practical performance of the method with numerical experiments and
show that for the problems we consider it typically outperforms alternative
schemes by orders of magnitude
Bayesian peak bagging analysis of 19 low-mass low-luminosity red giants observed with Kepler
The currently available Kepler light curves contain an outstanding amount of
information but a detailed analysis of the individual oscillation modes in the
observed power spectra, also known as peak bagging, is computationally
demanding and challenging to perform on a large number of targets. Our intent
is to perform for the first time a peak bagging analysis on a sample of 19
low-mass low-luminosity red giants observed by Kepler for more than four years.
This allows us to provide high-quality asteroseismic measurements that can be
exploited for an intensive testing of the physics used in stellar structure
models, stellar evolution and pulsation codes, as well as for refining existing
asteroseismic scaling relations in the red giant branch regime. For this
purpose, powerful and sophisticated analysis tools are needed. We exploit the
Bayesian code Diamonds, using an efficient nested sampling Monte Carlo
algorithm, to perform both a fast fitting of the individual oscillation modes
and a peak detection test based on the Bayesian evidence. We find good
agreement for the parameters estimated in the background fitting phase with
those given in the literature. We extract and characterize a total of 1618
oscillation modes, providing the largest set of detailed asteroseismic mode
measurements ever published. We report on the evidence of a change in regime
observed in the relation between linewidths and effective temperatures of the
stars occurring at the bottom of the RGB. We show the presence of a linewidth
depression or plateau around for all the red giants of the
sample. Lastly, we show a good agreement between our measurements of maximum
mode amplitudes and existing maximum amplitudes from global analyses provided
in the literature, useful as empirical tools to improve and simplify the future
peak bagging analysis on a larger sample of evolved stars.Comment: 78 pages, 46 figures, 22 tables. Accepted for publication in A&
High-precision acoustic helium signatures in 18 low-mass low-luminosity red giants. Analysis from more than four years of Kepler observations
High-precision frequencies of acoustic modes in red giant stars are now
available thanks to the long observing length and high-quality of the light
curves provided by the NASA Kepler mission, thus allowing to probe the interior
of evolved cool low-mass stars with unprecedented level of detail. We
characterize the acoustic signature of the helium second ionization zone in a
sample of 18 low-mass low-luminosity red giants by exploiting new mode
frequency measurements derived from more than four years of Kepler
observations. We analyze the second frequency differences of radial acoustic
modes in all the stars of the sample by using the Bayesian code Diamonds. We
find clear acoustic glitches due to the signature of helium second ionization
in all the stars of the sample. We measure the acoustic depth and the
characteristic width of the acoustic glitches with a precision level on average
around 2% and 8%, respectively. We find good agreement with
theoretical predictions and existing measurements from the literature. Lastly,
we derive the amplitude of the glitch signal at for the
second differences and for the frequencies with an average precision of
6%, obtaining values in the range 0.14-0.24 Hz, and 0.08-0.33
Hz, respectively, which can be used to investigate the helium abundance in
the stars.Comment: 12 pages, 19 figures, 3 tables. Accepted for publication in A&
Deep-inelastic Electron-Photon Scattering at High Q^2 : Neutral and Charged Current Reactions
We present the results of a calculation of deep inelastic electron-photon
scattering at a linear collider for very high virtuality of the intermediate
gauge boson up to NLO in perturbative QCD. The real photon is produced
unpolarized via the Compton back scattering of laser light of the incoming
beam. For values close to the masses squared of the Z and W gauge bosons,
the deep inelastic electron-photon scattering process receives important
contributions not only from virtual photon exchange but also from the exchange
of a Z-boson and a W-boson. We find that the total cross section for center of
mass energies above is at least of and has an
important charged current contribution.Comment: Talk given at the International Conference on the Structure and
Interactions of the Photon, PHOTON 99, Freiburg im Breisgau, Germany, May
23-27, 1999. To be published in the Proceedings. 6 pages, 6 postscript
figures. The complete paper, including figures, is also available via
anonymous ftp at ftp://ttpux2.physik.uni-karlsruhe.de/ttp99/ttp99-30/ or via
www at http://www-ttp.physik.uni-karlsruhe.de/Preprints
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