2,567 research outputs found
QCD NLO with Powheg matching and top threshold matching in WHIZARD
We present the status of the automation of NLO processes within the event
generator WHIZARD. The program provides an automated FKS subtraction and phase
space integration over the FKS regions, while the (QCD) NLO matrix element is
accessed via the Binoth Les Houches Interface from an externally linked
one-loop program. Massless and massive test cases and validation are shown for
several e+e- processes. Furthermore, we discuss work in progress and future
plans. The second part covers the matching of the NRQCD prediction with NLL
threshold resummation to the NLO continuum top pair production at lepton
colliders. Both the S-wave and P-wave production of the top pair are taken into
account in the resummation. The inclusion in WHIZARD allows to study more
exclusive observables than just the total cross section and automatically
accounts for important electroweak and relativistic corrections in the
threshold region.Comment: 9 pages, 3 figures, Talk given at 12th International Symposium on
Radiative Corrections (Radcor 2015) and LoopFest XIV (Radiative Corrections
for the LHC and Future Colliders); v2: reference adde
C II abundances in early-type stars: solution to a notorious non-LTE problem
We address a long-standing discrepancy between non-LTE analyses of the
prominent C II 4267 and 6578/82 A multiplets in early-type stars. A
comprehensive non-LTE model atom of C II is constructed based on critically
selected atomic data. This model atom is used for an abundance study of six
apparently slow-rotating main-sequence and giant early B-type stars.
High-resolution and high-S/N spectra allow us to derive highly consistent
abundances not only from the classical features but also from up to 18 further
C II lines in the visual - including two so far unreported emission features
equally well reproduced in non-LTE. These results require the stellar
atmospheric parameters to be determined with care. A homogeneous (slightly)
sub-solar present-day carbon abundance from young stars in the solar vicinity
(in associations and in the field) of log C/H +12= 8.29+/-0.03 is indicated.Comment: 8 pages, 5 figure
The Abundance of Interstellar Nitrogen
Using the HST Goddard High Resolution Spectrograph (GHRS), we have obtained
high S/N echelle observations of the weak interstellar N I 1160, 1161 A
absorption doublet toward the stars Gamma Cas, Lambda Ori, Iota Ori, Kappa Ori,
Delta Sco, and Kappa Sco. In combination with a previous GHRS measurement of N
I toward Zeta Oph, these new observations yield a mean interstellar gas phase
nitrogen abundance (per 10 H atoms) of 10 N/H = 75 +/- 4. There are no
statistically significant variations in the measured N abundances from
sightline to sightline and no evidence of density-dependent depletion from the
gas-phase. Since N is not expected to be depleted much into dust grains in
these diffuse sightlines, its gas-phase abundance should reflect the total
interstellar abundance. Consequently, the GHRS observations imply that the
abundance of interstellar nitrogen (gas plus grains) in the local Milky Way is
about 80% of the solar system value of 10 N/H = 93 +/- 16. Although this
interstellar abundance deficit is somewhat less than that recently found for
oxygen and krypton with GHRS, the solar N abundance and the N I oscillator
strengths are too uncertain to definitively rule out either a solar ISM N
abundance or a 2/3 solar ISM N abundance similar to that of O and Kr.Comment: 14 pages, LaTeX, 2 Postscript figures; ApJ Letters, in pres
Modern Particle Physics Event Generation with WHIZARD
We describe the multi-purpose Monte-Carlo event generator WHIZARD for the
simulation of high-energy particle physics experiments. Besides the
presentation of the general features of the program like SM physics, BSM
physics, and QCD effects, special emphasis will be given to the support of the
most accurate simulation of the collider environments at hadron colliders and
especially at future linear lepton colliders. On the more technical side, the
very recent code refactoring towards a completely object-oriented software
package to improve maintainability, flexibility and code development will be
discussed. Finally, we present ongoing work and future plans regarding
higher-order corrections, more general model support including the setup to
search for new physics in vector boson scattering at the LHC, as well as
several lines of performance improvements.Comment: 7 pages; contribution to the proceedings of the conference "ACAT 2014
(Advanced Computing and Analysis Techniques in physics)", Prague, Czech
Republic, September 201
Top Physics in WHIZARD
In this talk we summarize the top physics setup in the event generator
WHIZARD with a main focus on lepton colliders. This includes full six-, eight-
and ten-fermion processes, factorized processes and spin correlations. For
lepton colliders, QCD NLO processes for top quark physics are available and
will be discussed. A special focus is on the top-quark pair threshold, where a
special implementation combines a non-relativistic effective field theory
calculation augmented by a next-to-leading threshold logarithm resummation with
a continuum relativistic fixed-order QCD NLO simulation.Comment: 6 pages, 2 figures, Talk presented at the International Workshop on
Future Linear Colliders (LCWS15), Whistler, Canada, 2-6 November 201
Non-perturbative effects in semi-leptonic B_c decays
We discuss the impact of the soft degrees of freedom inside the B_c meson on
its rate in the semi-leptonic decay B_c -> X l nu_l where X denotes light
hadrons below the D^0 threshold. In particular we identify contributions
involving soft hadrons which are non-vanishing in the limit of massless
leptons. These contributions become relevant for a measurement of the purely
leptonic B_c decay rate, which due to helicity suppression involves a factor
m_l^2 and thus is much smaller than the contributions involving soft hadrons.Comment: LaTeX, 22 pages, 1 figur
Focusing a deterministic single-ion beam
We focus down an ion beam consisting of single 40Ca+ ions to a spot size of a
few mum using an einzel-lens. Starting from a segmented linear Paul trap, we
have implemented a procedure which allows us to deterministically load a
predetermined number of ions by using the potential shaping capabilities of our
segmented ion trap. For single-ion loading, an efficiency of 96.7(7)% has been
achieved. These ions are then deterministically extracted out of the trap and
focused down to a 1sigma-spot radius of (4.6 \pm 1.3)mum at a distance of 257mm
from the trap center. Compared to former measurements without ion optics, the
einzel-lens is focusing down the single-ion beam by a factor of 12. Due to the
small beam divergence and narrow velocity distribution of our ion source,
chromatic and spherical aberration at the einzel-lens is vastly reduced,
presenting a promising starting point for focusing single ions on their way to
a substrate.Comment: 16 pages, 7 figure
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