7,960 research outputs found
Top Quark Properties
Recent measurements of top-quark properties at the LHC and at the Tevatron
are presented. The results include precision measurements of standard model
parameters, such as the top-quark mass, the measurement of angular
distributions as well as the search for anomalous couplings.Comment: Conference proceedings for Lepton Photon, Ljubljana, 17-22 August
2015, 12 pages, 10 figure
Top Quark Production
Recent measurements of top quark pair and single top production are
presented. The results include inclusive cross sections as well as studies of
differential distributions. Evidence for single top quark production in
association with a W-boson in the final state is reported for the first time.
Calculations in perturbative QCD up to approximate next-to-next-to-leading
order show very good agreement with the data.Comment: Physics in Collision, Slovakia, 2012 PSNUM 0
Lattice calculation of the pion transition form factor with Wilson quarks
We present a lattice QCD calculation of the double-virtual neutral pion
transition form factor, with the goal to cover the kinematic range relevant to
hadronic light-by-light scattering in the muon . Several improvements have
been made compared to our previous work. First, we take into account the
effects of the strange quark by using the CLS gauge ensembles.
Secondly, we have implemented the on-shell -improvement of the
vector current to reduce the discretization effects associated with Wilson
quarks. Finally, in order to have access to a wider range of photon
virtualities, we have computed the transition form factor in a moving frame as
well as in the pion rest-frame. After extrapolating the form factor to the
continuum and to physical quark masses, we compare our results with
phenomenology. We extract the normalization of the form factor with a precision
of 3.5\% and confirm within our uncertainty previous somewhat conflicting
estimates for a low-energy constant that appears in chiral perturbation theory
for the decay at NLO. With additional input from
experiment and theory, we reproduce recent estimates for the decay width
. We also study the asymptotic large-
behavior of the transition form factor in the double-virtual case. Finally, we
provide as our main result a more precise model-independent lattice estimate of
the pion-pole contribution to hadronic light-by-light scattering in the muon
: . Using
in addition the normalization of the form factor obtained by the PrimEx
experiment, we get the lattice and data-driven estimate
.Comment: 29 pages, 14 figures. v2: minor corrections to match the published
version. A file with the transition form factor data at the physical pion
mass and in the continuum is included in the submissio
Top-Quark Physics at the LHC
The top quark is the heaviest of all known elementary particles. It was
discovered in 1995 by the CDF and D0 experiments at the Tevatron. With the
start of the LHC in 2009, an unprecedented wealth of measurements of the top
quark's production mechanisms and properties have been performed by the ATLAS
and CMS collaborations, most of these resulting in smaller uncertainties than
those achieved previously. At the same time, huge progress was made on the
theoretical side yielding significantly improved predictions up to
next-to-next-to-leading order in perturbative QCD. Due to the vast amount of
events containing top quarks, a variety of new measurements became feasible and
opened a new window to precisions tests of the Standard Model and to
contributions of new physics. In this review, originally written for a recent
book on the results of LHC Run 1, top-quark measurements obtained so far from
the LHC Run 1 are summarised and put in context with the current understanding
of the Standard Model.Comment: 35 pages, 25 figures. To appear in "The Large Hadron Collider --
Harvest of Run 1", Thomas Sch\"orner-Sadenius (ed.), Springer, 2015 (532
pages, 253 figures; ISBN 978-3-319-15000-0; eBook ISBN 978-3-319-15001-7, for
more details, see http://www.springer.com/de/book/9783319150000
Recycling probability and dynamical properties of germinal center reactions
We introduce a new model for the dynamics of centroblasts and centrocytes in
a germinal center. The model reduces the germinal center reaction to the
elements considered as essential and embeds proliferation of centroblasts,
point mutations of the corresponding antibody types represented in a shape
space, differentiation to centrocytes, selection with respect to initial
antigens, differentiation of positively selected centrocytes to plasma or
memory cells and recycling of centrocytes to centroblasts. We use exclusively
parameters with a direct biological interpretation such that, once determined
by experimental data, the model gains predictive power. Based on the experiment
of Han et al. (1995b) we predict that a high rate of recycling of centrocytes
to centroblasts is necessary for the germinal center reaction to work reliably.
Furthermore, we find a delayed start of the production of plasma and memory
cells with respect to the start of point mutations, which turns out to be
necessary for the optimization process during the germinal center reaction. The
dependence of the germinal center reaction on the recycling probability is
analyzed.Comment: 30 pages, 8 figure
A structural analysis of the A5/1 state transition graph
We describe efficient algorithms to analyze the cycle structure of the graph
induced by the state transition function of the A5/1 stream cipher used in GSM
mobile phones and report on the results of the implementation. The analysis is
performed in five steps utilizing HPC clusters, GPGPU and external memory
computation. A great reduction of this huge state transition graph of 2^64
nodes is achieved by focusing on special nodes in the first step and removing
leaf nodes that can be detected with limited effort in the second step. This
step does not break the overall structure of the graph and keeps at least one
node on every cycle. In the third step the nodes of the reduced graph are
connected by weighted edges. Since the number of nodes is still huge an
efficient bitslice approach is presented that is implemented with NVIDIA's CUDA
framework and executed on several GPUs concurrently. An external memory
algorithm based on the STXXL library and its parallel pipelining feature
further reduces the graph in the fourth step. The result is a graph containing
only cycles that can be further analyzed in internal memory to count the number
and size of the cycles. This full analysis which previously would take months
can now be completed within a few days and allows to present structural results
for the full graph for the first time. The structure of the A5/1 graph deviates
notably from the theoretical results for random mappings.Comment: In Proceedings GRAPHITE 2012, arXiv:1210.611
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