3,957 research outputs found
Cross Sections for Charm Production in Collisions: Massive versus Massless Scheme
The next--to--leading order inclusive cross section for large-
photoproduction of charm quarks at HERA is calculated in two different
approaches. In the first approach the charm quarks are treated as massive
objects which are strictly external to the proton and the photon while in the
second approach the charm mass is neglected and the quark is assumed to be
one of the active flavours in the proton and photon structure functions. We
present single-inclusive distributions in transverse momentum and rapidity
including direct and resolved photons. The cross section in the massless
approach is found to be significantly larger than in the massive scheme. The
deviation originates from several contributions which are disentangled. We
argue that large- photoproduction of charm quarks at HERA will be
sensitive to the charm content of the photon structure function.Comment: 11 pages, Latex, epsfig, 6 figures appended as uuencoded file
(hardcopy can be obtained upon request from [email protected]
Observation of plaquette fluctuations in the spin-1/2 honeycomb lattice
Quantum spin liquids are materials that feature quantum entangled spin
correlations and avoid magnetic long-range order at T = 0 K. Particularly
interesting are two-dimensional honeycomb spin lattices where a plethora of
exotic quantum spin liquids have been predicted. Here, we experimentally study
an effective S=1/2 Heisenberg honeycomb lattice with competing nearest and
next-nearest neighbor interactions. We demonstrate that YbBr avoids order
down to at least T=100 mK and features a dynamic spin-spin correlation function
with broad continuum scattering typical of quantum spin liquids near a quantum
critical point. The continuum in the spin spectrum is consistent with plaquette
type fluctuations predicted by theory. Our study is the experimental
demonstration that strong quantum fluctuations can exist on the honeycomb
lattice even in the absence of Kitaev-type interactions, and opens a new
perspective on quantum spin liquids.Comment: 32 pages, 7 Figure
More nonperturbative corrections to the fine and hyperfine splitting in the heavy quarkonium
The leading nonperturbative effects to the fine and hyperfine splitting were
calculated some time ago. Recently, they have been used in order to obtain
realistic numerical results for the lower levels in bottomonium systems. We
point out that a contribution of the same order has been overlooked. We calculate it in this paper.Comment: 9 pages, LaTeX, More self-contained and lengthier version without
changing physical outputs. To be published in Phys. Rev.
Hysteresis effects in rotating Bose-Einstein condensates
We study the formation of vortices in a dilute Bose-Einstein condensate
confined in a rotating anisotropic trap. We find that the number of vortices
and angular momentum attained by the condensate depends upon the rotation
history of the trap and on the number of vortices present in the condensate
initially. A simplified model based on hydrodynamic equations is developed, and
used to explain this effect in terms of a shift in the resonance frequency of
the quadrupole mode of the condensate in the presence of a vortex lattice.
Differences between the spin-up and spin-down response of the condensate are
found, demonstrating hysteresis phenomena in this system.Comment: 16 pages, 7 figures; revised after referees' report
High-resolution wide-band Fast Fourier Transform spectrometers
We describe the performance of our latest generations of sensitive wide-band
high-resolution digital Fast Fourier Transform Spectrometer (FFTS). Their
design, optimized for a wide range of radio astronomical applications, is
presented. Developed for operation with the GREAT far infrared heterodyne
spectrometer on-board SOFIA, the eXtended bandwidth FFTS (XFFTS) offers a high
instantaneous bandwidth of 2.5 GHz with 88.5 kHz spectral resolution and has
been in routine operation during SOFIA's Basic Science since July 2011. We
discuss the advanced field programmable gate array (FPGA) signal processing
pipeline, with an optimized multi-tap polyphase filter bank algorithm that
provides a nearly loss-less time-to-frequency data conversion with
significantly reduced frequency scallop and fast sidelobe fall-off. Our digital
spectrometers have been proven to be extremely reliable and robust, even under
the harsh environmental conditions of an airborne observatory, with
Allan-variance stability times of several 1000 seconds. An enhancement of the
present 2.5 GHz XFFTS will duplicate the number of spectral channels (64k),
offering spectroscopy with even better resolution during Cycle 1 observations.Comment: Accepted for publication in A&A (SOFIA/GREAT special issue
Supersymmetric NLO QCD Corrections to Resonant Slepton Production and Signals at the Tevatron and the LHC
We compute the total cross section and the transverse momentum distribution
for single charged slepton and sneutrino production at hadronic colliders
including NLO supersymmetric and non-supersymmetric QCD corrections. The
supersymmetric QCD corrections can be substantial. We also resum the gluon
transverse momentum distribution and compare our results with two Monte Carlo
generators. We compute branching ratios of the supersymmetric decays of the
slepton and determine event rates for the like-sign dimuon final state at the
Tevatron and at the LHC.Comment: 14 pages, LaTeX, 8 figures, uses REVTex
Enhancement of p-GaN conductivity using PECVD SiOx
A technique to enhance the hole concentration in activated Mg-doped p-type GaN epitaxial layers is described. The method consists of depositing a porous plasma-nhancedchemical vapor deposited SiOx layer on top of p-GaN after which the sample is heated to 950°C in nitrogen ambient for 1 min followed by the removal of the SiOx layer in a buffered HF solution. A significant improvement of the conductivity of the p-GaN layer has been obtained
Non-equilibrium hysteresis and spin relaxation in the mixed-anisotropy dipolar coupled spin-glass LiHoErF
We present a study of the model spin-glass LiHoErF using
simultaneous AC susceptibility, magnetization and magnetocaloric effect
measurements along with small angle neutron scattering (SANS) at sub-Kelvin
temperatures. All measured bulk quantities reveal hysteretic behavior when the
field is applied along the crystallographic c axis. Furthermore avalanche-like
relaxation is observed in a static field after ramping from the
zero-field-cooled state up to Oe. SANS measurements are employed to
track the microscopic spin reconfiguration throughout both the hysteresis loop
and the related relaxation. Comparing the SANS data to inhomogeneous mean-field
calculations performed on a box of one million unit cells provides a real-space
picture of the spin configuration. We discover that the avalanche is being
driven by released Zeeman energy, which heats the sample and creates positive
feedback, continuing the avalanche. The combination of SANS and mean-field
simulations reveal that the conventional distribution of cluster sizes is
replaced by one with a depletion of intermediate cluster sizes for much of the
hysteresis loop.Comment: 6 pages, 4 figure
Highly parallel multi-physics simulation of muscular activation and EMG
Simulation of skeletal muscle activation can help to interpret electromyographic measurements and infer the behavior of the muscle fibers. Existing models consider simplified geometries or a low number of muscle fibers to reduce the computation time. We demonstrate how to simulate a finely-resolved model of biceps brachii with a typical number of 270.000 fibers. We have used domain decomposition to run simulations on 27.000 cores of the supercomputer HazelHen at HLRS in Stuttgart, Germany. We present details on opendihu, our software framework. Its configurability, efficient data structures and modular software architecture target usability, performance and extensibility for future models. We present good parallel weak scaling of the simulations
Prediction of Aerodynamic Coefficients of Road Vehicles on Bridge Deck with and without Wind Protection by Means of CFD for Crosswind Stability Investigations
While planning a new bridge construction the risk of traffic accidents due to critical wind conditions should be carefully considered. The determination of aerodynamic forces and moments on vehicles is indispensable for stability investigations. However, the aerodynamic coefficients of vehicle-bridge systems depend on many factors which make it difficult to generalise the procedure. This paper is focusing on analysing a particular bridge geometry whereby aerodynamic coefficients were predicted by means of CFD. The accuracy of the numerical model was validated with the aid of experimental data from wind tunnel tests. Specifically, this work was conducted to investigate the effect of the wind barrier considering various wind flow angles and vehicle speeds. Mean forces and moments on the vehicle were analysed depending on both absolute and relative wind flows. The impact of performing relative motion between vehicle and bridge deck was investigated. Simulation results without wind barrier are qualitatively in good agreement with results found in literature. Nevertheless, the flow situation with wind barrier and relative motion is significantly more complex. Thus, CFD modelling has dominating advantages over wind tunnel tests in terms of both parameter variation and model accuracy. In this particular case CFD modelling is indeed essential in order to represent all possible wind flow angles and the relative motion between the vehicle and the bridge deck which remains difficult or rather hardly possible to perform in the wind tunnel
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