6,291 research outputs found
Making extreme computations possible with virtual machines
State-of-the-art algorithms generate scattering amplitudes for high-energy
physics at leading order for high-multiplicity processes as compiled code (in
Fortran, C or C++). For complicated processes the size of these libraries can
become tremendous (many GiB). We show that amplitudes can be translated to
byte-code instructions, which even reduce the size by one order of magnitude.
The byte-code is interpreted by a Virtual Machine with runtimes comparable to
compiled code and a better scaling with additional legs. We study the
properties of this algorithm, as an extension of the Optimizing Matrix Element
Generator (O'Mega). The bytecode matrix elements are available as alternative
input for the event generator WHIZARD. The bytecode interpreter can be
implemented very compactly, which will help with a future implementation on
massively parallel GPUs.Comment: 5 pages, 2 figures. arXiv admin note: substantial text overlap with
arXiv:1411.383
Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?
During the exothermic adsorption of molecules at solid surfaces dissipation
of the released energy occurs via the excitation of electronic and phononic
degrees of freedom. For metallic substrates the role of the nonadiabatic
electronic excitation channel has been controversially discussed, as the
absence of a band gap could favour an easy coupling to a manifold of
electronhole pairs of arbitrarily low energies. We analyse this situation for
the highly exothermic showcase system of molecular oxygen dissociating at
Pd(100), using time-dependent perturbation theory applied to first-principles
electronic-structure calculations. For a range of different trajectories of
impinging O2 molecules we compute largely varying electron-hole pair spectra,
which underlines the necessity to consider the high-dimensionality of the
surface dynamical process when assessing the total energy loss into this
dissipation channel. Despite the high Pd density of states at the Fermi level,
the concomitant non-adiabatic energy losses nevertheless never exceed about 5%
of the available chemisorption energy. While this supports an electronically
adiabatic description of the predominant heat dissipation into the phononic
system, we critically discuss the non-adiabatic excitations in the context of
the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed
V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin
asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)
The effect of configuration on strength, durability, and handle of Kevlar fabric-based materials
Five Kevlar based laminates and three Kevlar based coated materials were designed, hand made, and tested against comparative conventional Dacron based materials for strength, peel, tear, puncture, creases, and handle. Emphasis was placed on evaluating geometric orientation of constituents, use of elastomeric film in place of high modulus films, and the use of flying thread loom bias reinforcement of Kevlar yarns. Whereas, the performance of the Kevlar laminates was severely degraded by crease effects, significant gains in overall performance factors were shown for the coated Kevlar materials
Quantum Mechanics as a Gauge Theory of Metaplectic Spinor Fields
A hidden gauge theory structure of quantum mechanics which is invisible in
its conventional formulation is uncovered. Quantum mechanics is shown to be
equivalent to a certain Yang-Mills theory with an infinite-dimensional gauge
group and a nondynamical connection. It is defined over an arbitrary symplectic
manifold which constitutes the phase-space of the system under consideration.
The ''matter fields'' are local generalizations of states and observables; they
assume values in a family of local Hilbert spaces (and their tensor products)
which are attached to the points of phase-space. Under local frame rotations
they transform in the spinor representation of the metaplectic group Mp(2N),
the double covering of Sp(2N). The rules of canonical quantization are replaced
by two independent postulates with a simple group theoretical and differential
geometrical interpretation. A novel background-quantum split symmetry plays a
central role.Comment: 61 pages, late
Geometric Phases and Critical Phenomena in a Chain of Interacting Spins
The geometric phase can act as a signature for critical regions of
interacting spin chains in the limit where the corresponding circuit in
parameter space is shrunk to a point and the number of spins is extended to
infinity; for finite circuit radii or finite spin chain lengths, the geometric
phase is always trivial (a multiple of 2pi). In this work, by contrast, two
related signatures of criticality are proposed which obey finite-size scaling
and which circumvent the need for assuming any unphysical limits. They are
based on the notion of the Bargmann invariant whose phase may be regarded as a
discretized version of Berry's phase. As circuits are considered which are
composed of a discrete, finite set of vertices in parameter space, they are
able to pass directly through a critical point, rather than having to
circumnavigate it. The proposed mechanism is shown to provide a diagnostic tool
for criticality in the case of a given non-solvable one-dimensional spin chain
with nearest-neighbour interactions in the presence of an external magnetic
field.Comment: 7 Figure
Automated NLO QCD Corrections with WHIZARD
We briefly discuss the current status of NLO QCD automation in the Monte
Carlo event generator WHIZARD. The functionality is presented for the explicit
study of off-shell top quark production with associated backgrounds at a lepton
collider.Comment: 9 pages, 5 figures, to appear in the proceedings of the European
Physical Society Conference on High Energy Physics 2015 (EPS-HEP 2015),
Vienna, Austria, 22nd to 29th of July 201
Matching NLO QCD Corrections in WHIZARD with the POWHEG scheme
Building on the new automatic subtraction of NLO amplitudes in WHIZARD, we
present our implementation of the POWHEG scheme to match radiative corrections
consistently with the parton shower. We apply this general framework to two
linear collider processes, and
.Comment: 7 pages, 4 figures, to appear in the proceedings of the European
Physical Society Conference on High Energy Physics 2015 (EPS-HEP 2015),
Vienna, Austria, 22nd to 29th of July 201
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
A Spherical Plasma Dynamo Experiment
We propose a plasma experiment to be used to investigate fundamental
properties of astrophysical dynamos. The highly conducting, fast-flowing plasma
will allow experimenters to explore systems with magnetic Reynolds numbers an
order of magnitude larger than those accessible with liquid-metal experiments.
The plasma is confined using a ring-cusp strategy and subject to a toroidal
differentially rotating outer boundary condition. As proof of principle, we
present magnetohydrodynamic simulations of the proposed experiment. When a von
K\'arm\'an-type boundary condition is specified, and the magnetic Reynolds
number is large enough, dynamo action is observed. At different values of the
magnetic Prandtl and Reynolds numbers the simulations demonstrate either
laminar or turbulent dynamo action
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