1,879 research outputs found
Gluon Fragmentation into Heavy Quarkonium
The dominant production mechanism for heavy quark-antiquark bound states in
very high energy processes is fragmentation, the splitting of a high energy
parton into a quarkonium state and other partons. We show that the
fragmentation functions describing these processes can be calculated
using perturbative QCD. We calculate the fragmentation functions for a gluon to
split into S-wave quarkonium states to leading order in the QCD coupling
constant. The leading logarithms of , where is the factorization
scale and is the heavy quark mass, are summed up using Altarelli-Parisi
evolution equations.Comment: LateX 11 pages (3 figures available upon request). NUHEP-TH-92-2
Subleading Sudakov Logarithms in Electroweak Processes
Recent results for the asymptotic behavior of fermion scattering amplitudes
in the Sudakov limit are presented including next-to-leading logarithmic
corrections. These are used for the analysis of the dominant electroweak
corrections to the fermion-antifermion pair production in annihilation
at high energy.Comment: 6 pages, Latex, npb.sty is required (included). The complete paper is
also available via anonymous
ftp://ttpux2.physik.uni-karlsruhe.de/ttp00/ttp00-11/ or via www at
http://www-ttp.physik.uni-karlsruhe.de/Preprints
Crystal Structures and Electronic Properties of Haloform-Intercalated C60
Using density functional methods we calculated structural and electronic
properties of bulk chloroform and bromoform intercalated C60, C60 2CHX3
(X=Cl,Br). Both compounds are narrow band insulator materials with a gap
between valence and conduction bands larger than 1 eV. The calculated widths of
the valence and conduction bands are 0.4-0.6 eV and 0.3-0.4 eV, respectively.
The orbitals of the haloform molecules overlap with the orbitals of the
fullerene molecules and the p-type orbitals of halogen atoms significantly
contribute to the valence and conduction bands of C60 2CHX3. Charging with
electrons and holes turns the systems to metals. Contrary to expectation, 10 to
20 % of the charge is on the haloform molecules and is thus not completely
localized on the fullerene molecules. Calculations on different crystal
structures of C60 2CHCl3 and C60 2CHBr3 revealed that the density of states at
the Fermi energy are sensitive to the orientation of the haloform and C60
molecules. At a charging of three holes, which corresponds to the
superconducting phase of pure C60 and C60 2CHX3, the calculated density of
states (DOS) at the Fermi energy increases in the sequence DOS(C60) < DOS(C60
2CHCl3) < DOS(C60 2CHBr3).Comment: 11 pages, 7 figures, 4 table
Adapting Real Quantifier Elimination Methods for Conflict Set Computation
The satisfiability problem in real closed fields is decidable. In the context
of satisfiability modulo theories, the problem restricted to conjunctive sets
of literals, that is, sets of polynomial constraints, is of particular
importance. One of the central problems is the computation of good explanations
of the unsatisfiability of such sets, i.e.\ obtaining a small subset of the
input constraints whose conjunction is already unsatisfiable. We adapt two
commonly used real quantifier elimination methods, cylindrical algebraic
decomposition and virtual substitution, to provide such conflict sets and
demonstrate the performance of our method in practice
Next-to-Next-to-Leading Electroweak Logarithms in W-pair Production at ILC
We derive the high energy asymptotic behavior of gauge boson production cross
section in a spontaneously broken SU(2) gauge theory in the
next-to-next-to-leading logarithmic approximation. On the basis of this result
we obtain the logarithmically enhanced two-loop electroweak corrections to the
differential cross section of W-pair production at ILC/CLIC up to the second
power of the large logarithm.Comment: 17 pages, LaTeX, Eqs. (31) and (35) correcte
Sampled-data control of hybrid systems with discrete inputs and outputs
We address the control synthesis of hybrid systems with discrete inputs, disturbances and outputs. The control objective is to ensure that the events of the closed-loop system belong to the language of the control requirements. The controller is sampling-based and it is representable by a finite-state machine. We formalize the control problem and provide a theoretically sound solution. The solution is based on solving a discrete-event control problem for a finite-state abstraction of the plant. We propose a specific construction for the finite-state abstraction. This construction is not based on discretizing the state-space, but rather on converting the continuous-time hybrid system to a discrete-time one based on sampling. The construction works only for a specific class of hybrid systems. We describe this class of systems and we provide an example of such a system, inspired by an industrial use-case
Current in the light-front Bethe-Salpeter formalism II: Applications
We pursue applications of the light-front reduction of current matrix
elements in the Bethe-Salpeter formalism. The normalization of the reduced wave
function is derived from the covariant framework and related to non-valence
probabilities using familiar Fock space projection operators. Using a simple
model, we obtain expressions for generalized parton distributions that are
continuous. The non-vanishing of these distributions at the crossover between
kinematic regimes (where the plus component of the struck quark's momentum is
equal to the plus component of the momentum transfer) is tied to higher Fock
components. Moreover continuity holds due to relations between Fock components
at vanishing plus momentum. Lastly we apply the light-front reduction to
time-like form factors and derive expressions for the generalized distribution
amplitudes in this model.Comment: 12 pages, 6 figures, RevTex
Epibenthic and mobile species colonisation of a geotextile artificial surf reef on the south coast of England
With increasing coastal infrastructure and use of novel materials there is a need to investigate the colonisation of assemblages associated with new structures, how these differ to natural and other artificial habitats and their potential impact on regional biodiversity. The colonisation of Europe’s first artificial surf reef (ASR) was investigated at Boscombe on the south coast of England (2009–2014) and compared with assemblages on existing natural and artificial habitats. The ASR consists of geotextile bags filled with sand located 220m offshore on a sandy sea bed at a depth of 0-5m. Successional changes in epibiota were recorded annually on differently orientated surfaces and depths using SCUBA diving and photography. Mobile faunal assemblages were sampled using Baited Remote Underwater Video (BRUV). Distinct stages in colonisation were observed, commencing with bryozoans and green algae which were replaced by red algae, hydroids and ascidians, however there were significant differences in assemblage structure with depth and orientation. The reef is being utilised by migratory, spawning and juvenile life-history stages of fish and invertebrates. The number of non-native species was larger than on natural reefs and other artificial habitats and some occupied a significant proportion of the structure. The accumulation of 180 benthic and mobile taxa, recorded to date, appears to have arisen from a locally rich and mixed pool of native and non-native species. Provided no negative invasive impacts are detected on nearby protected reefs the creation of novel yet diverse habitats may be considered a beneficial outcome
Next-to-Next-to-Leading Logarithms in Four-Fermion Electroweak Processes at High Energy
We sum up the next-to-next-to-leading logarithmic virtual electroweak
corrections to the high energy asymptotics of the neutral current four-fermion
processes for light fermions to all orders in the coupling constants using the
evolution equation approach. From this all order result we derive finite order
expressions through next-to-next-to leading order for the total cross section
and various asymmetries. We observe an amazing cancellation between the sizable
leading, next-to-leading and next-to-next-to-leading logarithmic contributions
at TeV energies.Comment: latex, 20 pages; coupling constant in the fermionic contribution to
the electroweak corrections correcte
On the trace identity in a model with broken symmetry
Considering the simple chiral fermion meson model when the chiral symmetry is
explicitly broken, we show the validity of a trace identity -- to all orders of
perturbation theory -- playing the role of a Callan-Symanzik equation and which
allows us to identify directly the breaking of dilatations with the trace of
the energy-momentum tensor. More precisely, by coupling the quantum field
theory considered to a classical curved space background, represented by the
non-propagating external vielbein field, we can express the conservation of the
energy-momentum tensor through the Ward identity which characterizes the
invariance of the theory under the diffeomorphisms. Our ``Callan-Symanzik
equation'' then is the anomalous Ward identity for the trace of the
energy-momentum tensor, the so-called ``trace identity''.Comment: 11 pages, Revtex file, final version to appear in Phys.Rev.
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