69,187 research outputs found
Computation of in FDH and DRED: renormalization, operator mixing, and explicit two-loop results
The amplitude relevant for Higgs production via gluon fusion is
computed in the four-dimensional helicity scheme (FDH) and in dimensional
reduction (DRED) at the two-loop level. The required renormalization is
developed and described in detail, including the treatment of evanescent
-scalar contributions. In FDH and DRED there are additional
dimension-5 operators generating the vertices, where can either be
a gluon or an -scalar. An appropriate operator basis is given and the
operator mixing through renormalization is described. The results of the
present paper provide building blocks for further computations, and they allow
to complete the study of the infrared divergence structure of two-loop
amplitudes in FDH and DRED
SCET approach to regularization-scheme dependence of QCD amplitudes
We investigate the regularization-scheme dependence of scattering amplitudes
in massless QCD and find that the four-dimensional helicity scheme (FDH) and
dimensional reduction (DRED) are consistent at least up to NNLO in the
perturbative expansion if renormalization is done appropriately. Scheme
dependence is shown to be deeply linked to the structure of UV and IR
singularities. We use jet and soft functions defined in soft-collinear
effective theory (SCET) to efficiently extract the relevant anomalous
dimensions in the different schemes. This result allows us to construct
transition rules for scattering amplitudes between different schemes (CDR, HV,
FDH, DRED) up to NNLO in massless QCD. We also show by explicit calculation
that the hard, soft and jet functions in SCET are regularization-scheme
independent.Comment: 46 pages, 6 figure
A Multifunctional Processing Board for the Fast Track Trigger of the H1 Experiment
The electron-proton collider HERA is being upgraded to provide higher
luminosity from the end of the year 2001. In order to enhance the selectivity
on exclusive processes a Fast Track Trigger (FTT) with high momentum resolution
is being built for the H1 Collaboration. The FTT will perform a 3-dimensional
reconstruction of curved tracks in a magnetic field of 1.1 Tesla down to 100
MeV in transverse momentum. It is able to reconstruct up to 48 tracks within 23
mus in a high track multiplicity environment. The FTT consists of two hardware
levels L1, L2 and a third software level. Analog signals of 450 wires are
digitized at the first level stage followed by a quick lookup of valid track
segment patterns.
For the main processing tasks at the second level such as linking, fitting
and deciding, a multifunctional processing board has been developed by the ETH
Zurich in collaboration with Supercomputing Systems (Zurich). It integrates a
high-density FPGA (Altera APEX 20K600E) and four floating point DSPs (Texas
Instruments TMS320C6701). This presentation will mainly concentrate on second
trigger level hardware aspects and on the implementation of the algorithms used
for linking and fitting. Emphasis is especially put on the integrated CAM
(content addressable memory) functionality of the FPGA, which is ideally suited
for implementing fast search tasks like track segment linking.Comment: 6 pages, 4 figures, submitted to TN
Structure of self-assembled Mn atom chains on Si(001)
Mn has been found to self-assemble into atomic chains running perpendicular
to the surface dimer reconstruction on Si(001). They differ from other atomic
chains by a striking asymmetric appearance in filled state scanning tunneling
microscopy (STM) images. This has prompted complicated structural models
involving up to three Mn atoms per chain unit. Combining STM, atomic force
microscopy and density functional theory we find that a simple necklace-like
chain of single Mn atoms reproduces all their prominent features, including
their asymmetry not captured by current models. The upshot is a remarkably
simpler structure for modelling the electronic and magnetic properties of Mn
atom chains on Si(001).Comment: 5 pages, 4 figure
Ground States and Flux Configurations of the Two-dimensional Falicov-Kimball Model
The Falicov-Kimball model is a lattice model of itinerant spinless fermions
("electrons") interacting by an on-site potential with classical particles
("ions"). We continue the investigations of the crystalline ground states that
appear for various filling of electrons and ions, for large coupling. We
investigate the model for square as well as triangular lattices. New ground
states are found and the effects of a magnetic flux on the structure of the
phase diagram is studied. The flux phase problem where one has to find the
optimal flux configurations and the nuclei configurations is also solved in
some cases. Finaly we consider a model where the fermions are replaced by
hard-core bosons. This model also has crystalline ground states. Therefore
their existence does not require the Pauli principle, but only the on-site
hard-core constraint for the itinerant particles.Comment: 42 pages, uuencoded postscript file. Missing pages adde
The Mellin Transform Technique for the Extraction of the Gluon Density
A new method is presented to determine the gluon density in the proton from
jet production in deeply inelastic scattering. By using the technique of Mellin
transforms not only for the solution of the scale evolution equation of the
parton densities but also for the evaluation of scattering cross sections, the
gluon density can be extracted in next-to-leading order QCD. The method
described in this paper is, however, more general, and can be used in
situations where a repeated fast numerical evaluation of scattering cross
sections for varying parton distribution functions is required.Comment: 13 pages (LaTeX); 2 figures are included via epsfig; the
corresponding postscript files are uuencode
Saturation effects in the sub-Doppler spectroscopy of Cesium vapor confined in an Extremely Thin Cell
Saturation effects affecting absorption and fluorescence spectra of an atomic
vapor confined in an Extremely Thin Cell (cell thickness ) are
investigated experimentally and theoretically. The study is performed on the
line ( of and concentrates on the two
situations and , the most contrasted ones with
respect to the length dependence of the coherent Dicke narrowing. For , the Dicke-narrowed absorption profile simply broadens and
saturates in amplitude when increasing the light intensity, while for , sub-Doppler dips of reduced absorption at line-center appear on the
broad absorption profile. For a fluorescence detection at ,
saturation induces narrow dips, but only for hyperfine components undergoing a
population loss through optical pumping. These experimental results are
interpreted with the help of the various existing models, and are compared with
numerical calculations based upon a two-level modelling that considers both a
closed and an open system.Comment: 11 pages, 12 figure
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