4,934 research outputs found
Color-flow decomposition of QCD amplitudes
We introduce a new color decomposition for multi-parton amplitudes in QCD,
free of fundamental-representation matrices and structure constants. This
decomposition has a physical interpretation in terms of the flow of color,
which makes it ideal for merging with shower Monte-Carlo programs. The
color-flow decomposition allows for very efficient evaluation of amplitudes
with many quarks and gluons, many times faster than the standard color
decomposition based on fundamental-representation matrices. This will increase
the speed of event generators for multi-jet processes, which are the principal
backgrounds to signals of new physics at colliders.Comment: 23 pages, 11 figures, version to appear on Phys. Rev.
Momentum Correlations of Charmed Pairs Produced in Interactions at 230 GeV/c
We study the production characteristics of 557 pairs of charmed hadrons
produced in \ interactions at 230~GeV/c using a momentum estimator
for charmed hadrons with missing decay products. We find, the mean value of the
transverse momentum squared of the charmed pairs is GeV/c, the mean rapidity difference is , and the mean effective mass is GeV/c. Comparing these results with the next-to-leading order QCD
predictions we find an agreement for the \yd\ and \mef, whilst the measured
mean value of \pts\ is significantly larger than the predicted value.Comment: 10 pages, LaTeX, 4 (pages) postscript figure
MadEvent: Automatic Event Generation with MadGraph
We present a new multi-channel integration method and its implementation in
the multi-purpose event generator MadEvent, which is based on MadGraph. Given a
process, MadGraph automatically identifies all the relevant subprocesses,
generates both the amplitudes and the mappings needed for an efficient
integration over the phase space, and passes them to MadEvent. As a result, a
process-specific, stand-alone code is produced that allows the user to
calculate cross sections and produce unweighted events in a standard output
format. Several examples are given for processes that are relevant for physics
studies at present and forthcoming colliders.Comment: 11 pages, MadGraph home page at http://madgraph.physics.uiuc.ed
The Charm Content of W+1 Jet Events as a Probe of the Strange Quark Distribution Function
We investigate the prospects for measuring the strange quark distribution
function of the proton in associated plus charm quark production at the
Tevatron. The quark signal produced by strange quark -- gluon fusion,
and , is approximately 5\%
of the inclusive jet cross section for jets with a transverse momentum
~GeV. We study the sensitivity of the plus charm quark cross
section to the parametrization of the strange quark distribution function, and
evaluate the various background processes. Strategies to identify charm quarks
in CDF and D\O \ are discussed. For a charm tagging efficiency of about 10\%
and an integrated luminosity of 30~pb or more, it should be possible to
constrain the strange quark distribution function from production at the
Tevatron.Comment: submitted to Phys. Lett. B, Latex, 12 pages + 4 postscript figures
encoded with uufile, FSU-HEP-930812, MAD/TH/93-6, MAD/PH/788. A postscript
file with text and embedded figures is available via anonymous ftp at
hepsg1.physics.fsu.edu, file is /pub/keller/fsu-hep-930812.p
Monte Carlo studies of the jet activity in Higgs + 2 jet events
Tree-level studies have shown in the past that kinematical correlations
between the two jets in Higgs+2-jet events are direct probes of the Higgs
couplings, e.g. of their CP nature. In this paper we explore the impact of
higher-order corrections on the azimuthal angle correlation of the two leading
jets and on the rapidity distribution of extra jets. Our study includes
matrix-element and shower MC effects, for the two leading sources of Higgs plus
two jet events at the CERN LHC, namely vector-boson and gluon fusion. We show
that the discriminating features present in the previous leading-order matrix
element studies survive.Comment: 12 pages, 10 figures. Version to appear on JHEP. Figs. 5-8 replaced
with colour version
Small-x Physics with the ALICE experiment at the CERN-LHC
High energy p-p, p-Pb and Pb-Pb collisions at the CERN-LHC offer
unprecedented opportunities for studying wide variety of physics at small
Bjorken-x. Here we discuss capabilities of the ALICE experiment at the CERN-LHC
for probing small-x QCD physics. A new forward electromagnetic calorimeter is
being proposed as an ALICE upgrade to explore the small-x region in more
detail.Comment: Prepared for the Workshop on "Saturation, the Color Glass Condensate
and Glasma: What Have we Learned from RHIC?" - to be published in Nucl. Phys.
Calculation of HELAS amplitudes for QCD processes using graphics processing unit (GPU)
We use a graphics processing unit (GPU) for fast calculations of helicity
amplitudes of quark and gluon scattering processes in massless QCD. New HEGET
({\bf H}ELAS {\bf E}valuation with {\bf G}PU {\bf E}nhanced {\bf T}echnology)
codes for gluon self-interactions are introduced, and a C++ program to convert
the MadGraph generated FORTRAN codes into HEGET codes in CUDA (a C-platform for
general purpose computing on GPU) is created. Because of the proliferation of
the number of Feynman diagrams and the number of independent color amplitudes,
the maximum number of final state jets we can evaluate on a GPU is limited to 4
for pure gluon processes (), or 5 for processes with one or more
quark lines such as and . Compared with the usual
CPU-based programs, we obtain 60-100 times better performance on the GPU,
except for 5-jet production processes and the processes for which
the GPU gain over the CPU is about 20
Multi-gluon helicity amplitudes with one off-shell leg within high energy factorization
Basing on the Slavnov-Taylor identities, we derive a new prescription to
obtain gauge invariant tree-level scattering amplitudes for the process g*g->Ng
within high energy factorization. Using the helicity method, we check the
formalism up to several final state gluons, and we present analytical formulas
for the the helicity amplitudes for N=2. We also compare the method with
Lipatov's effective action approach.Comment: 25 pages, quite a few figures, an appendix added, typos correcte
- …