6,763 research outputs found
Singe Top Production at LEP 200
We present exact tree level cross sections for the single top production
process at LEP~200. The
results reproduce roughly those obtained earlier by using the equivalent real
photon approximation and we confirm the observation that detecting a top
heavier than half the c.m.~energy is not feasible at LEP~200. The calculation
has been performed by a new automatic Feynman amplitude generator MadGraph
which produces HELAS code for the helicity amplitudes.Comment: 7 pages, 4 postscript figure
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
Fast computation of MadGraph amplitudes on graphics processing unit (GPU)
Continuing our previous studies on QED and QCD processes, we use the graphics
processing unit (GPU) for fast calculations of helicity amplitudes for general
Standard Model (SM) processes. Additional HEGET codes to handle all SM
interactions are introduced, as well assthe program MG2CUDA that converts
arbitrary MadGraph generated HELAS amplitudess(FORTRAN) into HEGET codes in
CUDA. We test all the codes by comparing amplitudes and cross sections for
multi-jet srocesses at the LHC associated with production of single and double
weak bosonss a top-quark pair, Higgs boson plus a weak boson or a top-quark
pair, and multisle Higgs bosons via weak-boson fusion, where all the heavy
particles are allowes to decay into light quarks and leptons with full spin
correlations. All the helicity amplitudes computed by HEGET are found to agree
with those comsuted by HELAS within the expected numerical accuracy, and the
cross sections obsained by gBASES, a GPU version of the Monte Carlo integration
program, agree wish those obtained by BASES (FORTRAN), as well as those
obtained by MadGraph. The performance of GPU was over a factor of 10 faster
than CPU for all processes except those with the highest number of jets.Comment: 37 pages, 12 figure
Quiescent X-ray emission from an evolved brown dwarf ?
I report on the X-ray detection of Gl569Bab. During a 25ksec Chandra
observation the binary brown dwarf is for the first time spatially separated in
X-rays from the flare star primary Gl569A. Companionship to Gl569A constrains
the age of the brown dwarf pair to ~300-800 Myr. The observation presented here
is only the second X-ray detection of an evolved brown dwarf. About half of the
observing time is dominated by a large flare on Gl569Bab, the remainder is
characterized by weak and non-variable emission just above the detection limit.
This emission -- if not related to the afterglow of the flare -- represents the
first detection of a quiescent corona on a brown dwarf, representing an
important piece in the puzzle of dynamos in the sub-stellar regime.Comment: to appear in ApJ
Weak boson fusion production of supersymmetric particles at the LHC
We present a complete calculation of weak boson fusion production of
colorless supersymmetric particles at the LHC, using the new matrix element
generator SUSY-MadGraph. The cross sections are small, generally at the
attobarn level, with a few notable exceptions which might provide additional
supersymmetric parameter measurements. We discuss in detail how to consistently
define supersymmetric weak couplings to preserve unitarity of weak gauge boson
scattering amplitudes to fermions, and derive sum rules for weak supersymmetric
couplings.Comment: 24 p., 3 fig., 9 tab., published in PRD; numbers in Table IV
corrected to those with kinematic cuts cite
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
A statistical analysis of X-ray variability in pre-main sequence objects of the Taurus Molecular Cloud
This work is part of a systematic X-ray survey of the Taurus star forming
complex with XMM-Newton. We study the time series of all X-ray sources
associated with Taurus members, to statistically characterize their X-ray
variability, and compare the results to those for pre-main sequence stars in
the Orion Nebula Cluster and to expectations arising from a model where all the
X-ray emission is the result of a large number of stochastically occurring
flares. We find that roughly half of the detected X-ray sources show
variability above our sensitivity limit, and in ~ 26 % of the cases this
variability is recognized as flares. Variability is more frequently detected at
hard than at soft energies. The variability statistics of cTTS and wTTS are
undistinguishable, suggesting a common (coronal) origin for their X-ray
emission. We have for the first time applied a rigorous maximum likelihood
method in the analysis of the number distribution of flare energies on pre-main
sequence stars. In its differential form this distribution follows a power-law
with index alpha = 2.4 +- 0.5, in the range typically observed on late-type
stars and the Sun. The flare energy distribution is probably steep enough to
explain the heating of stellar coronae by nano-flares (alpha > 2), albeit
associated with a rather large uncertainty that leaves some doubt on this
conclusion.Comment: accepted for publication in Astronomy & Astrophysics; to appear in a
Special Section dedicated to the XMM-Newton Extended Survey of the Taurus
Molecular Cloud (XEST
X-Shooter spectroscopy of young stellar objects in Lupus. Atmospheric parameters, membership and activity diagnostics
A homogeneous determination of basic stellar parameters of young stellar
object (YSO) candidates is needed to confirm their evolutionary stage,
membership to star forming regions (SFRs), and to get reliable values of the
quantities related to chromospheric activity and accretion. We used the code
ROTFIT and synthetic BT-Settl spectra for the determination of the atmospheric
parameters (Teff and logg), the veiling, the radial (RV) and projected
rotational velocity (vsini), from X-Shooter spectra of 102 YSO candidates in
the Lupus SFR. We have shown that 13 candidates can be rejected as Lupus
members based on their discrepant RV with respect to Lupus and/or the very low
logg values. At least 11 of them are background giants. The spectral
subtraction of inactive templates enabled us to measure the line fluxes for
several diagnostics of both chromospheric activity and accretion. We found that
all Class-III sources have H fluxes compatible with a pure
chromospheric activity, while objects with disks lie mostly above the boundary
between chromospheres and accretion. YSOs with transitional disks displays both
high and low H fluxes. We found that the line fluxes per unit surface
are tightly correlated with the accretion luminosity () derived
from the Balmer continuum excess. This rules out that the relationships between
and line luminosities found in previous works are simply due to
calibration effects. We also found that the CaII-IRT flux ratio,
, is always small, indicating an optically thick emission
source. The latter can be identified with the accretion shock near the stellar
photosphere. The Balmer decrement reaches instead, for several accretors, high
values typical of optically thin emission, suggesting that the Balmer emission
originates in different parts of the accretion funnels with a smaller optical
depth.Comment: 28 pages, 26 figures, accepted by A&
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