1,054 research outputs found
Challenges in the development of the orbiter active thermal control subsystem
A number of major challenges were faced in the design and development of the Orbiter Active Thermal Control Subsystem (ATCS). At the system level, the initial challenges were to define an approach that would interface dual Freon coolant loops with multiple coolant loops from other vehicle subsystems with the lowest weight penalty to the Orbiter; and to provide highly responsive vehicle heat rejection throughout all of the Orbiter mission phases. Optimized heat exchangers, representing an advance in the state-of-the-art in heat exchanger design, were developed to transfer heat between the orbiter Freon coolant loops and five other vehicle systems. Flash evaporation was selected as a highly efficient and responsive means for cooling the Orbiter Freon loops during ascent and entry. The Flash Evaporator Subsystem (FES) utilizes cyclic water spray cooling in a chamber maintained at or below the water triple point pressure. A summary of the basic heat transfer research conducted to identify the fundamental heat transfer processes involved in water spray cooling in support of the FES design is given. The high fidelity dynamic analytical model of the FES that was generated to aid in the design of control logic, evaluate performance and simulate ground test and flight anomalies is discussed. A description of the FES and Integrated ATCS testing conducted in the SESL chamber A at NASA-JSC is also presented
Azimuthal Dependence of the Heavy Quark Initiated Contributions to DIS
We analyze the azimuthal dependence of the heavy-quark-initiated
contributions to the lepton-nucleon deep inelastic scattering (DIS). First we
derive the relations between the parton level semi-inclusive structure
functions and the helicity cross sections in the case of
arbitrary values of the heavy quark mass. Then the azimuth-dependent lepton-quark DIS is calculated in the helicity basis. Finally,
we investigate numerically the properties of the and
distributions caused by the photon-quark scattering (QS) contribution. It turns
out that, contrary to the basic photon-gluon fusion (GF) component, the QS
mechanism is practically -independent. This fact implies that
measurements of the azimuthal distributions in charm leptoproduction could
directly probe the charm density in the proton.Comment: 11 pages, 4 figures, revtex4, published versio
Detecting an Invisibly Decaying Higgs Boson at a Hadron Supercollider
We demonstrate that an invisibly decaying Higgs boson with Standard Model
coupling strength to top--anti-top can be detected at the LHC for masses up to
about 250 GeV.Comment: 7 pages, requires phyzzx.tex and tables.tex, revised to convert
results from SSC to LHC and include additional top quark mass cases, full
postscript file including embedded figure available via anonymous ftp at
ucdhep.ucdavis.edu as [anonymous.gunion]hinvisible_revised.ps, preprint
UCD-93-2
A Positive-Weight Next-to-Leading-Order Monte Carlo for Z Pair Hadroproduction
We present a first application of a previously published method for the
computation of QCD processes that is accurate at next-to-leading order, and
that can be interfaced consistently to standard shower Monte Carlo programs. We
have considered Z pair production in hadron-hadron collisions, a process whose
complexity is sufficient to test the general applicability of the method. We
have interfaced our result to the HERWIG and PYTHIA shower Monte Carlo
programs. Previous work on next-to-leading order corrections in a shower Monte
Carlo (the MC@NLO program) may involve the generation of events with negative
weights, that are avoided with the present method. We have compared our results
with those obtained with MC@NLO, and found remarkable consistency. Our method
can also be used as a standalone, alternative implementation of QCD
corrections, with the advantage of positivity, improved convergence, and
next-to-leading logarithmic accuracy in the region of small transverse momentum
of the radiated parton.Comment: 33 pages, 10 figure
Next-to-next-to-leading soft-gluon corrections for the top quark cross section and transverse momentum distribution
I present results for top quark production in hadronic collisions at LHC and
Tevatron energies. The soft-gluon corrections to the differential cross section
are resummed at next-to-next-to-leading-logarithm (NNLL) accuracy via the
two-loop soft anomalous dimension matrices. Approximate
next-to-next-to-leading-order (NNLO) differential and total cross sections are
calculated. Detailed theoretical predictions are shown for the t tbar cross
section and the top quark p_T distribution at the Tevatron and the LHC.Comment: 23 pages, 14 figures; additional results and figure
Improving NLO-parton shower matched simulations with higher order matrix elements
In recent times the algorithms for the simulation of hadronic collisions have
been subject to two substantial improvements: the inclusion, within parton
showering, of exact higher order tree level matrix elements (MEPS) and,
separately, next-to-leading order corrections (NLOPS). In this work we examine
the key criteria to be met in merging the two approaches in such a way that the
accuracy of both is preserved, in the framework of the POWHEG approach to
NLOPS. We then ask to what extent these requirements may be fulfilled using
existing simulations, without modifications. The result of this study is a
pragmatic proposal for merging MEPS and NLOPS events to yield much improved
MENLOPS event samples. We apply this method to W boson and top quark pair
production. In both cases results for distributions within the remit of the NLO
calculations exhibit no discernible changes with respect to the pure NLOPS
prediction; conversely, those sensitive to the distribution of multiple hard
jets assume, exactly, the form of the corresponding MEPS results.Comment: 38 pages, 17 figures. v2: added citations and brief discussion of
related works, MENLOPS prescription localized in a subsection. v3: cited 4
more MEPS works in introduction
J/psi Production via Fragmentation at the Tevatron
The production of \jpsi at large transverse momenta (\pt > M_\jpsi) in
collisions is considered by including the mechanism of fragmentation.
Both contributions of fragmentation to \jpsi and of fragmentation to
states followed by radiative decay to \jpsi are taken into account. The
latter is found to be dominant and larger than direct production. The overall
theoretical estimate is shown to be nearly consistent with the experimental
observation.Comment: LaTeX, 7 pages. Preprint FNT/T-94/13, LNF-94/024(P). Data taken from
a reference were incorrect and have been changed. Complete postscript file
available via anonymous ftp at cobra1.pv.infn.it, as pub/jpsi.ps(.Z)(.gz
Prompt atmospheric neutrinos and muons: dependence on the gluon distribution function
We compute the next-to-leading order QCD predictions for the vertical flux of
atmospheric muons and neutrinos from decays of charmed particles, for different
PDF's (MRS-R1, MRS-R2, CTEQ-4M and MRST) and different extrapolations of these
at small partonic momentum fraction x. We find that the predicted fluxes vary
up to almost two orders of magnitude at the largest energies studied, depending
on the chosen extrapolation of the PDF's. We show that the spectral index of
the atmospheric leptonic fluxes depends linearly on the slope of the gluon
distribution function at very small x. This suggests the possibility of
obtaining some bounds on this slope in ``neutrino telescopes'', at values of x
not reachable at colliders, provided the spectral index of atmospheric leptonic
fluxes could be determined.Comment: 20 pages including 8 figure
Analytic Calculation of Prompt Photon plus Associated Heavy Flavor at Next-to-Leading Order in QCD
Contributions through second order, , in perturbative quantum
chromodynamics are calculated analytically for inclusive associated production
of a prompt photon and a charm quark at large values of transverse momentum in
high energy hadron-hadron collisions. Seven partonic subprocesses contribute at
order . We find important corrections to the lowest order,
, subprocess . We demonstrate to what
extent data from may serve to measure
the charm quark density in the nucleon.Comment: 34 pages RevTex plus 9 figures submitted as uuencoded ps files;
figures replaced and text revised to include one additional referenc
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