2,427 research outputs found
The Perturbative Resummed Series for Top Production
Our calculation of the total cross section for inclusive production of
pairs in hadron collisions is summarized. The principal ingredient
of this calculation is resummation of the universal leading-logarithm effects
of gluon radiation to all orders in the quantum chromodynamics coupling
strength, restricted to the region of phase space that is manifestly
perturbative. We present predictions of the physical cross section as a
function of top quark mass in proton-antiproton reactions at center-of-mass
energies of 1.8 and 2.0 TeV.Comment: 9 pages. Latex file plus 6 ps files of figures submitted togther in
uuencoded form; invited talk presented by Harry Contopanagos at the XXXI
Rencontres de Moriond on QCD and High Energy Hadronic Interactions, Les Arcs,
France, March, 1996. New version to correct problem with unsuccessful posting
of figures on May
Top Quark Production Dynamics in QCD
A calculation of the total cross section for top quark production in
hadron-hadron collisions is presented based on an all-orders perturbative
resummation of initial-state gluon radiative contributions to the basic quantum
chromodynamics subprocesses. Principal-value resummation is used to evaluate
all relevant large threshold contributions. In this method there are no
arbitrary infrared cutoffs, and the perturbative regime of applicability is
well defined, two attributes that significantly reduce the estimated
uncertainty of the results. For collisions at center-of-mass energy
TeV and a top mass of 175 GeV, we obtain
, in agreement with experiment.
Predicted cross sections are provided as a function of top mass in
collisions at TeV and in collisions at CERN LHC energies.Comment: 12 pages. Latex file plus 8 ps files of figures submitted altogther
in uuencoded form; Invited talk presented by E. L. Berger at the
International Symposium on Heavy Flavor and Electroweak Theory, Beijing,
August 16 - 19, 199
Threshold Resummation and the Total Cross Section for Top Quark Production
We discuss the motivation for resummation of the effects of initial-state
soft-gluon radiation, to all orders in the strong coupling strength, for
processes in which the near-threshold region in the partonic subenergy is
important. We summarize our calculation of the total cross section for top
quark production at hadron colliders. Comments are included on the differences
between our treatment of subleading logarithmic terms and other methods.Comment: 7 pages, 1 figure, uses aipproc.sty; Paper presented by E. L. Berger,
to be published in the Proceedings of DIS'97, Fifth International Workshop on
Deep Inelastic Scattering and QCD, Chicago, IL, April 14 - 18, 199
Calculation of the Cross Section for Top Quark Production
We summarize calculations of the cross section for top quark production at
hadron colliders within the context of perturbative quantum chromodynamics,
including resummation of the effects of initial-state soft gluon radiation to
all orders in the strong coupling strength. In our approach we resum the
universal leading-logarithm contributions, and we restrict the calculation to
the region of phase space that is demonstrably perturbative. We compare our
approach with other methods. We present predictions of the physical cross
section as a function of the top quark mass in proton-antiproton reactions at
center-of-mass energies of 1.8 and 2.0 TeV, and we discuss estimated
uncertainties.Comment: 13 pages, LaTeX, including 6 ps files of figures. Invited paper
presented by E. L. Berger at the XIth Topical Workshop on Hadron Collider
Physics, Abano Terme, Padova, Italy, May, 199
Future scenarios for the charity sector in 2045
Rapid change is affecting the demography, technology and availability of resources (both financial and volunteer) on which charities draw. This paper presents four different scenarios that could describe the charity sector one generation from now as it responds to a different world. We highlight the dangers if any one scenario becomes dominant. While it is inevitable that change will occur, these drawbacks should be minimized and it is important that public funders and policy makers steer intelligently through this changing world. Also, charity leaders must prepare and plan for inevitable change in the sector
Homotransplantation of multiple visceral organs
It was technically possible to perform simultaneous homotransplantation of multiple visceral organs including the liver, spleen, pancreas, omentum and the entire gastrointestinal tract. Arterialization of the cooled graft was accomplished through the donor aorta which was removed with the graft and attached to that of the recipient dog. Gastrointestinal hemorrhage after surgery accounted for a high operative mortality and was thought to be due to denervation of the graft. The five dogs which survived the immediate trauma of surgery lived for five and a half to nine days. After the second day, these animals were physically active and able to resume oral alimentation. In three dogs, there was metabolic evidence of rejection of the liver. In two others, jaundice did not develop. These observations were compared with chemical, hematologic and pathologic data obtained in previous experiments involving homotransplantation of the liver alone. In some cases, there was less evidence of host versus graft rejection after the multiple organ transplants. Other data in the present study suggested the possibility that a significant graft versus host reaction may have been an important contributory cause of death. © 1962
Localizing compact binary inspirals on the sky using ground-based gravitational wave interferometers
The inspirals and mergers of compact binaries are among the most promising
events for ground-based gravitational-wave (GW) observatories. The detection of
electromagnetic (EM) signals from these sources would provide complementary
information to the GW signal. It is therefore important to determine the
ability of gravitational-wave detectors to localize compact binaries on the
sky, so that they can be matched to their EM counterparts. We use Markov Chain
Monte Carlo techniques to study sky localization using networks of ground-based
interferometers. Using a coherent-network analysis, we find that the Laser
Interferometer Gravitational Wave Observatory (LIGO)-Virgo network can localize
50% of their ~8 sigma detected neutron star binaries to better than 50 sq.deg.
with 95% confidence region. The addition of the Large Scale Cryogenic
Gravitational Wave Telescope (LCGT) and LIGO-Australia improves this to 12
sq.deg.. Using a more conservative coincident detection threshold, we find that
50% of detected neutron star binaries are localized to 13 sq.deg. using the
LIGO-Virgo network, and to 3 sq.deg. using the LIGO-Virgo-LCGT-LIGO-Australia
network. Our findings suggest that the coordination of GW observatories and EM
facilities offers great promise.Comment: 6 pages, 4 figures, 1 table, matches published version in ApJ
(incorporates referee's comments
Recommended from our members
Sentiment bubbles
We examine cumulative changes in investor sentiment and find that these changes relate to extended periods of increasing overvaluation, followed by price corrections. The relation between sentiment and returns is path dependent — short-term increases in sentiment precede strong positive returns, while prolonged periods of increasing sentiment precede negative returns. Positive short-run returns are consistent with bubble dynamics and mitigate the backwards induction conundrum described by Abreu and Brunnermeier (2003). Our results hold for the market portfolio, and are especially strong for opaque portfolios with high levels of uncertainty, as well as portfolios with greater market frictions that limit arbitrage.Keywords: Bubbles, Investor sentiment, Price-correctio
Constraints on the Heating of High Temperature Active Region Loops: Observations from Hinode and SDO
We present observations of high temperature emission in the core of a solar
active region using instruments on Hinode and SDO. These multi-instrument
observations allow us to determine the distribution of plasma temperatures and
follow the evolution of emission at different temperatures. We find that at the
apex of the high temperature loops the emission measure distribution is
strongly peaked near 4 MK and falls off sharply at both higher and lower
temperatures. Perhaps most significantly, the emission measure at 0.5 MK is
reduced by more than two orders of magnitude from the peak at 4 MK. We also
find that the temporal evolution in broad-band soft X-ray images is relatively
constant over about 6 hours of observing. Observations in the cooler SDO/AIA
bandpasses generally do not show cooling loops in the core of the active
region, consistent with the steady emission observed at high temperatures.
These observations suggest that the high temperature loops observed in the core
of an active region are close to equilibrium. We find that it is possible to
reproduce the relative intensities of high temperature emission lines with a
simple, high-frequency heating scenario where heating events occur on time
scales much less than a cooling time. In contrast, low-frequency heating
scenarios, which are commonly invoked to describe nanoflare models of coronal
heating, do not reproduce the relative intensities of high temperature emission
lines and predict low-temperature emission that is approximately an order of
magnitude too large. We also present an initial look at images from the SDO/AIA
94 A channel, which is sensitive to Fe XVIII.Comment: Movies are available at
http://tcrb.nrl.navy.mil/~hwarren/temp/papers/active_region_core/ Paper has
been refereed and revise
Toward an optimal search strategy of optical and gravitational wave emissions from binary neutron star coalescence
Observations of an optical source coincident with gravitational wave emission
detected from a binary neutron star coalescence will improve the confidence of
detection, provide host galaxy localisation, and test models for the
progenitors of short gamma ray bursts. We employ optical observations of three
short gamma ray bursts, 050724, 050709, 051221, to estimate the detection rate
of a coordinated optical and gravitational wave search of neutron star mergers.
Model R-band optical afterglow light curves of these bursts that include a
jet-break are extrapolated for these sources at the sensitivity horizon of an
Advanced LIGO/Virgo network. Using optical sensitivity limits of three
telescopes, namely TAROT (m=18), Zadko (m=21) and an (8-10) meter class
telescope (m=26), we approximate detection rates and cadence times for imaging.
We find a median coincident detection rate of 4 yr^{-1} for the three bursts.
GRB 050724 like bursts, with wide opening jet angles, offer the most optimistic
rate of 13 coincident detections yr^{-1}, and would be detectable by Zadko up
to five days after the trigger. Late time imaging to m=26 could detect off-axis
afterglows for GRB 051221 like bursts several months after the trigger. For a
broad distribution of beaming angles, the optimal strategy for identifying the
optical emissions triggered by gravitational wave detectors is rapid response
searches with robotic telescopes followed by deeper imaging at later times if
an afterglow is not detected within several days of the trigger.Comment: 6 pages, 1 figure, Accepted for publication in MNRAS Letters (2011
April 22
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