4,961 research outputs found
Admit your weakness: Verifying correctness on TSO architectures
“The final publication is available at http://link.springer.com/chapter/10.1007%2F978-3-319-15317-9_22 ”.Linearizability has become the standard correctness criterion for fine-grained non-atomic concurrent algorithms, however, most approaches assume a sequentially consistent memory model, which is not always realised in practice. In this paper we study the correctness of concurrent algorithms on a weak memory model: the TSO (Total Store Order) memory model, which is commonly implemented by multicore architectures. Here, linearizability is often too strict, and hence, we prove a weaker criterion, quiescent consistency instead. Like linearizability, quiescent consistency is compositional making it an ideal correctness criterion in a component-based context. We demonstrate how to model a typical concurrent algorithm, seqlock, and prove it quiescent consistent using a simulation-based approach. Previous approaches to proving correctness on TSO architectures have been based on linearizabilty which makes it necessary to modify the algorithm’s high-level requirements. Our approach is the first, to our knowledge, for proving correctness without the need for such a modification
Where is the jet quenching in Pb+Pb collisions at 158 AGeV?
Because of the rapidly falling particle spectrum at large from jet
fragmentation at the CERN SPS energy, the high- hadron distribution should
be highly sensitive to parton energy loss inside a dense medium as predicted by
recent perturbative QCD (pQCD) studies. A careful analysis of recent data from
CERN SPS experiments via pQCD calculation shows little evidence of energy loss.
This implies that either the life-time of the dense partonic matter is very
short or one has to re-think about the problem of parton energy loss in dense
matter. The hadronic matter does not seem to cause jet quenching in
collisions at the CERN SPS. High- two particle correlation in the
azimuthal angle is proposed to further clarify this issue.Comment: 4 pages with 2 ps figures. Minors changes are made in the text with
updated references. Revised version to appear in Phys. Rev. Letter
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The radial width of a coronal mass ejection between 0.1 and 0.4 AU estimated from the heliospheric imager on STEREO
On 15-17 February 2008, a CME with an approximately circular cross section was tracked through successive images obtained by the Heliospheric Imager (HI) instrument onboard the STEREO-A spacecraft. Reasoning that an idealised flux rope is cylindrical in shape with a circular cross-section, best fit circles are used to determine the radial width of the CME. As part of the process the radial velocity and longitude of propagation are determined by fits to elongation-time maps as 252±5 km/s and 70±5° respectively. With the longitude known, the radial size is calculated from the images, taking projection effects into account. The radial width of the CME, S (AU), obeys a power law with heliocentric distance, R, as the CME travels between 0.1 and 0.4 AU, such that S=0.26 R0.6±0.1. The exponent value obtained is compared to published studies based on statistical surveys of in situ spacecraft observations of ICMEs between 0.3 and 1.0 AU, and general agreement is found. This paper demonstrates the new opportunities provided by HI to track the radial width of CMEs through the previously unobservable zone between the LASCO field of view and Helios in situ measurements
Jet Quenching in the Opposite Direction of a Tagged Photon in High-Energy Heavy-Ion Collisions
We point out that events associated with large direct photons in
high-energy heavy-ion collisions can be used to study jet energy loss in dense
matter. In such events, the spectrum of charged hadrons from jet
fragmentation in the opposite direction of the tagged photon is estimated to be
well above the background which can be reliably subtracted at moderately large
. We demonstrate that comparison between the extracted fragmentation
function in and collisions can be used to determine the jet energy
loss and the interaction mean-free-path in the dense matter produced in
high-energy heavy-ion collisions.Comment: 4 pages in RevTex twocolumn with embedded psfigure
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Do the legs of magnetic clouds contain twisted flux-rope magnetic fields?
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) characterised primarily by a smooth rotation in the magnetic field direction indicative of the presence of a magnetic flux rope. Energetic particle signatures suggest MC flux ropes remain magnetically connected to the Sun at both ends, leading to widely used model of global MC structure as an extended flux rope, with a loop-like axis stretching out from the Sun into the heliosphere and back to the Sun. The time of flight of energetic particles, however, suggests shorter magnetic field line lengths than such a continuous twisted flux rope would produce. In this study, two simple models are compared with observed flux rope axis orientations of 196 MCs to show that the flux rope structure is confined to the MC leading edge. The magnetic cloud “legs,” which magnetically connect the flux rope to the Sun, are not recognisable as MCs and thus are unlikely to contain twisted flux rope fields. Spacecraft encounters with these non-flux rope legs may provide an explanation for the frequent observation of non-magnetic cloud ICMEs
Temperature-dependent proximity magnetism in Pt
We experimentally demonstrate the existence of magnetic coupling between two
ferromagnets separated by a thin Pt layer. The coupling remains ferromagnetic
regardless of the Pt thickness, and exhibits a significant dependence on
temperature. Therefore, it cannot be explained by the established mechanisms of
magnetic coupling across nonmagnetic spacers. We show that the experimental
results are consistent with the presence of magnetism induced in Pt in
proximity to ferromagnets, in direct analogy to the well-known proximity
effects in superconductivity.Comment: 4 pages, 3 figure
Diphoton Production at Hadron Colliders and New Contact Interactions
We explore the capability of the Tevatron and LHC to place limits on the
possible existence of flavor-independent contact
interactions which can lead to an excess of diphoton events with large
invariant masses. Assuming no departure from the Standard Model is observed, we
show that the Tevatron will eventually be able to place a lower bound of
0.5-0.6 TeV on the scale associated with this new contact interaction. At the
LHC, scales as large as 3-6 TeV may be probed with suitable detector cuts and
an integrated luminosity of .Comment: LaTex, 12pages plus 5 figures(available on request), SLAC-PUB-657
Systematic Study of High p_T Hadron Spectra in pp, pA and AA Collisions from SPS to RHIC Energies
High- particle spectra in (), and
collisions are calculated within a QCD parton model in which intrinsic
transverse momentum, its broadening due to initial multiple parton scattering,
and jet quenching due to parton energy loss inside a dense medium are included
phenomenologically. The intrinsic and its broadening in and
collisions due to initial multiple parton scattering are found to be very
important at low energies ( GeV). Comparisons with ,
and data with different centrality cuts show that the differential
cross sections of large transverse momentum pion production ( GeV/)
in collisions scale very well with the number of binary nucleon-nucleon
collisions (modulo effects of multiple initial scattering). This indicates that
semi-hard parton scattering is the dominant particle production mechanism
underlying the hadron spectra at moderate GeV/.
However, there is no evidence of jet quenching or parton energy loss. Within
the parton model, one can exclude an effective parton energy loss
GeV/fm and a mean free path fm from the
experimental data of collisions at the SPS energies. Predictions for high
particle spectra in and collisions with and without jet
quenching at the RHIC energy are also given. Uncertainties due to initial
multiple scattering and nuclear shadowing of parton distributions are also
discussed.Comment: 13 pages in RevTex with 14 figures, the final published version (with
some typos corrected
kt Effects in Direct-Photon Production
We discuss the phenomenology of initial-state parton-kt broadening in
direct-photon production and related processes in hadron collisions. After a
brief summary of the theoretical basis for a Gaussian-smearing approach, we
present a systematic study of recent results on fixed-target and collider
direct-photon production, using complementary data on diphoton and pion
production to provide empirical guidance on the required amount of kt
broadening. This approach provides a consistent description of the observed
pattern of deviation of next-to-leading order QCD calculations relative to the
direct-photon data, and accounts for the shape and normalization difference
between fixed-order perturbative calculations and the data. We also discuss the
uncertainties in this phenomenological approach, the implications of these
results on the extraction of the gluon distribution of the nucleon, and the
comparison of our findings to recent related work.Comment: LaTeX, uses revtex and epsf, 37 pages, 15 figure
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Ensemble downscaling in coupled solar wind-magnetosphere modeling for space weather forecasting
Advanced forecasting of space weather requires simulation of the whole Sun-to-Earth system, which necessitates driving magnetospheric models with the outputs from solar wind models. This presents a fundamental difficulty, as the magnetosphere is sensitive to both large-scale solar wind structures, which can be captured by solar wind models, and small-scale solar wind “noise,” which is far below typical solar wind model resolution and results primarily from stochastic processes. Following similar approaches in terrestrial climate modeling, we propose statistical “downscaling” of solar wind model results prior to their use as input to a magnetospheric model. As magnetospheric response can be highly nonlinear, this is
preferable to downscaling the results of magnetospheric modeling. To demonstrate the benefit of this approach, we first approximate solar wind model output by smoothing solar wind observations with an 8 h filter, then add small-scale structure back in through the addition of random noise with the observed spectral characteristics. Here we use a very simple parameterization of noise based upon the observed probability distribution functions of solar wind parameters, but more sophisticated methods will be developed in the future. An ensemble of results from the simple downscaling scheme are tested using a
model-independent method and shown to add value to the magnetospheric forecast, both improving the best estimate and quantifying the uncertainty. We suggest a number of features desirable in an operational solar wind downscaling scheme
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