130 research outputs found
Energy losses in a hot plasma revisited
We present a mini-review of the problem of evaluating the energy loss of a
ultrarelativistic charged particle crossing a thermally equilibrated high
temperature e+e- or quark-gluon plasma.Comment: 63 pages. Some references added, and a few small changes in the text,
in conformity with the published versio
Comover Enhancement of Quarkonium Production
Quarkonium data suggest an enhancement of the hadroproduction rate from
interactions of the heavy quark pair with a comoving color field generated in
the hard gg -> Q\bar{Q} subprocess. We review the motivations and principal
consequences of this comover enhancement scenario (CES).Comment: 7 pages, 1 eps figure, to appear in the proceedings of the CERN
2001-2002 workshop `Hard Probes in Heavy Ion Collisions at the LHC
Turbulence, Complexity, and Solar Flares
The issue of predicting solar flares is one of the most fundamental in
physics, addressing issues of plasma physics, high-energy physics, and
modelling of complex systems. It also poses societal consequences, with our
ever-increasing need for accurate space weather forecasts. Solar flares arise
naturally as a competition between an input (flux emergence and rearrangement)
in the photosphere and an output (electrical current build up and resistive
dissipation) in the corona. Although initially localised, this redistribution
affects neighbouring regions and an avalanche occurs resulting in large scale
eruptions of plasma, particles, and magnetic field. As flares are powered from
the stressed field rooted in the photosphere, a study of the photospheric
magnetic complexity can be used to both predict activity and understand the
physics of the magnetic field. The magnetic energy spectrum and multifractal
spectrum are highlighted as two possible approaches to this.Comment: 2 figure
Non-universality of transverse Coulomb exchange at small x
Within an explicit scalar QED model we compare, at fixed x << 1, the
leading-twist K_T-dependent `quark' distribution f_q(x, K_T) probed in deep
inelastic scattering and Drell-Yan production, and show that the model is
consistent with the universality of f_q(x, K_T). The extension of the model
from the aligned-jet to the 'symmetric' kinematical regime reveals interesting
properties of the physics of Coulomb rescatterings when comparing DIS and DY
processes. At small x the transverse momentum induced by multiple
scattering on a single centre is process dependent, as well as the transverse
momentum broadening occurring in collisions on a finite size nuclear target.Comment: 28 pages, 3 eps figure
Degrees of Freedom of the Quark Gluon Plasma, tested by Heavy Mesons
Heavy quarks (charm and bottoms) are one of the few probes which are
sensitive to the degrees of freedom of a Quark Gluon Plasma (QGP), which cannot
be revealed by lattice gauge calculations in equilibrium. Due to the rapid
expansion of the QGP energetic heavy quarks do not come to an equilibrium with
the QGP. Their energy loss during the propagation through the QGP medium
depends strongly on the modelling of the interaction of the heavy quarks with
the QGP quarks and gluons, i.e. on the assuption of the degrees of freedom of
the plasma. Here we compare the results of different models, the pQCD based
Monte-Carlo (MC@sHQ), the Dynamical Quasi Particle Model (DQPM) and the
effective mass approach, for the drag force in a thermalized QGP and discuss
the sensitivity of heavy quark energy loss on the properties of the QGP as well
as on non-equilibrium dynamicsComment: proceedings symposion "New Horizons" Makutsi, South Africa, Nov 201
Hard probes in heavy ion collisions at the LHC: heavy flavour physics
We present the results from the heavy quarks and quarkonia working group.
This report gives benchmark heavy quark and quarkonium cross sections for
and collisions at the LHC against which the rates can be compared in
the study of the quark-gluon plasma. We also provide an assessment of the
theoretical uncertainties in these benchmarks. We then discuss some of the cold
matter effects on quarkonia production, including nuclear absorption,
scattering by produced hadrons, and energy loss in the medium. Hot matter
effects that could reduce the observed quarkonium rates such as color screening
and thermal activation are then discussed. Possible quarkonium enhancement
through coalescence of uncorrelated heavy quarks and antiquarks is also
described. Finally, we discuss the capabilities of the LHC detectors to measure
heavy quarks and quarkonia as well as the Monte Carlo generators used in the
data analysis.Comment: 126 pages Latex; 96 figures included. Subgroup report, to appear in
the CERN Yellow Book of the workshop: Hard Probes in Heavy Ion Collisions at
the LHC. See also http://a.home.cern.ch/f/frixione/www/hvq.html for a version
with better quality for a few plot
Precision surface characterization for finish cylindrical milling with dynamic tool displacements model
In this work a new approach to surface roughness parameters estimation during finish cylindrical end milling is presented. The proposed model includes the influence of cutting parameters, the tool’s static run out and dynamic phenomena related to instantaneous tool deflections. The modeling procedure consists of two parts. In the first stage, tool working part instantaneous displacements are estimated using an analytical model which considers tool dynamic deflections and static errors of the machine – tool-holder – tool system. The obtained height of the tool’s displacement envelope is then applied in the second stage to the calculation of surface roughness parameters. These calculations assume that in the cylindrical milling process, two different mechanisms of surface profile formation exist. Which mechanism is present is dependent on the feed per tooth and the maximum height of the tool’s displacement envelope. The developed model is validated during cylindrical milling of hardened hot-work tool steel 55NiCrMoV6 using a stylus profiler and scanning laser vibrometer over a range of cutting parameters. The surface roughness values predicted by the developed model are in good agreement with measured values. It is found that the employment of a model which includes only the effect of static displacements gives an inferior estimation of surface roughness compared to the model incorporating dynamic tool deflection
Predictions for Pb Collisions at TeV: Comparison with Data
Predictions made in Albacete {\it et al} prior to the LHC Pb run at
TeV are compared to currently available data. Some
predictions shown here have been updated by including the same experimental
cuts as the data. Some additional predictions are also presented, especially
for quarkonia, that were provided to the experiments before the data were made
public but were too late for the original publication are also shown here.Comment: 55 pages 35 figure
System size dependence of nuclear modification and azimuthal anisotropy of jet quenching
We investigate the system size dependence of jet-quenching by analyzing
transverse momentum spectra of neutral pions in Au+Au and Cu+Cu collisions at
=200 GeV for different centralities. The fast partons
are assumed to lose energy by radiating gluons as they traverse the plasma and
undergo multiple collisions. The energy loss per collision, , is
taken as proportional to (where is the energy of the parton),
proportional to , or a constant depending on whether the formation
time of the gluon is less than the mean path, greater than the mean free path
but less than the path length, or greater than the path length of the partons,
respectively. NLO pQCD is used to evaluate pion production by modifying the
fragmentation function to account for the energy loss. We reproduce the nuclear
modification factor by treating as the only free
parameter, depending on the centrality and the mechanism of energy loss. These
values are seen to explain the nuclear modification of prompt photons, caused
by the energy lost by final state quarks before they fragment into photons.
These also reproduce the azimuthal asymmetry of transverse momentum
distribution for pions within a factor of two and for prompt photons in a fair
agreement with experimental data.Comment: 26 pages, 17 figures. One more figure added. Discussion expanded.
Typographical corrections done, several references added. To appear in
Journal of Physics
The molecular characterisation of Escherichia coli K1 isolated from neonatal nasogastric feeding tubes
Background: The most common cause of Gram-negative bacterial neonatal meningitis is E. coli K1. It has a mortality rate of 10–15%, and neurological sequelae in 30– 50% of cases. Infections can be attributable to nosocomial sources, however the pre-colonisation of enteral feeding tubes has not been considered as a specific risk factor. Methods: Thirty E. coli strains, which had been isolated in an earlier study, from the residual lumen liquid and biofilms of neonatal nasogastric feeding tubes were genotyped using pulsed-field gel electrophoresis, and 7-loci multilocus sequence typing. Potential pathogenicity and biofilm associated traits were determined using specific PCR probes, genome analysis, and in vitro tissue culture assays. Results: The E. coli strains clustered into five pulsotypes, which were genotyped as sequence types (ST) 95, 73, 127, 394 and 2076 (Achman scheme). The extra-intestinal pathogenic E. coli (ExPEC) phylogenetic group B2 ST95 serotype O1:K1:NM strains had been isolated over a 2 week period from 11 neonates who were on different feeding regimes. The E. coli K1 ST95 strains encoded for various virulence traits associated with neonatal meningitis and extracellular matrix formation. These strains attached and invaded intestinal, and both human and rat brain cell lines, and persisted for 48 h in U937 macrophages. E. coli STs 73, 394 and 2076 also persisted in macrophages and invaded Caco-2 and human brain cells, but only ST394 invaded rat brain cells. E. coli ST127 was notable as it did not invade any cell lines. Conclusions: Routes by which E. coli K1 can be disseminated within a neonatal intensive care unit are uncertain, however the colonisation of neonatal enteral feeding tubes may be one reservoir source which could constitute a serious health risk to neonates following ingestion
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