558 research outputs found
Leptonic and charged kaon decay modes of the meson measured in heavy-ion collisions at the CERN SPS
We report a measurement of meson production in central Pb+Au
collisions at E/A=158 GeV. For the first time in heavy-ion collisions,
mesons were reconstructed in the same experiment both in the KK
and the dilepton decay channel. Near mid-rapidity, this yields rapidity
densities, corrected for production at the same rapidity value, of 2.05 +-
0.14(stat) +- 0.25(syst) and 2.04 +- 0.49(stat)+-{0.32}(syst), respectively.
The shape of the measured transverse momentum spectra is also in close
agreement in both decay channels. The data rule out a possible enhancement of
the yield in the leptonic over the hadronic channel by a factor larger
than 1.6 at 95% CL.Comment: 11 pages, 4 figures,submitted to Phys. Rev. Let
Elliptic flow of charged pions, protons and strange particles emitted in Pb+Au collisions at top SPS energy
Differential elliptic flow spectra v2(pT) of \pi-, K0short, p, \Lambda have
been measured at \sqrt(s NN)= 17.3 GeV around midrapidity by the
CERN-CERES/NA45 experiment in mid-central Pb+Au collisions (10% of
\sigma(geo)). The pT range extends from about 0.1 GeV/c (0.55 GeV/c for
\Lambda) to more than 2 GeV/c. Protons below 0.4 GeV/c are directly identified
by dE/dx. At higher pT, proton elliptic flow v2(pT) is derived as a
constituent, besides \pi+ and K+, of the elliptic flow of positive pion
candidates. The retrieval requires additional inputs: (i) of the particle
composition, and (ii) of v2(pT) of positive pions. For (i), particle ratios
obtained by NA49 were adapted to CERES conditions; for (ii), the measured
v2(pT) of negative pions is substituted, assuming \pi+ and \pi- elliptic flow
magnitudes to be sufficiently close. The v2(pT) spectra are compared to
ideal-hydrodynamics calculations. In synopsis of the series \pi- - K0short - p
- \Lambda, flow magnitudes are seen to fall with decreasing pT progressively
even below hydro calculations with early kinetic freeze-out (Tf= 160 MeV)
leaving not much time for hadronic evolution. The proton v2(pT) data show a
downward swing towards low pT with excursions into negative v2 values. The
pion-flow isospin asymmetry observed recently by STAR at RHIC, invalidating in
principle our working assumption, is found in its impact on proton flow
bracketed from above by the direct proton flow data, and not to alter any of
our conclusions. Results are discussed in perspective of recent viscous
dynamics studies which focus on late hadronic stages.Comment: 38 pages, 27 figures, 2 tables. Abstract and parts of introduction
made more comprehensible; corrected typos; acknowledgement added. To appear
in Nucl.Phys.
Azimuthal dependence of pion source radii in Pb+Au collisions at 158 A GeV
We present results of a two-pion correlation analysis performed with the
Au+Pb collision data collected by the upgraded CERES experiment in the fall of
2000. The analysis was done in bins of the reaction centrality and the pion
azimuthal emission angle with respect to the reaction plane. The pion source,
deduced from the data, is slightly elongated in the direction perpendicular to
the reaction plane, similarly as was observed at the AGS and at RHIC.Comment: 5 pages, 2 figure
Lambda production in 40 A GeV/c Pb-Au collisions
During the 1999 lead run, CERES has measured hadron and electron-pair
production at 40 A GeV/c beam momentum with the spectrometer upgraded by the
addition of a radial TPC. Here the analysis of lambda and antilambda will be
presented.Comment: 8 pages, 8 figures. 6th International Conference on Strange Quarks in
Matter, Frankfurt 200
Effective Scenario of Loop Quantum Cosmology
Semiclassical states in isotropic loop quantum cosmology are employed to show
that the improved dynamics has the correct classical limit. The effective
Hamiltonian for the quantum cosmological model with a massless scalar field is
thus obtained, which incorporates also the next to leading order quantum
corrections. The possibility that the higher order correction terms may lead to
significant departure from the leading order effective scenario is revealed. If
the semiclassicality of the model is maintained in the large scale limit, there
are great possibilities for Friedmann expanding universe to undergo a
collapse in the future due to the quantum gravity effect. Thus the quantum
bounce and collapse may contribute a cyclic universe in the new scenario.Comment: 4 pages, 2 figures; version published in PR
Recent results from Pb-Au collisions at 158 GeV/c per nucleon obtained with the CERES spectrometer
During the 1996 lead run time, CERES has accumulated 42 million events,
corresponding to a factor of 5 more statistics than in 1995 and 2.5 million
events of a special photon-run. We report on the results of the low-mass
ee-pair analysis. Since the most critical item is the poor
signal-to-background ratio we also discuss the understanding of this
background, in absolute terms, with the help of a detailed Monte Carlo
simulation. We show preliminary results of the photon analysis and summarize
the results of the hadron analysis preliminarily reported on already at QM'97Comment: 10 pages, 9 figures, Proceedings of the XIV Int. Conf. on
Nucleus-Nucleus Collisions,Quark Matter 99, Torino, Italy, May 10 - 15, 199
New Results on Pb-Au Collisions at 40 AGeV from the CERES/NA45 Experiment
In 1999 the CERES/NA45 ran at the CERN SPS with a beam energy of 40
GeV/nucleon. The data set comprises about 8.7 millions Pb-Au events with a
trigger selection corresponding to approximately the most central 30% of the
geometrical cross section. Results on low-mass electron pair analysis are
presented.
The upgrade of the experimental setup with the radial drift TPC has allowed
to enhance hadron physics capabilities of the experiment. New results on hadron
spectra (including Lambda) and flow are presented.Comment: Talk at the International Nuclear Physics Conference INPC2001,
Berkeley, CA, July 29th - August 3rd 200
Point defects and clustering in uranium dioxide by LSDA+U calculations
A comprehensive investigation on point defects and their clustering behavior
in nonstoichiometric uranium dioxide UO2+x is carried out using LSDA+U method
based on density functional theory. Accurate energetic information and charge
transfers available so far are obtained. With these energies that have improved
more than 50% over that of pure GGA and LDA, we show the density functional
theory predicts the predominance of oxygen defects over uranium ones at any
compositions, which is possible only after treated the localized 5f electrons
properly. Calculations also suggest an upper bound of x~0.03 for oxygen
clusters to start off. The volume change induced by point uranium defects is
monotonic but nonlinear, whereas for oxygen defects, increase x always reduces
the system volume linearly, except dimers that require extra space for
accommodation, which has been identified as meta-stable ionic molecule. Though
oxygen dimers usually occupy Willis O'' sites and mimic a single oxygen in
energetics and charge state, they are rare at ambient conditions. Its
decomposition process and vibrational properties have been studied carefully.
To obtain a general clustering mechanism in anion-excess fluorites
systematically, we also analyze the local stabilities of possible basic
clustering modes of oxygen defects. The result shows an unified way to
understand the structure of Willis type and cuboctahedral clusters in UO2+x and
beta-U4O9. Finally we generalize the point defect model to the independent
clusters approximation to include clustering effects, the impact on defect
populations is discussed.Comment: 20 pages, 12 figure
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Initial Commissioning Experience With the LCLS Injector
The Linac Coherent Light Source (LCLS) is a SASE xray Free-Electron Laser (FEL) project presently under construction at SLAC [1]. The injector section, from drive-laser and RF photocathode gun through first bunch compressor chicane, was installed in fall 2006. Initial system commissioning with an electron beam is taking place during the spring and summer of 2007. The second phase of construction, including second bunch compressor and full linac, will begin later, in the fall of 2007. We report here on experience gained during the first phase of machine commissioning, including RF photocathode gun, linac booster section, S-band and X-band RF systems, first bunch compressor, and the various beam diagnostics
Abnormal cortical responses to somatosensory stimulation in medication-overuse headache
BACKGROUND: Medication-overuse headache (MOH) is a frequent, disabling disorder. Despite a controversial pathophysiology convincing evidence attributes a pivotal role to central sensitization. Most patients with MOH initially have episodic migraine without aura (MOA) characterized interictally by an absent amplitude decrease in cortical evoked potentials to repetitive stimuli (habituation deficit), despite a normal initial amplitude (lack of sensitization). Whether central sensitization alters this electrophysiological profile is unknown. We therefore sought differences in somatosensory evoked potential (SEP) sensitization and habituation in patients with MOH and episodic MOA. METHODS: We recorded median-nerve SEPs (3 blocks of 100 sweeps) in 29 patients with MOH, 64 with MOA and 42 controls. Episodic migraineurs were studied during and between attacks. We measured N20-P25 amplitudes from 3 blocks of 100 sweeps, and assessed sensitization from block 1 amplitude, and habituation from amplitude changes between the 3 sequential blocks. RESULTS: In episodic migraineurs, interictal SEP amplitudes were normal in block 1, but thereafter failed to habituate. Ictal SEP amplitudes increased in block 1, then habituated normally. Patients with MOH had larger-amplitude block 1 SEPs than controls, and also lacked SEP habituation. SEP amplitudes were smaller in triptan overusers than in patients overusing nonsteroidal anti-inflammatory drugs (NSAIDs) or both medications combined, lowest in patients with the longest migraine history, and highest in those with the longest-lasting headache chronification. CONCLUSIONS: In patients with MOH, especially those overusing NSAIDs, the somatosensory cortex becomes increasingly sensitized. Sensory sensitization might add to the behavioral sensitization that favors compulsive drug intake, and may reflect drug-induced changes in central serotoninergic transmission
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