295 research outputs found
Event-by-event background in estimates of the chiral magnetic effect
In terms of the parton-hadron-string-dynamics (PHSD) approach - including the
retarded electromagnetic field - we investigate the role of fluctuations of the
correlation function in the azimuthal angle of charged hadrons that is
expected to be a sensitive signal of local strong parity violation. For the
early time we consider fluctuations in the position of charged spectators
resulting in electromagnetic field fluctuations as well as in the position of
participant baryons defining the event plane. For partonic and hadronic phases
in intermediate stages of the interaction we study the possible formation of
excited matter in electric charge dipole and quadrupole form as generated by
fluctuations. The role of the transverse momentum and local charge conservation
laws in the observed azimuthal asymmetry is investigated, too. All these
above-mentioned effects are incorporated in our analysis based on
event-by-event PHSD calculations. Furthermore, the azimuthal angular
correlations from Au+Au collisions observed in the recent STAR measurements
within the RHIC Beam-Energy-Scan (BES) program are studied. It is shown that
the STAR correlation data at the collision energies of = 7.7
and 11.5 GeV can be reasonably reproduced within the PHSD. At higher energies
the model fails to describe the correlation data resulting in an
overestimation of the partonic scalar field involved. We conclude that an
additional transverse anisotropy fluctuating source is needed which with a
comparable strength acts on both in- and out-of-plane components.Comment: 20 pages, 19 figures, to be published in Phys. Rev.
Charged particle directed flow in Pb-Pb collisions at sqrt{s_NN} = 2.76 TeV measured with ALICE at the LHC
Charged particle directed flow at midrapidity, |eta|<0.8, and forward
rapidity, 1.7 < |eta|<5.1, is measured in Pb-Pb collisions at sqrt{s_NN} = 2.76
TeV with ALICE at the LHC. Directed flow is reported as a function of collision
centrality, charged particle transverse momentum, and pseudo-rapidity. Results
are compared to measurements at RHIC and recent model calculations for LHC
energies.Comment: Talk given at the XXII International Conference on Ultrarelativistic
Nucleus-Nucleus Collisions (Quark Matter 2011), 23-28 May 2011, Annecy,
France; 4 pages, 3 figure
Two-particle correlations in azimuthal angle and pseudorapidity in inelastic p + p interactions at the CERN Super Proton Synchrotron
Results on two-particle ΔηΔϕ correlations in inelastic p + p interactions at 20, 31, 40, 80, and 158 GeV/c are presented. The measurements were performed using the large acceptance NA61/SHINE hadron spectrometer at the CERN Super Proton Synchrotron. The data show structures which can be attributed mainly to effects of resonance decays, momentum conservation, and quantum statistics. The results are compared with the Epos and UrQMD models.ISSN:1434-6044ISSN:1434-605
Transverse-momentum correlations on from mean- fluctuations in Au-Au collisions at 200 GeV
We present first measurements of the pseudorapidity and azimuth
bin-size dependence of event-wise mean transverse momentum
fluctuations for Au-Au collisions at GeV. We invert that
dependence to obtain autocorrelations on differences
interpreted to represent velocity/temperature
distributions on (). The general form of the autocorrelations
suggests that the basic correlation mechanism is parton fragmentation. The
autocorrelations vary strongly with collision centrality, which suggests that
fragmentation is strongly modified by a dissipative medium in the more central
Au-Au collisions relative to peripheral or p-p collisions. \\Comment: 7 pages, 3 figure
Probing dense baryon-rich matter with virtual photons
International audienceAbout 10 μs after the Big Bang, the universe was filled—in addition to photons and leptons—with strong-interaction matter consisting of quarks and gluons, which transitioned to hadrons at temperatures close to kT = 150 MeV and densities several times higher than those found in nuclei. This quantum chromodynamics (QCD) matter can be created in the laboratory as a transient state by colliding heavy ions at relativistic energies. The different phases in which QCD matter may exist depend for example on temperature, pressure or baryochemical potential, and can be probed by studying the emission of electromagnetic radiation. Electron–positron pairs emerge from the decay of virtual photons, which immediately decouple from the strong interaction, and thus provide information about the properties of QCD matter at various stages. Here, we report the observation of virtual photon emission from baryon-rich QCD matter. The spectral distribution of the electron–positron pairs is nearly exponential, providing evidence for a source of temperature in excess of 70 MeV with constituents whose properties have been modified, thus reflecting peculiarities of strong-interaction QCD matter. Its bulk properties are similar to the dense matter formed in the final state of a neutron star merger, as apparent from recent multimessenger observation
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