242 research outputs found
Beam energy dependent two-pion interferometry and the freeze-out eccentricity of pions in heavy ion collisions at STAR
We present results of analyses of two-pion interferometry in Au+Au collisions
at = 7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV measured in the
STAR detector as part of the RHIC Beam Energy Scan program. The extracted
correlation lengths (HBT radii) are studied as a function of beam energy,
azimuthal angle relative to the reaction plane, centrality, and transverse mass
() of the particles. The azimuthal analysis allows extraction of the
eccentricity of the entire fireball at kinetic freeze-out. The energy
dependence of this observable is expected to be sensitive to changes in the
equation of state. A new global fit method is studied as an alternate method to
directly measure the parameters in the azimuthal analysis. The eccentricity
shows a monotonic decrease with beam energy that is qualitatively consistent
with the trend from all model predictions and quantitatively consistent with a
hadronic transport model.Comment: 27 pages; 27 figure
Isolation of Flow and Nonflow Correlations by Two- and Four-Particle Cumulant Measurements of Azimuthal Harmonics in 200 GeV Au+Au Collisions
A data-driven method was applied to measurements of Au+Au collisions at
200 GeV made with the STAR detector at RHIC to isolate
pseudorapidity distance -dependent and -independent
correlations by using two- and four-particle azimuthal cumulant measurements.
We identified a component of the correlation that is -independent,
which is likely dominated by anisotropic flow and flow fluctuations. It was
also found to be independent of within the measured range of
pseudorapidity . The relative flow fluctuation was found to be for particles of transverse momentum
less than GeV/. The -dependent part may be attributed to
nonflow correlations, and is found to be relative to the
flow of the measured second harmonic cumulant at
Beam-energy dependence of charge separation along the magnetic field in Au+Au collisions at RHIC
Local parity-odd domains are theorized to form inside a Quark-Gluon-Plasma
(QGP) which has been produced in high-energy heavy-ion collisions. The local
parity-odd domains manifest themselves as charge separation along the magnetic
field axis via the chiral magnetic effect (CME). The experimental observation
of charge separation has previously been reported for heavy-ion collisions at
the top RHIC energies. In this paper, we present the results of the beam-energy
dependence of the charge correlations in Au+Au collisions at midrapidity for
center-of-mass energies of 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV from the STAR
experiment. After background subtraction, the signal gradually reduces with
decreased beam energy, and tends to vanish by 7.7 GeV. The implications of
these results for the CME will be discussed.Comment: 6 pages, 4 figures, accepted by Phys. Rev. Lett (more model
comparisons have been added in version 2
Charged-to-neutral correlation at forward rapidity in Au+Au collisions at =200 GeV
Event-by-event fluctuations of the ratio of inclusive charged to photon
multiplicities at forward rapidity in Au+Au collision at =200
GeV have been studied. Dominant contribution to such fluctuations is expected
to come from correlated production of charged and neutral pions. We search for
evidences of dynamical fluctuations of different physical origins. Observables
constructed out of moments of multiplicities are used as measures of
fluctuations. Mixed events and model calculations are used as baselines.
Results are compared to the dynamical net-charge fluctuations measured in the
same acceptance. A non-zero statistically significant signal of dynamical
fluctuations is observed in excess to the model prediction when charged
particles and photons are measured in the same acceptance. We find that, unlike
dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated
by correlation due to particle decay. Results are compared to the expectations
based on the generic production mechanism of pions due to isospin symmetry, for
which no significant (<1%) deviation is observed.Comment: 14 pages, 6 figure
Observation of meson nuclear modifications in Au+Au collisions at = 200 GeV
We report the first measurement of charmed-hadron () production via the
hadronic decay channel () in Au+Au collisions at
= 200\,GeV with the STAR experiment. The charm
production cross-section per nucleon-nucleon collision at mid-rapidity scales
with the number of binary collisions, , from + to central Au+Au
collisions. The meson yields in central Au+Au collisions are strongly
suppressed compared to those in + scaled by , for transverse
momenta GeV/, demonstrating significant energy loss of charm
quarks in the hot and dense medium. An enhancement at intermediate is
also observed. Model calculations including strong charm-medium interactions
and coalescence hadronization describe our measurements.Comment: 7 pages including author list, 4 figures, submit to PRL with revised
versio
Elliptic flow of electrons from heavy-flavor hadron decays in Au+Au collisions at 200, 62.4, and 39 GeV
We present measurements of elliptic flow () of electrons from the decays
of heavy-flavor hadrons () by the STAR experiment. For Au+Au collisions
at 200 GeV we report , for transverse momentum
() between 0.2 and 7 GeV/c using three methods: the event plane method
({EP}), two-particle correlations ({2}), and four-particle
correlations ({4}). For Au+Au collisions at = 62.4 and
39 GeV we report {2} for GeV/c. {2} and {4} are
non-zero at low and intermediate at 200 GeV, and {2} is consistent
with zero at low at other energies. The {2} at the two lower beam
energies is systematically lower than at 200 GeV for
GeV/c. This difference may suggest that charm quarks interact less
strongly with the surrounding nuclear matter at those two lower energies
compared to GeV.Comment: Version accepted by PR
Measurement of longitudinal spin asymmetries for weak boson production in polarized proton-proton collisions at RHIC
We report measurements of single- and double- spin asymmetries for
and boson production in longitudinally polarized collisions
at GeV by the STAR experiment at RHIC. The asymmetries for
were measured as a function of the decay lepton pseudorapidity, which
provides a theoretically clean probe of the proton's polarized quark
distributions at the scale of the mass. The results are compared to
theoretical predictions, constrained by recent polarized deep inelastic
scattering measurements, and show a preference for a sizable, positive up
antiquark polarization in the range .Comment: 7 pages, 5 figures, Submitted to Physical Review Letters; replaced
with published versio
Precision Measurement of the Longitudinal Double-spin Asymmetry for Inclusive Jet Production in Polarized Proton Collisions at GeV
We report a new high-precision measurement of the mid-rapidity inclusive jet
longitudinal double-spin asymmetry, , in polarized collisions at
center-of-mass energy GeV. The STAR data place stringent
constraints on polarized parton distribution functions extracted at
next-to-leading order from global analyses of inclusive deep inelastic
scattering (DIS), semi-inclusive DIS, and RHIC data. The measured
asymmetries provide evidence for positive gluon polarization in the Bjorken-
region .Comment: 7 pages, 3 figure
Centrality and transverse momentum dependence of elliptic flow of multi-strange hadrons and meson in Au+Au collisions at = 200 GeV
We present high precision measurements of elliptic flow near midrapidity
() for multi-strange hadrons and meson as a function of
centrality and transverse momentum in Au+Au collisions at center of mass energy
200 GeV. We observe that the transverse momentum dependence of
and is similar to that of and , respectively,
which may indicate that the heavier strange quark flows as strongly as the
lighter up and down quarks. This observation constitutes a clear piece of
evidence for the development of partonic collectivity in heavy-ion collisions
at the top RHIC energy. Number of constituent quark scaling is found to hold
within statistical uncertainty for both 0-30 and 30-80 collision
centrality. There is an indication of the breakdown of previously observed mass
ordering between and proton at low transverse momentum in the
0-30 centrality range, possibly indicating late hadronic interactions
affecting the proton .Comment: 7 pages and 4 figures, Accepted for publication in Physical Review
Letter
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