We present the first measurements of charge-dependent correlations on angular difference variables η1−η2 (pseudorapidity) and ϕ1−ϕ2 (azimuth) for primary charged hadrons with transverse momentum ...0.15⩽pt⩽2 GeV/c and |η|⩽1.3 from Au–Au collisions at sNN=130 GeV. We observe correlation structures not predicted by theory but consistent with evolution of hadron emission geometry with increasing centrality from one-dimensional fragmentation of color strings along the beam direction to an at least two-dimensional hadronization geometry along the beam and azimuth directions of a hadron-opaque bulk medium.
We present results for the measurement of {phi} meson production via its charged kaon decay channel {phi}{yields}K{sup +}K{sup -} in Au+Au collisions at {radical}(s{sub NN})=62.4,130, and 200 GeV, ...and in p+p and d+Au collisions at {radical}(s{sub NN})=200 GeV from the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC). The midrapidity (|y|<0.5) {phi} meson transverse momentum (p{sub T}) spectra in central Au+Au collisions are found to be well described by a single exponential distribution. On the other hand, the p{sub T} spectra from p+p, d+Au, and peripheral Au+Au collisions show power-law tails at intermediate and high p{sub T} and are described better by Levy distributions. The constant {phi}/K{sup -} yield ratio vs beam species, collision centrality, and colliding energy is in contradiction with expectations from models having kaon coalescence as the dominant mechanism for {phi} production at RHIC. The {omega}/{phi} yield ratio as a function of p{sub T} is consistent with a model based on the recombination of thermal s quarks up to p{sub T}{approx}4 GeV/c, but disagrees at higher transverse momenta. The measured nuclear modification factor, R{sub dAu}, for the {phi} meson increases above unity at intermediate p{sub T}, similar to that for pions and protons, while R{sub AA} is suppressed due to the energy loss effect in central Au+Au collisions. Number of constituent quark scaling of both R{sub cp} and v{sub 2} for the {phi} meson with respect to other hadrons in Au+Au collisions at {radical}(s{sub NN})=200 GeV at intermediate p{sub T} is observed. These observations support quark coalescence as being the dominant mechanism of hadronization in the intermediate p{sub T} region at RHIC.
Parity-odd domains, corresponding to nontrivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge ...separation of quarks along the system's orbital momentum axis. We investigate a three-particle azimuthal correlator which is a P even observable, but directly sensitive to the charge separation effect. We report measurements of charged hadrons near center-of-mass rapidity with this observable in Au+Au and Cu+Cu collisions at sq root(s{sub NN})=200 GeV using the STAR detector. A signal consistent with several expectations from the theory is detected. We discuss possible contributions from other effects that are not related to parity violation.
Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au+Au and p+p collisions at sq root(s{sub NN})=200 GeV. Strong short- and long-range correlations ...(LRC) are seen in central Au+Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short-range correlations are observed in peripheral Au+Au collisions. Both the dual parton model (DPM) and the color glass condensate (CGC) predict the existence of the long-range correlations. In the DPM, the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC, longitudinal color flux tubes generate the LRC. The data are in qualitative agreement with the predictions of the DPM and indicate the presence of multiple parton interactions.
We present the multiplicity and pseudorapidity distributions of photons produced in Au
+
Au and Cu
+
Cu collisions at
s
NN
=
62.4
and 200 GeV. The photons are measured in the region
−
3.7
<
η
<
−
2.3
...using the photon multiplicity detector in the STAR experiment at RHIC. The number of photons produced per average number of participating nucleon pairs increases with the beam energy and is independent of the collision centrality. For collisions with similar average numbers of participating nucleons the photon multiplicities are observed to be similar for Au
+
Au and Cu
+
Cu collisions at a given beam energy. The ratios of the number of charged particles to photons in the measured pseudorapidity range are found to be
1.4
±
0.1
and
1.2
±
0.1
for
s
NN
=
62.4
and 200 GeV, respectively. The energy dependence of this ratio could reflect varying contributions from baryons to charged particles, while mesons are the dominant contributors to photon production in the given kinematic region. The photon pseudorapidity distributions normalized by average number of participating nucleon pairs, when plotted as a function of
η
−
y
beam
, are found to follow a longitudinal scaling independent of centrality and colliding ion species at both beam energies.
In ultraperipheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a ...{rho}{sup 0}. The {rho}{sup 0} production occurs in two well-separated (median impact parameters of 20 and 40 F for the cases considered here) nuclei, so the system forms a two-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing {rho}{sup 0} production. Since the {rho}{sup 0} decays before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled nonlocal wave function, and is thus an example of the Einstein-Podolsky-Rosen paradox. We observe this suppression in 200 GeV per nucleon-pair gold-gold collisions. The interference is 87%{+-}5%(stat.){+-}8%(syst.) of the expected level. This translates into a limit on decoherence due to wave function collapse or other factors of 23% at the 90% confidence level.
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