Comparisons of particle production from high-energy ion collisions with next-to-leading order perturbative QCD calculations show good agreement down to moderate transverse momentum values. Distributions of azimuthal angle differences between coincident hadrons in these collisions support a partonic origin to the particle production again down to moderate transverse momentum values. The rapidity dependence of inclusive and coincident particle production can therefore be used to probe parton distribution functions down to small momentum fractions where theory anticipates that parton saturation could be present. This paper describes how such experiments could be completed. © Springer-Verlag 2005.

We present high precision measurements of elliptic flow near midrapidity (|y|<1.0) for multistrange hadrons and ϕ meson as a function of centrality and transverse momentum in Au+Au collisions at center of mass energy √[sNN]=200  GeV. We observe that the transverse momentum dependence of ϕ and Ω v2 is similar to that of π and p, 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 ϕ and proton v2 at low transverse momentum in the 0%-30% centrality range, possibly indicating late hadronic interactions affecting the proton v2.

We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in p^{↑}+p collisions at RHIC. These correlations directly probe quark transversity distributions. We measure signals in excess of 5 standard deviations at high transverse momenta, at high pseudorapidities η>0.5, and for pair masses around the mass of the ρ meson. This is the first direct transversity measurement in p+p collisions.

Dihadron correlations are analyzed in √sNN = 200 GeV d + Au collisions classified by forward charged particle multiplicity and zero-degree neutral energy in the Au-beam direction. It is found that the jetlike correlated yield increases with the event multiplicity. After taking into account this dependence, the non-jet contribution on the away side is minimal, leaving little room for a back-to-back ridge in these collisions.

Measurements of two-particle correlations on transverse momentum pt for Au-Au collisions at sqrt sNN = 130 GeV are presented. Significant large-momentum-scale correlations are observed for charged primary hadrons with 0.15 <_ pt <_ 2 GeV/c and pseudorapidity lbar eta rbar <_ 1.3. Such correlations were not observed in a similar study at lower energy and are not predicted by theoretical collision models. Their direct relation to mean-pt fluctuations measured in the same angular acceptance is demonstrated. Positive correlations are observed for pairs of particles which have large pt values while negative correlations occur for pairs in which one particle has large pt and the other has much lower pt. The correlation amplitudes per final state particle increase with collision centrality. The 3 observed correlations are consistent with a scenario in which the transverse momentum of hadrons

A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the BNL Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical Kπ, pπ, and Kp fluctuations as measured by the STAR experiment in central 0-5% Au + Au collisions from center-of-mass collision energies sNN=7.7 to 200 GeV are presented. The observable νdyn was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the Kπ, pπ, and Kp pairs. The energy dependences of these fluctuations from central 0-5% Au + Au collisions all demonstrate a smooth evolution with collision energy.

The STAR Collaboration presents for the first time two-dimensional di-hadron correlations with identified leading hadrons in 200 GeV central Au. +. Au and minimum-bias d. +. Au collisions to explore hadronization mechanisms in the quark gluon plasma. The enhancement of the jet-like yield for leading pions in Au. +. Au data with respect to the d. +. Au reference and the absence of such an enhancement for leading non-pions (protons and kaons) are discussed within the context of a quark recombination scenario. The correlated yield at large angles, specifically in the ridge region, is found to be significantly higher for leading non-pions than pions. The consistencies of the constituent quark scaling, azimuthal harmonic model and a mini-jet modification model description of the data are tested, providing further constraints on hadronization.

Balance functions have been measured in terms of relative pseudorapidity (Δη) for charged particle pairs at the BNL Relativistic Heavy Ion Collider from Au + Au collisions at sNN=7.7GeV to 200 GeV using the STAR detector. These results are compared with balance functions measured at the CERN Large Hadron Collider from Pb + Pb collisions at sNN=2.76TeV by the ALICE Collaboration. The width of the balance function decreases as the collisions become more central and as the beam energy is increased. In contrast, the widths of the balance functions calculated using shuffled events show little dependence on centrality or beam energy and are larger than the observed widths. Balance function widths calculated using events generated by UrQMD are wider than the measured widths in central collisions and show little centrality dependence. The measured widths of the balance functions in central collisions are consistent with the delayed hadronization of a deconfined quark gluon plasma (QGP). The narrowing of the balance function in central collisions at sNN=7.7 GeV implies that a QGP is still being created at this relatively low energy.