We present STAR measurements of azimuthal anisotropy by means of the two- and four-particle cumulants $v_2$ ($v_2{2}$ and $v_2{4}$) for Au+Au and Cu+Cu collisions at center of mass energies $sqrt{s_{_{mathrm{NN}}}} = 62.4$ and 200 GeV. The difference
between $v_2{2}^2$ and $v_2{4}^2$ is related to $v_{2}$ fluctuations ($sigma_{v_2}$) and nonflow $(delta_{2})$. We present an upper limit to $sigma_{v_2}/v_{2}$. Following the assumption that eccentricity fluctuations $sigma_{epsilon}$ dominate $v_2$ fluctuations $frac{sigma_{v_2}}{v_2} approx frac{sigma_{epsilon}}{epsilon}$ we deduce the nonflow implied for several models of eccentricity fluctuations that would be required for consistency with $v_2{2}$ and $v_2{4}$. We also present results on the ratio of $v_2$ to eccentricity.
We present two-dimensional (2D) two-particle angular correlations on relative pseudorapidity $eta$ and azimuth $phi$ for charged particles from Au-Au collisions at $sqrt{s_{rm NN}} = 62$ and 200 GeV with transverse momentum $p_t geq 0.15$ GeV/$c$, $|
eta| leq 1$ and $2pi$ azimuth. Observed correlations include a {same-side} (relative azimuth $< pi/2$) 2D peak, a closely-related away-side azimuth dipole, and an azimuth quadrupole conventionally associated with elliptic flow. The same-side 2D peak and away-side dipole are explained by semihard parton scattering and fragmentation (minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those structures follow N-N binary-collision scaling in Au-Au collisions until mid-centrality where a transition to a qualitatively different centrality trend occurs within a small centrality interval. Above the transition point the number of same-side and away-side correlated pairs increases rapidly {relative to} binary-collision scaling, the $eta$ width of the same-side 2D peak also increases rapidly ($eta$ elongation) and the $phi$ width actually decreases significantly. Those centrality trends are more remarkable when contrasted with expectations of jet quenching in a dense medium. Observed centrality trends are compared to {sc hijing} predictions and to the expected trends for semihard parton scattering and fragmentation in a thermalized opaque medium. We are unable to reconcile a semihard parton scattering and fragmentation origin for the observed correlation structure and centrality trends with heavy ion collision scenarios which invoke rapid parton thermalization. On the other hand, if the collision system is effectively opaque to few-GeV partons the observations reported here would be inconsistent with a minijet picture.
We report first results from an analysis based on a new multi-hadron correlation technique, exploring jet-medium interactions and di-jet surface emission bias at RHIC. Pairs of back-to-back high transverse momentum hadrons are used for triggers to st
udy associated hadron distributions. In contrast with two- and three-particle correlations with a single trigger with similar kinematic selections, the associated hadron distribution of both trigger sides reveals no modification in either relative pseudo-rapidity or relative azimuthal angle from d+Au to central Au+Au collisions. We determine associated hadron yields and spectra as well as production rates for such correlated back-to-back triggers to gain additional insights on medium properties.
We report on K*0 production at mid-rapidity in Au+Au and Cu+Cu collisions at sqrt{s_{NN}} = 62.4 and 200 GeV collected by the Solenoid Tracker at RHIC (STAR) detector. The K*0 is reconstructed via the hadronic decays K*0 to K+ pi- and bar{K*0} to K-p
i+. Transverse momentum, pT, spectra are measured over a range of pT extending from 0.2 GeV/c to 5 GeV/c. The center of mass energy and system size dependence of the rapidity density, dN/dy, and the average transverse momentum, <pT>, are presented. The measured N(K*0)/N(K) and N(phi)/N(K*0) ratios favor the dominance of re-scattering of decay daughters of K*0 over the hadronic regeneration for the K*0 production. In the intermediate pT region (2.0 < pT < 4.0 GeV/c), the elliptic flow parameter, v2, and the nuclear modification factor, RCP, agree with the expectations from the quark coalescence model of particle production.
Yields, correlation shapes, and mean transverse momenta pt{} of charged particles associated with intermediate to high-pt{} trigger particles ($2.5 < pt < 10$ GeVc) in d+Au and Au+Au collisions at $snn=200$ GeV are presented. For associated particles
at higher $pt gtrsim 2.5$ GeVc, narrow correlation peaks are seen in d+Au and Au+Au, indicating that the main production mechanism is jet fragmentation. At lower associated particle $pt < 2$ GeVc, a large enhancement of the near- ($dphi sim 0$) and away-side ($dphi sim pi$) associated yields is found, together with a strong broadening of the away-side azimuthal distributions in Au+Au collisions compared to d+Au measurements, suggesting that other particle production mechanisms play a role. This is further supported by the observed significant softening of the away-side associated particle yield distribution at $dphi sim pi$ in central Au+Au collisions.
We present a systematic analysis of two-pion interferometry in Au+Au collisions at $sqrt{s_{rm{NN}}}$ = 62.4 GeV and Cu+Cu collisions at $sqrt{s_{rm{NN}}}$ = 62.4 and 200 GeV using the STAR detector at RHIC. The multiplicity and transverse momentum d
ependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.
