Anomalous centrality variation of minijet angular correlations in Au-Au collisions at 62 and 200 GeV from STAR

We have measured 2D autocorrelations for all charged hadrons in STAR with $p_{t}$ $>$ 0.15 GeV/c and $| \eta |$ $<$ 1 from Au+Au collisions at 62 and 200 GeV. The correlation structure is dominated by a peak centered at zero relative opening angles on $\eta$ and $\phi$ which we hypothesize is caused by minimum-bias jets (minijets). We observe a large excess of minijet correlations in more-central Au-Au collisions relative to binary-collision scaling (more correlated pairs than expected from surface emission or even volume emission). We also observe a sudden increase of the minijet peak amplitude and $\eta$ width relative to binary-collision scaling of scattered partons which occurs at an energy-dependent centrality point. There is a possible scaling of the transition point with transverse particle density.Comment: Quark Matter 2008 proceedings, submitted to Journal of Physics G. v2: added STAR Collaboration to author lis

K* production in Cu+Cu and Au+Au collisions at sqrt(s_{NN}) = 62.4 GeV and 200 GeV in STAR

We report the measurements of $p_T$ spectra of $K^*$ up to intermediate $p_T$ region in mid-rapidity through its hadronic decay channel using the STAR detector in Au+Au and Cu+Cu collisions at $\sqrt{s_{\mathrm{NN}}}$= 62.4 GeV and 200 GeV. Particle ratios such as $K^{*}/K$ and $K^{*}/\phi$ is used to understand the rescattering and regeneration effect on $K^{*}$ production in the hadronic medium. The $K^*$ $v_{2}$ measurement using a high statistics Au+Au 200 GeV dataset and nuclear modification factor measurement supports the quark coalescence model of particle production in the intermediate $p_T$ range.Comment: 5 page

Strangeness and bulk freeze-out properties at RHIC

Identified charged kaon, pion, and proton spectra and ratios from sqrt(s_NN) = 200 and 62.4 GeV Cu+Cu collisions are studied with a hydro-motivated blast-wave and a statistical model framework in order to explore the strangeness production at RHIC and characterize the bulk freeze-out properties of the created system. The spectra are measured at mid-rapidity |y|<0.1 over the transverse momentum range of 0.25 < p_T < 1.2 GeV/c with particle identification derived from the ionization energy loss in the STAR Time Projection Chamber. The multi-dimensional systematic study of pi(+/-), K(+/-), p and pbar production in Cu+Cu, Au+Au, d+Au and p+p collisions is used to discuss the energy, system size and inferred energy density dependence of freeze-out parameters and strangeness production. The new data from Cu+Cu collisions bridge the gap between the smaller d+Au and larger Au+Au systems, allowing a detailed study of the onset of strangeness equilibration at RHIC.Comment: 6 pages, 5 figures, proceedings to Strangeness in Quark Matter 200

STAR's measurement of Long-range forward-backward multiplicity correlations as the signature of "dense partonic matter" in the Heavy Ion collisions at sNN=\sqrt{s_{NN}}=200 GeV.}

Forward-backward multiplicity correlations have been measured with the STAR detector for Au+Au, Cu+Cu and {\it p+p} collisions at $\sqrt{s_{NN}}$ = 200 GeV. A strong, long-range correlation is observed for central heavy ion collisions that vanishes in semi-peripheral events and {\it pp} collisions. There is no apparent scaling of correlation strength with the number of participants involved in the collision. Both the Dual Parton Model and the Color Glass condensate indicate that the long range correlations are due to multiple parton interactions. This suggests that the dense partonic matter might have been created in mid-central and central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.Comment: 4 pages, 3 figures,presented at the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, "Quark Matter 2008", Jaipur, India, February 4-10, 200

Υ\Upsilon production in p+p and Au+Au collisions in STAR

The study of quarkonium production in relativistic heavy ion collisions provides insight into the properties of the produced medium. The lattice studies show a sequential suppression of quarkonia states when compared to normal nuclear matter; which further affirms that a full spectroscopy including bottomonium can provide us a better thermometer for the matter produced under extreme conditions in relativistic heavy ion collisions. With the completion of the STAR Electromagnetic Calorimeter and with the increased luminosity provided by RHIC in Run 6 and 7, the study of $\Upsilon$ production via the di-electron channel becomes possible. We present the results on $\Upsilon$ measurements in p+p collisions (from Run 6) along with the first results from Au+Au collisions (in Run 7) at $\sqrt{s_{\rm{NN}}} = 200$ GeV from the STAR experiment.Comment: 4 pages and 2 figures, Oral Presentation at QM2008 February 4-10, 2008: Jaipur India. submitted to Journal Of Physics-G:Nuclear and Particle Physic

Reaction Plane Dependent Away-side Modification and Near-side Ridge in Au+Au Collisions

STAR preliminary results of di-hadron correlations versus $\phi_{s}$, the trigger particle azimuthal angle relative to the constructed event plane are reported for mid-central Au+Au collisions and compared to central Au+Au as well as minimum bias d+Au collisions. The correlations are observed to vary with $\phi_{s}$ on both the near and away side of the trigger particle. The away-side correlation evolves from single- to double-peak with increasing $\phi_{s}$. The near-side correlation is separated into 'jet' and 'ridge': the ridge is found to decrease with $\phi_{s}$ while the jet remains relatively constant.Comment: 5 pages, 5 figure

STAR's Measurement of Energy and System Size Dependence of K/π\pi Fluctuations at RHIC

Strangeness enhancement has been predicted to be one of the signatures of the formation of the quark gluon plasma (QGP) and event-by-event fluctuations of strangeness may reveal the nature of such a phase transition. We report results for $K/\pi$ fluctuations from Cu+Cu at $\sqrt{s_{NN}}$ = 62.4 and 200 GeV and Au+Au collisions at $\sqrt{s_{NN}}$ = 62.4, 130 and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. We compare our results with the results observed in central Pb+Pb collisions at SPS energies at $\sqrt{s_{NN}}$ = 6.3, 7.6, 8.8, 12.3, and 17.3 GeV. We observe the fluctuation strength measured at the highest SPS energy point by the NA49 collaboration in Pb+Pb collisions is similar to our results in Cu+Cu and Au+Au collisions. We also compare our results with HIJING, a statistical hadronization model and a multi-phase transport model.Comment: To be published in the proceedings of Quark Matter, 2008, held at Jaipur, Indi

J/psi production at high pT in p+p and A+A collisions at STAR

The preliminary results of J/psi spectra at high transverse momentum (5<p_T<14 GeV/c) in p+p and Cu+Cu collisions at s_NN = 200 GeV are reported. The nuclear modification factor is measured to be 0.9+/-0.2 at p_T>5 GeV/c. The correlations between J/psi and charged hadrons are also studied in p+p collisions to understand the J/psi production mechanism at high p_T.Comment: 4 pages, 3 figures. Proceedings of the Quark Matter 2008 conference, Jaipur, India, February 4-10, 2008. Submitted to J. Phys. G: Nucl. and Part. Phy

Direct photon-charged hadron azimuthal correlations

Azimuthal correlations of direct photons at high transverse energy (8 $<$ E$_T$ $<$ 16 GeV) with away-side hadrons of transverse momentum (3 $<$ p$_T$ $<$ 6 GeV/c) have been measured over a broad range of centrality for $Au+Au$ collisions and $p+p$ collisions at $\sqrt{s_{NN}}$ = 200 GeV. The presented results are the first measurements at RHIC for $\gamma$-hadron azimuthal correlations in $Au+Au$ collisions.Comment: 4 pages, 3 figure