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

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

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

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

Baryon to Meson Ratios on the Near and Away-Side of Jets and their Centrality Dependence at STAR

We measure relative abundances of $K_{S}^{0}$, $\Lambda$ and $\bar{\Lambda}$ in near-side and away-side cones correlated with triggered high-$p_{T}$ particles in $^{197}$Au + $^{197}$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The centrality dependence of identified particles in the triggered particle cones is also presented. Particle yields and ratios are extracted on the near-side and away-side of the trigger particle. The associate-particle ratios are studied as a function of the angle relative to the trigger particle azimuth $\Delta\phi$. Such studies should help elucidate the origin of the modifications in the jet like correlations observed in Au+Au collisions relative to p+p collisions. And these studies also will help understand the variation of local parton densities at the away side. We discuss how these measurements might be related to several scenarios for interactions of fast partons with the medium created in Au+Au collision.Comment: 6 pages, 4 figures, Proceedings for SQM 2007 Talk, Levoca, Slovakia, June 24-29, 200

Heavy-flavor correlation measurements via electron azimuthal correlations with open charm mesons

We report first STAR measurement on two heavy-flavor particle correlations in p+p collisions at RHIC. Heavy-flavor (charm and bottom) events are identified and separated on a statistical basis by the azimuthal correlation of their decay electrons and open charm mesons, which yield important information about the underlying production mechanism. The azimuthal correlation distribution exhibits a two-peak structure which can be attributed to B decays on the near-side and predominantly charm pair production on the away-side. These assumptions are supported by dedicated simulations using PYTHIA and MC@NLO event generators. This novel correlation technique will allow comprehensive energy-loss measurements of heavy quarks in heavy-ion collisions.Comment: 6 pages, 3 figures. Invited talk at the International Conference on Strangeness in Quark Matter, Levoca, Slovakia, June 24-29, 200

K* production in Heavy Ion Collisions at RHIC

Study of resonances with their short life-times provides useful tools to probe the properties of hot and dense matter produced in relativistic heavy ion collisions. The high density and/or high temperature of the medium can modify resonance properties such as mass and width. Therefore, measurement of these properties can reveal important information about the evolution dynamics in heavy ion collisions. We report the measurements of $K^{*}$ transverse momentum($p_T$) spectra at mid-rapidity via its hadronic decay channel up to intermediate $p_{T}$ of 2.9 GeV/c using the STAR detector in Au+Au and Cu+Cu collisions at $\sqrt{s_{\mathrm{NN}}}$= 62.4 GeV and 200 GeV. These results are compared to previously reported $K^{*}$ results from Au+Au collisions at RHIC. Integrated yield ratios of $K^{*}/K$ and $K^{*}/\phi$ are used to understand the rescattering and regeneration effects on $K^{*}$ production.Comment: 6 pages, SQM-07 proceeding

Non-photonic electron-hadron correlations at STAR

We present STAR's measurements of azimuthal correlations between non-photonic electrons and charged hadrons in p+p, Cu+Cu and Au+Au collisions at 200 GeV. In p+p collisions, from the non-photonic e-h correlation we have extracted the relative B meson semi-leptonic decay contributions to non-photonic electrons up to electron $p_t \sim 9$ GeV/$c$. In central Cu+Cu and Au+Au collisions where a dense medium is created, we find that the e-h correlation on the away side of the trigger non-photonic electron has been modified in comparison with the expectation from PYTHIA simulations.Comment: 4 pages, 3 figures, Quark Matter 2008 proceedings, presented in the parallel Session XII