Event by event di-hadron correlations in Pb-Pb 2.76 TeV collisions from the ALICE experiment

The large multiplicities at the LHC may permit flow harmonics to be determined on an event by event basis in Pb-Pb collisions. We extract these harmonics from event by event di-hadron correlations. Within a fine centrality bin, we find the correlation function varies substantially on an event by event basis, indicating large fluctuations in the initial conditions for a given impact parameter. Such large fluctuations lead to some events being highly triangular or highly elliptical, where the angular correlation function is completely dominated by the respective second and third Fourier harmonics. We will show unfolded $v_{2}$ distributions for various centralities, and implications for our understanding of the initial conditions.Comment: 4 pages, Hot Quarks 201

Untriggered di-hadron correlations in Pb-Pb collisions at sNN=\sqrt{s_{NN}} = 2.76 TeV from ALICE

We present measurements of untriggered di-hadron correlations as a function of centrality in Pb-Pb \sNN collisions, for charged hadrons with $p_{T} > 0.15$ GeV$/c$. These measurements provide a map of the bulk correlation structures in heavy-ion collisions. Contributions to these structures may come from jets, initial density fluctuations, elliptic flow, resonances, and/or momentum conservation. We decompose the measured correlation functions via a multi-parameter fit in order to extract the nearside Gaussian, the longer range $\Delta \eta$ correlation often referred to as the soft ridge. The effect of including higher harmonics ($v_{3}$ and $v_{4}$) in this procedure will be discussed. We investigate how the nearside Gaussian scales with the number of binary collisions. Finally, we show the charge dependence of the nearside Gaussian.Comment: 4 pages, QM 2011 proceeding

The Centrality Dependence of Strange Baryon and Meson Production in Cu+Cu and Au+Au sqrt(s_NN) = 200 GeV collisions

Transverse momentum spectra of Lambda and K0Short particles are presented for Cu+Cu sqrt(s_NN) = 200 GeV collisions observed at STAR, and compared to Au+Au measurements at the same energy. For both systems, a number of observables are shown to increase at mid-rapidity (|y| < 0.5) with increasing centrality. These are the integrated Lambda and K0Short yields, the integrated Lambda and K0Short yields per participating nucleon, and mid-pT (1 GeV/c -> 4.5 GeV/c) Lambda/KK0Short ratios. The RCP ratio is found to be higher for the Lambda yields at mid-pT compared to the K0Short yields for both the Cu+Cu and Au+Au data. In contrast, when similar numbers of participating nucleons are considered for the Cu+Cu and Au+Au data, an indication of increased bulk strangeness production and a higher mid-pT (1 -> 4.5 GeV/c) Lambda/K0Short ratio are found, for Cu+Cu.Comment: Quark Matter 2006 Proceeding

Strangeness Production in Heavy-Ion Collisions at STAR

We report an overview of strangeness production in Cu+Cu and Au+Au collisions at the energies $\sqrt{s_{NN}} =$ 62.4 and 200 GeV. We show new mid-rapidly $dN/dy$ results for the $K^{0}_{S}$, $\Lambda$, $\Xi$, $\Omega$ particles in Cu+Cu \sNN{62} collisions and compare to results in Au+Au \sNN{62} collisions. We show new results for mid-$p_T$ $\Lambda/K^{0}_{S}$ ratios in Cu+Cu \sNN{62} collisions and again compare to ratios in Au+Au \sNN{62} collisions. Finally, we show the high-$p_{T}$ ($\sim 6.2$ GeV/c) $R_{AA}(K^{0}_{S})$ as a function of system size in Au+Au \sNN{200} collisions and compare to $R_{AA}(\pi)$.Comment: 4 pages, 3 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Selected results on Strong and Coulomb-induced correlations from the STAR experiment

Using recent high-statistics STAR data from Au+Au and Cu+Cu collisions at full RHIC energy I discuss strong and Coulomb-induced final state interaction effects on identical ($\pi-\pi$) and non-identical ($\pi-\Xi$) particle correlations. Analysis of $\pi-\Xi$ correlations reveals the strong and Coulomb-induced FSI effects allowing for the first time to estimate space extension of $\pi$ and $\Xi$ sources and average shift between them. Source imaging technique providing clean separation of these effects from effects due to the source function itself is applied to one-dimensional relative momentum correlation function of identical pions. For low momentum pions and/or non-central collisions large departure from a single-Gaussian shape is observed

Longitudinal Spin Asymmetry and Cross Section of Inclusive pi0 Production in Polarized p+p Collisions at RHIC

We present the first measurement of the cross section and the double longitudinal spin asymmetry of inclusive pi0 production in polarized p+p collisions at Sqrt(s) = 200 GeV at mid-rapidity with the STAR detector, using the barrel electromagnetic calorimeter. The measured cross section is compared to NLO pQCD calculations and can provide constraints on the pion fragmentation functions. Fragmentation is studied directly by measuring the momentum fraction of pi0 in jets, a quantity that is affected by the fragmentation process and jet reconstruction effects. The double longitudinal spin asymmetry is compared to NLO pQCD calculations based on different assumptions for the gluon polarization in the nucleon to provide constraints on delta g/g. At the present level of statistics the measured asymmetry disfavors a large positive gluon polarization, but can not yet distinguish between other scenarios.Comment: 4 pages, 2 figures, submitted to the proceedings of the 17th International Spin Physics Symposium (SPIN2006), Kyoto, Japan, October 2 to 7, 200

Recent results of the STAR high-energy polarized proton-proton program at RHIC at BNL

The STAR experiment at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) is carrying out a spin physics program colliding transverse or longitudinal polarized proton beams at $\sqrt{s}=200-500$GeV to gain a deeper insight into the spin structure and dynamics of the proton. These studies provide fundamental tests of Quantum Chromodynamics (QCD). One of the main objectives of the STAR spin physics program is the determination of the polarized gluon distribution function through a measurement of the longitudinal double-spin asymmetry, $A_{LL}$, for various processes. Recent results will be shown on the measurement of $A_{LL}$ for inclusive jet production, neutral pion production and charged pion production at $\sqrt{s}=200$GeV. In addition to these measurements involving longitudinal polarized proton beams, the STAR collaboration has performed several important measurements employing transverse polarized proton beams. New results on the measurement of the transverse single-spin asymmetry, $A_{N}$, for forward neutral pion production and the first measurement of $A_{N}$ for mid-rapidity di-jet production will be discussed.Comment: 8 pages, 5 figures, Invited talk given at the 17th International Spin Physics Symposium (SPIN 2006), October 2006, Kyoto, Japa

Measurement of Sivers Asymmetries for Di-jets in \sqrt{s}=200 GeV pp Collisions at STAR

Measurement of the transverse spin dependence of the di-jet opening angle in pp collisions at sqrt{s}=200 GeV has been performed by the STAR collaboration. An analyzing power consistent with zero has been observed over a broad range in pseudorapidity sum of the two jets with respect to the polarized beam direction. A non-zero (Sivers) correlation between transverse momentum direction of partons in the initial state and transverse spin orientation of the parent proton has been previously observed in semi-inclusive deep inelastic scattering (SIDIS). The present measurements are much smaller than deduced from predictions made for STAR di-jets based on non-zero quark Sivers functions deduced from SIDIS, and furthermore indicate that gluon Sivers asymmetries are comparably small.Comment: 4 pages, 3 figures, talk presented at SPIN 2006, Kyoto, October 200