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

Event Structure at RHIC from p-p to Au-Au

Several correlation analysis techniques are applied to p-p and Au-Au collisions at RHIC. Strong large-momentum-scale correlations are observed which can be related to local charge and momentum conservation during hadronization and to minijet (minimum-bias parton fragment) correlations.Comment: 6 pages, 6 figures, 20th Winter Workshop on Nuclear Dynamics, Trelawny Beach, Jamaica, March 15-20, 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

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

Proton - Lambda correlations in Au-Au Collisions at sNN=200\sqrt{s_{NN}} = 200 GeV from the STAR experiment

The space-time evolution of the source of particles formed in the collision of nuclei can be studied through particle correlations. The STAR experiment is dedicated to study ultra-relativistic heavy ions collisions and allows to measure non-identical strange particle correlations. The source size can be extracted by studying $p-\Lambda$, $\bar{p}-\bar{\Lambda}$, $\bar{p}-\Lambda$ and $p-\bar{\Lambda}$ correlation functions. Strong interaction potential has been studied for these systems using an analytical model. Final State Interaction (FSI) parameters have been determined and has shown a significant annihilation process present in $\bar{p}-\Lambda$ and $p-\bar{\Lambda}$ systems not present in $p-\Lambda$ and $\bar{p}-\bar{\Lambda}$.Comment: contribution to the 20th Winter Workshop on Nuclear Dynamic

Event by Event fluctuation in K/pi ratio at RHIC

We present the preliminary results from our analysis of event by event fluctuation in K/pi ratio in Au+Au collision at \sqrt s_{NN} = 200 GeV and at 62.4 GeV using STAR detector at RHIC. Two different methods have been used to extract the strength of dynamical fluctuation and the centrality dependence of that. The results from the study of energy and centrality dependence of the dynamical fluctuation are presented. From the excitation function it is seen that at two RHIC energies the measure of dynamical fluctuation is constant with values very close to that at 12.3 GeV at SPS. The dynamical fluctuation is found to be positive and decreasing with increasing centrality at RHIC. The results are compared with HIJING model calculation with jets. Results from HIJING are found to be very close to data from central collisions whereas it over predicts the data for peripheral events.Comment: 4 pages, 3 figures, for ICPAQGP - 2005 (Recalculated the errors shown in Fig 2 and Table 1

Anisotropic flow at RHIC

We present the first measurement of directed flow ($v_1$) at the Relativistic Heavy Ion Collider (RHIC). $v_1$ is found to be consistent with zero at pseudorapidities $\eta$ from -1.2 to 1.2, then rises to the level of a couple of percent over the range $2.4 < |\eta| < 4$. The latter observation is similar to that from NA49 if the SPS rapidities are shifted by the difference in beam rapidity between RHIC and SPS. We studied the evolution of elliptic flow from p+p collisions through d+Au collision, and onto Au+Au collisions. Measurements of higher harmonics are presented and discussed.Comment: 6 pages, 3 figures. Proceeding for the 20th Winter Workshop on Nuclear Dynamics, Jamaic

First Results from Photon Multiplicity Detector at RHIC

We present the first measurement of multiplicity and pseudorapidity distributions of photons in the pseudorapidity region 2.3 < eta < 3.7 for different centralities in Au + Au collisions at sqrt{s_NN} = 62.4 GeV. The pseudorapidity distribution of photons, dominated by neutral pion decays, has been compared to those of identified charged pions, photons, and inclusive charged particles from heavy ion and nucleon-nucleon collisions at various energies. Scaling of photon yield with number of participating nucleons and limiting fragmentation scenario for inclusive photon production has been studied.Comment: Talk given at 5th International Conference on Physics and Astrophysics of Quark Gluon Plasma (February 8 - 12, 2005); 4 pages and 6 figure

Recent Results on Searches for Pentaquark States from STAR at RHIC

We present results on pentaquark searches from nuclear collisions at RHIC with the STAR detector system. An intriguing peak has been observed in the invariant mass distribution of $pK^{+}+\overline{p}K^{-}$ from 18.6 Million d+Au collision events at $\sqrt{s_{_{NN}}} = 200$~GeV. The peak centers at a mass $1528 \pm 2 \pm 5$~MeV/c$^{2}$ and the FWHM $\sim15$ MeV/c$^{2}$ limited by detector responses. The statistical significance of the peak is $4.2\sigma$. Such a state if confirmed is manifestly exotic and implies a family of isospin one states. A weak signal of less statistical significance ($\sim3\sigma$) has been observed in 5.6M Au+Au collision events at 62.4 GeV. Searches in 10.7M Au+Au collision events at 200 GeV yield no significant signal. The Au+Au results neither confirm nor rule out the d+Au observation as a possible state.Comment: To appear in proceedings of International Conference on QCD and Hadron Physics at Beijing, June 200

Baryonic Resonance Studies with STAR

Yields and spectra of $\Sigma(1385)$ are measured in $p+p$, d+Au and Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV . The nuclear modification factors in d+Au collisions are presented. The $p_{T}$ dependent medium effects are investigated via the nuclear modification factors. The implications of these results on various models are discussed.Comment: Strange Quark Matter 2006 Conference Talk Proceeding