Longitudinal spin transfer of Lambda and anti-Lambda in polarized pp collisions at \sqrt s=200 GeV at STAR

We report our measurement on longitudinal spin transfer, D_LL, from high energy polarized protons to $\Lambda$ and $\bar{\Lambda}$ hyperons in proton-proton collisions at $\sqrt{s} = 200 GeV$ with the STAR detector at RHIC. The current measurements cover $\Lambda$, $\bar\Lambda$ pseudorapidity $|\eta| < 1.2$ and transverse momenta $p_T$ up to $4GeV/c$ using the data taken in 2005. The longitudinal spin transfer is found to be D_LL= -0.03\pm 0.13 (stat) \pm 0.04(syst)$for inclusive$\Lambda$and$D_{LL} = -0.12 \pm 0.08(stat) \pm 0.03(syst)$for inclusive$\bar{\Lambda}$hyperons with$ = 0.5$and$ = 3.7 GeV/c$. The prospects with 2009 data and the future measurements are also given.Comment: 6 pages, 3 figures, presentation at the SPIN2010 International Symposium, Juelich (Germany), Sep. 27-Oct. 2, 201

Test of Chemical freeze-out at RHIC

We present the results of a systematic test applying statistical thermal model fits in a consistent way for different particle ratios, and different system sizes using the various particle yields measured in the STAR experiment. Comparison between central and peripheral Au+Au and Cu+Cu collisions with data from p+p collisions provides an interesting tool to verify the dependence with the system size. We also present a study of the rapidity dependence of the thermal fit parameters using available data from RHIC in the forward rapidity regions and also using different parameterization for the rapidity distribution of different particles.Comment: SQM2008 conference proceeding

Open heavy flavor production at RHIC

The study of heavy flavor production in relativistic heavy ion collisions is an extreme experimental challenge but provides important information on the properties of the Quark-Gluon Plasma (QGP) created in Au+Au collisions at RHIC. Heavy-quarks are believed to be produced in the initial stages of the collision, and are essential on the understanding of parton energy loss in the dense medium created in such environment. Moreover, heavy-quarks can help to investigate fundamental properties of QCD in elementary p+p collisions. In this work we review recent results on heavy flavor production and their interaction with the hot and dense medium at RHIC.Comment: Quark Matter 2006 proceedings, 8 pages, 5 figure

High pT correlations with strange particles in STAR

We present the highlights of the current identified strange particles DeltaPhi and DeltaEta correlations analyses, including system-size and trigger-pT of the jet and ridge, jet, ridge and away-side meson/baryon ratios, and the current state of the multi-strange baryon analysis. We see clear azimuthal peaks of comparable strength for all strange baryons and K0-short mesons. We see no observable species dependence on the same-side jet or ridge yields as a function of pT. However, while the away side and the ridge have Lambda to K0-short ratio similar to that of the bulk, the jet-only ratio is similar to that in $p+p$. The implications of these findings on current in-medium jet theoretical explanations are discussed.Comment: Proceedings for the Strangeness in Quark Matter 2007 Conference in Levoca, Slovaki

Heavy Flavor Production at STAR

e present measurements on $D^0$ meson production via direct reconstruction of its hadronic decay channel $D^0\to K\pi$ in minimum bias $d$+Au and Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV with $p_T$ up to $\sim$3 GeV/$c$. Non-photonic electron spectra from the charm semi-leptonic decays are analyzed from the same data set as well as in $p$+$p$ collision at $\sqrt{s}$=200 GeV using the STAR Time-of-Flight (TOF) and Barrel EMC (BEMC) detectors, respectively. Results of the charm-decayed single muon (prompt muon) spectra are also presented at low $p_T$ in Au+Au collisions measured by the TOF detector. The charm production total cross-section per nucleon-nucleon collision is measured to be 1.26$\pm$0.09(stat.)$\pm$0.23(sys.) mb in minimum bias Au+Au collisions, which is consistent with the $N_{bin}$ scaling compared to 1.4$\pm0.2\pm$0.4 mb in minimum bias $d$+Au collisions, and supports the idea that charm quarks should be produced mostly via parton fusion at early stage in relativistic heavy-ion collisions. A Blast-Wave model fit to the low $p_T$ ($<2$ GeV/c) non-photonic electrons, prompt muons and $D^0$ spectra shows that charm hadrons may kinetically freeze-out earlier than light hadrons with a smaller collective velocity. The nuclear modification factors ($R_{AA}$) of the non-photonic electrons in central Au+Au collisions are significantly below unity at $p_T>\sim$2 GeV/$c$, which indicates a significant amount of energy loss for heavy quarks in Au+Au collisions. The charm transverse momentum distribution must have been modified by the hot and dense matter created in central Au+Au collisions at RHIC.Comment: 7 pages, 4 figures, proceedings for the Strange Quark Matter 2006 conferenc

On saturation of charged hadron production in pp collisions at LHC

First results on charged hadron transverse momentum spectra in pp collisions obtained by the CMS Collaboration at LHC were analyzed in z-scaling approach. The first LHC data confirm z-scaling. The saturation regime of the scaling function psi(z) observed in pp and antp-pp interactions at lower energy sqrt s = 19-1960 GeV is verified. The saturation of psi(z) for charged hadrons is found down to z=0.05 at the highest energy sqrt s = 2360 GeV reached till now at colliders. A microscopic scenario of hadron production is discussed in connection with search for new signatures of phase transitions in hadron matter. Constituent energy loss and its dependencies on the transverse momentum of charged hadrons and collision energy are estimated. The beam energy scan at LHC in the saturation region is suggested.Comment: LaTeX, 6 pages, 6 figure

Overview of Strangeness Production at the STAR Experiment

We present an overview of recent STAR results on strangeness production in p+p and heavy-ion collisions at RHIC. In both Cu+Cu and Au+Au collisions we show the centrality dependencies of bulk yield and mid-$p_{T}$ spectrum measurements with new comparisons to theory. The latest $v_{2}$ results for strange particles are presented and prospects for strangeness production in the low energy scan program will be outlined. Finally, we report new measurements of strangeness fragmentation functions for jets in p+p collisions.Comment: 8 pages, 7 figures, proceedings for SQM 200

STAR Results on High Transverse Momentum, Heavy Flavor and Electromagnetic Probes

We summarize here recent results from the STAR collaboration focusing on processes involving large momentum transfers. Measurements of angular correlations of di-hadrons are explored in both the pseudorapidity (eta) and azimuthal (phi) projections. In central Au+Au, an elongated structure is found in the eta projection which persists up to the highest measured pT. After quantifying the particle yield in this structure and subtracting it from the near-side yield, we observe that the remainder exhibits a behavior strikingly similar to that of the near-side yield in d+Au. For heavy flavor production, using electron-hadron correlations in p+p collisions, we obtain an estimate of the b-quark contribution to the non-photonic electrons in the pT region 3-6 GeV/c, and find it consistent with FONLL calculations. Together with the observed suppression of non-photonic electrons in Au+Au, this strongly suggests suppression of b-quark production in Au+Au collisions. We discuss results on the mid-rapidity Upsilon cross-section in p+p collisions. Finally, we present a proof-of-principle measurement of photon-hadron correlations in p+p collisions, paving the way for the tomographic study of the matter produced in central Au+Au via gamma-jet measurements.Comment: 8 pages, 4 figures. Proceedings of "Quark Matter 2006", 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collision

Radial flow afterburner for event generators and the baryon puzzle

A simple afterburner including radial flow to the randomized transverse momentum obtained from event generators, Pythia and Hijing, has been implemented to calculate the $p/\pi$ ratios and compare them with available data. A coherent trend of qualitative agreement has been obtained in $pp$ collisions and in $Au+Au$ for various centralities. Those results indicate that the radial flow does play an important role in the so called baryon puzzle anomaly.Comment: 11 pages, 5 figures. To appear in Journal of Physics

Exploring Early Parton Momentum Distribution with the Ridge from the Near-Side Jet

In a central nucleus-nucleus collision at high-energies, medium partons kicked by a near-side jet acquire a momentum along the jet direction and subsequently materialize as the observed ridge particles. They carry direct information on the early parton momentum distribution which can be extracted by using the ridge data for central AuAu collisions at \sqrt{s_{NN}}=200 GeV. The extracted parton momentum distribution has a thermal-like transverse momentum distribution but a non-Gaussian, relatively flat rapidity distribution at mid-rapidity with sharp kinematic boundaries at large rapidities that depend on the transverse momentum.Comment: In Proceedings of 20th International Conference on Ultra-Relativistic Nucleus Nucleus Collisions, Jaipur, India, Feb. 4-10, 200