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Register a new userStrangeness in STAR at RHIC
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Shusu Shi
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We present the recent results of strangeness production at the mid-rapidity in Au + Au collisions at RHIC, from $sqrt{s_{rm NN}}$ = 7.7 to 200 GeV. The $v_2$ of multi-strange baryon $Omega$ and $phi$ mesons are similar to that of pions and protons in the intermediate $p_T$ range (2 - 5 GeV/$c$) in $sqrt{s_{rm NN}}$ = 200 GeV Au + Au collisions, indicating that the major part of collective flow has been built up at partonic stage. The breaking of mass ordering between $phi$ mesons and protons in the low $p_T$ range ($<$ 1 GeV/$c$) is consistent with a picture that $phi$ mesons are less sensitive to later hadronic interaction. The nuclear modification factor $R_{rm CP}$ and baryon to meson ratio change dramatically when the collision energy is lower than 19.6 GeV. It suggests a possible change of medium property of the system compared to those from high energies.
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The production of $W$ bosons in polarized $p+p$ collisions at RHIC provides an excellent tool to probe the protons sea quark distributions. At leading order $W^{-(+)}$ bosons are produced in $bar{u}+d,(bar{d}+u)$ collisions, and parity-violating single-spin asymmetries measured in longitudinally polarized $p+p$ collisions give access to the flavor-separated light quark and antiquark helicity distributions. In this proceedings we report preliminary results for the single-spin asymmetry, $A_L$ from data collected in 2012 by the STAR experiment at RHIC with an integrated luminosity of 72 pb$^{-1}$ at $sqrt{s}=510$ GeV and an average beam polarization of 56%.
STAR collected data in proton-proton collisions at sqrt(s)=200 GeV with transverse and longitudinal beam polarizations during the initial running periods in 2002--2004 at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. Results on the single transverse spin asymmetries in the production of high energy forward neutral pions and of forward charged hadrons will be presented. Data have been obtained for double longitudinal asymmetries in inclusive jet production in 2003 and 2004. These data provide sensitivity to the polarization of gluons in the proton. In the future, we aim to determine the gluon polarization over a wide kinematic range using coincidences of direct photons and jets. Furthermore, we aim to determine the polarizations of the u, bar(u), d and bar(d) quarks in the proton by measuring single longitudinal spin asymmetries in the production of weak bosons at sqrt(s) = 500$ GeV.
We determine chemical freeze-out conditions from strangeness observables measured at RHIC beam energies. Based on a combined analysis of lowest-order net-Kaon fluctuations and strange anti-baryon over baryon yield ratios we obtain visibly enhanced freeze-out conditions at high beam energies compared to previous studies which analyzed net-proton and net-charge fluctuations. Our findings are in qualitative agreement with the recent study [1] which utilizes the net-Kaon fluctuation data in combination with information from lattice QCD. Our complimentary approach shows that also strange hadron yield ratios are described by such enhanced freeze-out conditions.
We report the energy and centrality dependence of dynamical kurtosis for Au + Au collisions at $sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV at RHIC. The dynamical kurtosis of net-proton is compared to that of total-proton. The results are also compared with AMPT model calculations.
We present STAR measurements of K^{0}_{S}, phi, Lambda, Xi, and Omega at mid-rapidity from Au+Au collisions at sqrt{s_{NN}} = 7.7, 11.5, 19.6, 27, and 39 GeV from the Beam Energy Scan (BES) program at the BNL Relativistic Heavy Ion Collider (RHIC). Nuclear modification factors and baryon-to-meson ratios are measured to understand recombination and parton energy loss mechanisms. Implications on partonic versus hadronic dynamics at low beam energies are discussed.
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