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Register a new userSpin Physics with STAR at RHIC
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J. Kiryluk
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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.
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Recent soft physics results from collisions of ultra-relativistic nuclei at Relativistic Heavy Ion Collider (RHIC) operating at Brookhaven National Laboratory (BNL) are reviewed. Topics discussed cover the Beam Energy Scan program with some emphasis on anisotropic particle flow.
The RHIC high energy collision of species ranging from p+p, p(d)+A to A+A provide access to the {small-x} component of the hadron wave function. The RHIC program has brought renewed interest in that subject with its ability to reach values of the parton momentum fraction smaller than 0.01 with studies of particle production at high rapidity. Furthermore, the use of heavy nuclei in the p(d)+A collisions facilitates the study of saturation effects in the gluonic component of the nuclei because the appropriate scale for that regime grows as A^1/3. We review the experimental results of the RHIC program that have relevance to {small-x} emphasizing the physics extracted from d+Au collisions and their comparison to p+p collisions at the same energy.
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.
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%.
The RHIC program was intended to identify and study the quark-gluon plasma formed in the collision of heavy nuclei. The discovery of the perfect liquid is an essential step towards the understanding of the medium formed in these collisions. Much of data relevant to this was provided by the study of soft observables, which involve many particles of low momentum produced in nearly every event, rather than high momentum particles produced in rare events. The main results related to soft physics at RHIC are discussed, as well as their implications for the physics of the LHC heavy ion program.
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