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Qi-Ye Shou
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The chiral magnetic wave (CMW) has been theorized to propagate in the Quark-Gluon Plasma formed in high-energy heavy-ion collisions. It could cause a finite electric quadrupole moment of the collision system, and may be observed as a dependence of elliptic flow, $v_{2}$, on the asymmetry between positively and negatively charged hadrons, $A_{rm ch}$. However, non-CMW mechanisms, such as local charge conservation (LCC) and hydrodynamics with isospin effect, could also contribute to the experimental observations. Here we present the STAR measurements of elliptic flow $v_{2}$ and triangular flow $v_{3}$ of charged pions, along with $v_{2}$ of charged kaons and protons, as functions of $A_{rm ch}$ in Au+Au collisions at $sqrt{s_{rm NN}}$ = 200 GeV. The slope parameters of $Delta v_{2}$($A_{rm ch}$) and $Delta v_{3}$($A_{rm ch}$) are reported and compared to investigate the LCC background. The similarity between pion and kaon slopes suggests that the hydrodynamics is not the dominant mechanism. The difference between the normalized $Delta v_{2}$ and $Delta v_{3}$ slopes, together with the small slopes in p+Au and d+Au collisions at $sqrt{s_{rm NN}}$ = 200 GeV, suggest that the CMW picture remains a viable interpretation at RHIC.
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The chiral magnetic effect (CME) is predicted to occur as a consequence of a local violation of $cal P$ and $cal CP$ symmetries of the strong interaction amidst a strong electro-magnetic field generated in relativistic heavy-ion collisions. Experimen
tal manifestation of the CME involves a separation of positively and negatively charged hadrons along the direction of the magnetic field. Previous measurements of the CME-sensitive charge-separation observables remain inconclusive because of large background contributions. In order to better control the influence of signal and backgrounds, the STAR Collaboration performed a blind analysis of a large data sample of approximately 3.8 billion isobar collisions of $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr at $sqrt{s_{rm NN}}=200$ GeV. Prior to the blind analysis, the CME signatures are predefined as a significant excess of the CME-sensitive observables in Ru+Ru collisions over those in Zr+Zr collisions, owing to a larger magnetic field in the former. A precision down to 0.4% is achieved, as anticipated, in the relative magnitudes of the pertinent observables between the two isobar systems. Observed differences in the multiplicity and flow harmonics at the matching centrality indicate that the magnitude of the CME background is different between the two species. No CME signature that satisfies the predefined criteria has been observed in isobar collisions in this blind analysis.
An observable sensitive to the chiral magnetic wave (CMW) is the charge asymmetry dependence of the $pi^{-}$ and $pi^{+}$ anisotropic flow difference, $Delta v_{n}(A_{rm ch})$. We show that, due to non-flow correlations, the flow measurements by the Q-cumulant method using all charged particles as reference introduce a trivial linear term to $Delta v_{n}(A_{rm ch})$. The trivial slope contribution to the triangle flow difference $Delta v_{3}(A_{rm ch})$ can be negative if the non-flow is dominated by back-to-back pairs. This can explain the observed negative $Delta v_{3}(A_{rm ch})$ slope in the preliminary STAR data. We further find that the non-flow correlations give rise to additional backgrounds to the slope of $Delta v_{2}(A_{rm ch})$ from the competition among different pion sources and from the larger multiplicity dilution to $pi^{+}$ ($pi^{-}$) at positive (negative) $A_{rm ch}$.
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.
Recent results on identified hadrons from the PHENIX experiment in Au+Au collisions at mid-rapidity at $sqrt{s_{NN}}$ = 200 GeV are presented. The centrality dependence of transverse momentum distributions and particle ratios for identified charged hadrons are studied. The transverse flow velocity and freeze-out temperature are extracted from $p_{T}$ spectra within the framework of a hydrodynamic collective flow model. Two-particle HBT correlations for charged pions are measured in different centrality selections for a broad range of transverse momentum of the pair. Results on elliptic flow measurements with respect to the reaction plane for identified particles are also presented.
We have implemented the Tsallis statistics in a Blast-Wave model and applied it to mid-rapidity transverse-momentum spectra of identified particles measured at RHIC. This new Tsallis Blast-Wave function fits the RHIC data very well for $p_T<$3 GeV/$c$. We observed that the collective flow velocity starts from zero in p+p and peripheral Au+Au collisions growing to 0.470 $pm$ 0.009($c$) in central Au+Au collisions. The $(q-1)$ parameter, which characterizes the degree of non-equilibrium in a system, changes from $0.100pm0.003$ in p+p to $0.015pm0.005$ in central Au+Au collisions, indicating an evolution from a highly non-equilibrated system in p+p collisions toward an almost thermalized system in central Au+Au collisions. The temperature and collective velocity are well described by a quadratic dependence on $(q-1)$. Two sets of parameters in our Tsallis Blast-Wave model are required to describe the meson and baryon groups separately in p+p collisions while one set of parameters appears to fit all spectra in central Au+Au collisions.
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