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Register a new userPercolation of Color Sources and the determination of the Equation of State of the Quark-Gluon Plasma (QGP) produced in central Au-Au collisions at sqrt S_{NN}= 200 GeV
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The Color String Percolation Model (CSPM) is used to determine the equation of state (EOS) of the QGP produced in central Au-Au collisions at $sqrt{s_{NN}}$ = 200 A GeV using STAR data at RHIC. When the initial density of interacting colored strings exceeds the 2D percolation threshold a cluster is formed, which defines the onset of color deconfinement. These interactions also produce fluctuations in the string tension which transforms the Schwinger particle (gluon) production mechanism into a maximum entropy thermal distribution. The single string tension is determined by identifying the known value of the universal hadron limiting temperature $T_{c}$ = 167.7 $pm$ 2.6 MeV with the CSPM percolation temperature at the critical threshold $xi_{c}$ =1.2. At mid-rapidity the initial Bjorken energy density and the initial temperature determine the number of degrees of freedom consistent with the formation of a $sim$ 2+1 flavor QGP. An analytic expression for the equation of state, the sound velocity $C_{s}^{2}(xi)$ is obtained in CSPM. The CSPM $C_{s}^{2}(xi)$ and the bulk thermodynamic values $varepsilon /T^{4}$ and $s /T^{3}$ are in excellent agreement in the phase transition region with recent lattice QCD simulations (LQCD) by the HotQCD Collaboration.
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We report preliminary results of hypertriton observation in heavy-ion collisions at RHIC. We have identified 157 +- 30 candidates in the current sample containing ~10^8 Au+Au events at sqrt{s_{NN}} = 200 GeV. The production rate of hypertriton is close to that of helium 3. No extra penalty factor is observed for hypertriton, in contrast to results observed at the AGS.
We present measurements of $e^+e^-$ production at midrapidity in Au$+$Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of mass ($m_{ee} <$ 5 GeV/$c^2$) and pair transverse momentum ($p_T$ $<$ 5 GeV/$c$), for minimum bias and for five centrality classes. The ee yield is compared to the expectations from known sources. In the low-mass region ($m_{ee}=0.30$--0.76 GeV/$c^2$) there is an enhancement that increases with centrality and is distributed over the entire pair pt range measured. It is significantly smaller than previously reported by the PHENIX experiment and amounts to $2.3pm0.4({rm stat})pm0.4({rm syst})pm0.2^{rm model}$ or to $1.7pm0.3({rm stat})pm0.3({rm syst})pm0.2^{rm model}$ for minimum bias collisions when the open-heavy-flavor contribution is calculated with {sc pythia} or {sc mc@nlo}, respectively. The inclusive mass and $p_T$ distributions as well as the centrality dependence are well reproduced by model calculations where the enhancement mainly originates from the melting of the $rho$ meson resonance as the system approaches chiral symmetry restoration. In the intermediate-mass region ($m_{ee}$ = 1.2--2.8 GeV/$c^2$), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.
The Multi-Phase Transport model, AMPT, and the Anomalous Viscous Fluid Dynamics model, AVFD, are used to assess a possible chiral-magnetically-driven charge separation ($Delta S$) recently measured with the ${R_{Psi_2}(Delta S)}$ correlator in Au+Au collisions at $sqrt{s_{mathrm{NN}}}=200$ GeV. The Comparison of the experimental and simulated ${R_{Psi_2}(Delta S)}$ distributions indicates that background-driven charge separation is insufficient to account for the measurements. The AVFD model calculations, which explicitly account for CME-driven anomalous transport in the presence of background, indicate a CME signal quantified by the $P$-odd Fourier dipole coefficient ${a_1}approx 0.5%$ in mid-central collisions. A similar evaluation for the $Deltagamma$ correlator suggests that only a small fraction of this signal ($f_{rm CME}=Deltagamma_{rm CME}/Deltagamma approx 25%$) is measurable with this correlator in the same collisions. The related prediction for signal detection in isobaric collisions of Ru+Ru and Zr+Zr are also presented.
We have measured the distributions of protons and deuterons produced in high energy heavy ion Au+Au collisions at RHIC over a very wide range of transverse and longitudinal momentum. Near mid-rapidity we have also measured the distribution of anti-protons and anti-deuterons. We present our results in the context of coalescence models. In particular we extract the volume of homogeneity and the average phase-space density for protons and anti-protons. Near central rapidity the coalescence parameter $B_2(p_T)$ and the space averaged phase-space density $<f> (p_T)$ are very similar for both protons and anti-protons. For protons we see little variation of either $B_2(p_T)$ or the space averaged phase-space density as the rapidity increases from 0 to 3. However both these quantities depend strongly on $p_T$ at all rapidities. These results are in contrast to lower energy data where the proton and anti-proton phase-space densities are different at $y$=0 and both $B_2$ and $f$ depend strongly on rapidity.
Global polarization of $Lambda$ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV, with no significant difference between $Lambda$ and $bar{Lambda}$. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at $sqrt{s_{_{NN}}}$ = 7.7 -- 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperons transverse momentum and pseudorapidity $eta^H$ within $|eta^H|<1$. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the $2sigma$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.
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