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Register a new userBulk Properties of the System Formed in Au+Au Collisions at $sqrt{s_{mathrm{NN}}}$ = 14.5 GeV
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We report systematic measurements of bulk properties of the system created in Au+Au collisions at $sqrt{s_{mathrm{NN}}}$ = 14.5 GeV recorded by the STAR detector at the Relativistic Heavy Ion Collider (RHIC).The transverse momentum spectra of $pi^{pm}$, $K^{pm}$ and $p(bar{p})$ are studied at mid-rapidity ($|y| < 0.1$) for nine centrality intervals. The centrality, transverse momentum ($p_T$),and pseudorapidity ($eta$) dependence of inclusive charged particle elliptic flow ($v_2$), and rapidity-odd charged particles directed flow ($v_{1}$) results near mid-rapidity are also presented. These measurements are compared with the published results from Au+Au collisions at other energies, and from Pb+Pb collisions at $sqrt{s_{mathrm{NN}}}$ = 2.76 TeV. The results at $sqrt{s_{mathrm{NN}}}$ = 14.5 GeV show similar behavior as established at other energies and fit well in the energy dependence trend. These results are important as the 14.5 GeV energy fills the gap in $mu_B$, which is of the order of 100 MeV,between $sqrt{s_{mathrm{NN}}}$ =11.5 and 19.6 GeV. Comparisons of the data with UrQMD and AMPT models show poor agreement in general.
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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 report measurements of the nuclear modification factor, $R_{ mathrm{CP}}$, for charged hadrons as well as identified $pi^{+(-)}$, $K^{+(-)}$, and $p(overline{p})$ for Au+Au collision energies of $sqrt{s_{_{ mathrm{NN}}}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, and 62.4 GeV. We observe a clear high-$p_{mathrm{T}}$ net suppression in central collisions at 62.4 GeV for charged hadrons which evolves smoothly to a large net enhancement at lower energies. This trend is driven by the evolution of the pion spectra, but is also very similar for the kaon spectra. While the magnitude of the proton $R_{ mathrm{CP}}$ at high $p_{mathrm{T}}$ does depend on collision energy, neither the proton nor the anti-proton $R_{ mathrm{CP}}$ at high $p_{mathrm{T}}$ exhibit net suppression at any energy. A study of how the binary collision scaled high-$p_{mathrm{T}}$ yield evolves with centrality reveals a non-monotonic shape that is consistent with the idea that jet-quenching is increasing faster than the combined phenomena that lead to enhancement.
We report on the measurements of directed flow $v_1$ and elliptic flow $v_2$ for hadrons ($pi^{pm}$, $K^{pm}$, $K_{S}^0$, $p$, $phi$, $Lambda$ and $Xi^{-}$) from Au+Au collisions at $sqrt{s_{NN}}$ = 3 GeV and $v_{2}$ for ($pi^{pm}$, $K^{pm}$, $p$ and $overline{p}$) at 27 and 54.4 GeV with the STAR experiment. While at the two higher energy midcentral collisions the number-of-constituent-quark (NCQ) scaling holds, at 3 GeV the midrapidity $v_{2}$ is negative for all hadrons and the NCQ scaling is absent. In addition, the midrapidity $v_1$ slopes for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. The features of negative $v_2$ and positive $v_1$ slope at 3 GeV can be reproduced with a baryonic mean-field in transport model calculations. These results imply that in 3 GeV Au+Au collisions, the medium is characterized by baryonic interactions.
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.
Event-by-event fluctuations of the ratio of inclusive charged to photon multiplicities at forward rapidity in Au+Au collision at $sqrt{s_{NN}}$=200 GeV have been studied. Dominant contribution to such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidences of dynamical fluctuations of different physical origins. Observables constructed out of moments of multiplicities are used as measures of fluctuations. Mixed events and model calculations are used as baselines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A non-zero statistically significant signal of dynamical fluctuations is observed in excess to the model prediction when charged particles and photons are measured in the same acceptance. We find that, unlike dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated by correlation due to particle decay. Results are compared to the expectations based on the generic production mechanism of pions due to isospin symmetry, for which no significant (<1%) deviation is observed.
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