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Register a new userKs, Lambda and Xi production at intermediate to high pT from Au+Au collisions at sqrt{s_{NN}} = 39, 11.5 and 7.7 GeV
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Xianglei Zhu
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We report on the pT dependence of nuclear modification factors ($R_{CP}$) for Ks, Lambda, Xi and the Anti-Lambda/Ks ratios at mid-rapidity from Au+Au collisions at sqrt{s_{NN}} = 39, 11.5 and 7.7 GeV. At sqrt{s_{NN}} = 39 GeV, the $R_{CP}$ data shows a baryon/meson separation at intermediate pT and a suppression for Ks for pT up to 4.5 GeV/$c$; the Anti-Lambda/Ks shows baryon enhancement in the most central collisions. However, at sqrt{s_{NN}} = 11.5 and 7.7 GeV, $R_{CP}$ shows much less baryon/meson separation and Anti-Lambda/Ks shows almost no baryon enhancement. These observations indicate that the matter created in Au+Au collisions at sqrt{s_{NN}} = 11.5 or 7.7 GeV might be distinct from that created at sqrt{s_{NN}} = 39 GeV.
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We present STAR measurements of strange hadron ($mathrm{K}^{0}_{mathrm S}$, $Lambda$, $overline{Lambda}$, $Xi^-$, $overline{Xi}^+$, $Omega^-$, $overline{Omega}^+$, and $phi$) production at mid-rapidity ($|y| < 0.5$) in Au+Au collisions at $sqrt{s_{_{mathrm{NN}}}}$ = 7.7 - 39 GeV from the Beam Energy Scan Program at the Relativistic Heavy Ion Collider (RHIC). Transverse momentum spectra, averaged transverse mass, and the overall integrated yields of these strange hadrons are presented versus the centrality and collision energy. Antibaryon-to-baryon ratios ($overline{Lambda}$/$Lambda$, $overline{Xi}^+$/$Xi^-$, $overline{Omega}^+$/$Omega^-$) are presented as well, and used to test a thermal statistical model and to extract the temperature normalized strangeness and baryon chemical potentials at hadronic freeze-out ($mu_{B}/T_{rm ch}$ and $mu_{S}/T_{rm ch}$) in central collisions. Strange baryon-to-pion ratios are compared to various model predictions in central collisions for all energies. The nuclear modification factors ($R_{textrm{CP}}$) and antibaryon-to-meson ratios as a function of transverse momentum are presented for all collision energies. The $mathrm{K}^{0}_{mathrm S}$ $R_{textrm{CP}}$ shows no suppression for $p_{rm T}$ up to 3.5 $mathrm{GeV} / c$ at energies of 7.7 and 11.5 GeV. The $overline{Lambda}$/$mathrm{K}^{0}_{mathrm S}$ ratio also shows baryon-to-meson enhancement at intermediate $p_{rm T}$ ($approx$2.5 $mathrm{GeV} / c$) in central collisions at energies above 19.6 GeV. Both observations suggest that there is likely a change of the underlying strange quark dynamics at collision energies below 19.6 GeV.
A systematic study is presented for centrality, transverse momentum ($p_T$) and pseudorapidity ($eta$) dependence of the inclusive charged hadron elliptic flow ($v_2$) at midrapidity($|eta| < 1.0$) in Au+Au collisions at $sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27 and 39 GeV. The results obtained with different methods, including correlations with the event plane reconstructed in a region separated by a large pseudorapidity gap and 4-particle cumulants ($v_2{4}$), are presented in order to investigate non-flow correlations and $v_2$ fluctuations. We observe that the difference between $v_2{2}$ and $v_2{4}$ is smaller at the lower collision energies. Values of $v_2$, scaled by the initial coordinate space eccentricity, $v_{2}/varepsilon$, as a function of $p_T$ are larger in more central collisions, suggesting stronger collective flow develops in more central collisions, similar to the results at higher collision energies. These results are compared to measurements at higher energies at the Relativistic Heavy Ion Collider ($sqrt{s_{NN}}$ = 62.4 and 200 GeV) and at the Large Hadron Collider (Pb + Pb collisions at $sqrt{s_{NN}}$ = 2.76 TeV). The $v_2(p_T)$ values for fixed $p_T$ rise with increasing collision energy within the $p_T$ range studied ($< 2 {rm GeV}/c$). A comparison to viscous hydrodynamic simulations is made to potentially help understand the energy dependence of $v_{2}(p_{T})$. We also compare the $v_2$ results to UrQMD and AMPT transport model calculations, and physics implications on the dominance of partonic versus hadronic phases in the system created at Beam Energy Scan (BES) energies are discussed.
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 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.
Light nuclei production in relativistic $^{197}$Au + $^{197}$Au collisions from 7.7 to 80 GeV is investigated within the Ultra-relativistic-Quantum-Molecular-Dynamics model (UrQMD) with a naive coalescence approach. The results of the production of light nuclei at midrapidity can essentially match up the experimental data and a slight enhancement of combined ratio of ${N_{p}N_{t}}/{N_{d}^{2}}$ where $N_p, N_d$ and $N_t$ represent respectively the yields of proton, deuteron and triton, which is sensitive to the neutron density fluctuations, occurs around 20 GeV. However, this enhanced ${N_{p}N_{t}}/{N_{d}^{2}}$ ratio should not be over-understood considering that the present UrQMD model is a cascade version without equation of state (EoS), i.e. there is an absence of critical end point mechanism. Furthermore, within different rapidity regions, the kinetic temperatures of different light nuclei are extracted by the Blast-wave model analysis and ratios among different light nuclei are also discussed.
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