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Register a new userMeasurement of elliptic flow of light nuclei at $sqrt{s_{NN}}$ = 200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV at RHIC
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We present measurements of 2$^{nd}$ order azimuthal anisotropy ($v_{2}$) at mid-rapidity $(|y|<1.0)$ for light nuclei d, t, $^{3}$He (for $sqrt{s_{NN}}$ = 200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV) and anti-nuclei $bar{rm d}$ ($sqrt{s_{NN}}$ = 200, 62.4, 39, 27, and 19.6 GeV) and $^{3}bar{rm He}$ ($sqrt{s_{NN}}$ = 200 GeV) in the STAR (Solenoidal Tracker at RHIC) experiment. The $v_{2}$ for these light nuclei produced in heavy-ion collisions is compared with those for p and $bar{rm p}$. We observe mass ordering in nuclei $v_{2}(p_{T})$ at low transverse momenta ($p_{T}<2.0$ GeV/$c$). We also find a centrality dependence of $v_{2}$ for d and $bar{rm d}$. The magnitude of $v_{2}$ for t and $^{3}$He agree within statistical errors. Light-nuclei $v_{2}$ are compared with predictions from a blast wave model. Atomic mass number ($A$) scaling of light-nuclei $v_{2}(p_{T})$ seems to hold for $p_{T}/A < 1.5$ GeV/$c$. Results on light-nuclei $v_{2}$ from a transport-plus-coalescence model are consistent with the experimental measurements.
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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 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.
Measurements of the elliptic flow, $v_{2}$, of identified hadrons ($pi^{pm}$, $K^{pm}$, $K_{s}^{0}$, $p$, $bar{p}$, $phi$, $Lambda$, $bar{Lambda}$, $Xi^{-}$, $bar{Xi}^{+}$, $Omega^{-}$, $bar{Omega}^{+}$) in Au+Au collisions at $sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV are presented. The measurements were done at mid-rapidity using the Time Projection Chamber and the Time-of-Flight detectors of the STAR experiment during the Beam Energy Scan program at RHIC. A significant difference in the $v_{2}$ values for particles and the corresponding anti-particles was observed at all transverse momenta for the first time. The difference increases with decreasing center-of-mass energy, $sqrt{s_{NN}}$ (or increasing baryon chemical potential, $mu_{B}$) and is larger for the baryons as compared to the mesons. This implies that particles and anti-particles are no longer consistent with the universal number-of-constituent quark (NCQ) scaling of $v_{2}$ that was observed at $sqrt{s_{NN}}=$ 200 GeV. However, for the group of particles NCQ scaling at $(m_{T}-m_{0})/n_{q}>$ 0.4 GeV/$c^{2}$ is not violated within $pm$10%. The $v_{2}$ values for $phi$ mesons at 7.7 and 11.5 GeV are approximately two standard deviations from the trend defined by the other hadrons at the highest measured $p_{T}$ values.
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 present measurements of the elliptic flow, $v_2$, as a function of transverse momentum ($p_T$), pseudorapidity ($eta$), and centrality in $d$$+$Au collisions at $sqrt{s_{_{NN}}}=$200, 62.4, 39, and 19.6 GeV. The beam-energy scan of $d$$+$Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero $v_2$ signal at all four collision energies, which, at midrapidity and low $p_T$, is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the {sc ampt} Monte Carlo generator) show good agreement with the data at midrapidity to forward ($d$-going) rapidities and low $p_T$. At backward (Au-going) rapidities and $p_T>1.5$ GeV/$c$, the data diverges from {sc ampt} calculations of $v_2$ relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity ($dN_{rm ch}/deta$) as a function of $eta$ in central $d$$+$Au collisions at the same energies. We find that in $d$$+$Au collisions at $sqrt{s_{_{NN}}}=$200 GeV the $v_2$ scales with $dN_{rm ch}/deta$ over all $eta$ in the PHENIX acceptance. At $sqrt{s_{_{NN}}}=$62.4, and 39 GeV, $v_2$ scales with $dN_{rm ch}/deta$ at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the $dN_{rm ch}/deta$ scaling may be a further indication of nonflow effects dominating at backward rapidity.
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