Showing 10 of 53 results
The deconfined quark-gluon plasma (QGP) created in relativistic heavy-ion collisions enables the exploration of the fundamental properties of matter under extreme conditions. Non-central collisions can produce strong magnetic fields on the order of $10^{18}$ Gauss, which offers a probe into the electrical conductivity of the QGP. In particular, quarks and anti-quarks carry opposite charges and receive contrary electromagnetic forces that alter their momenta. This phenomenon can be manifested in the collective motion of final-state particles, specifically in the rapidity-odd directed flow, denoted as $v_1(\mathsf{y})$. Here we present the charge-dependent measurements of $dv_1/d\mathsf{y}$ near midrapidities for $\pi^{\pm}$, $K^{\pm}$, and $p(\bar{p})$ in Au+Au and isobar ($_{44}^{96}$Ru+$_{44}^{96}$Ru and $_{40}^{96}$Zr+$_{40}^{96}$Zr) collisions at $\sqrt{s_{\rm NN}}=$ 200 GeV, and in Au+Au collisions at 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the $v_1$ signal on collision system, particle species, and collision centrality can be qualitatively and semi-quantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the $u$ and $d$ quarks transported from initial-state nuclei, those in peripheral events reveal the impacts of the electromagnetic field on the QGP. Our data put valuable constraints on the electrical conductivity of the QGP in theoretical calculations.
Global polarizations ($P$) of $\Lambda$ ($\bar{\Lambda}$) hyperons have been observed in non-central heavy-ion collisions. The strong magnetic field primarily created by the spectator protons in such collisions would split the $\Lambda$ and $\bar{\Lambda}$ global polarizations ($\Delta P = P_{\Lambda} - P_{\bar{\Lambda}} < 0$). Additionally, quantum chromodynamics (QCD) predicts topological charge fluctuations in vacuum, resulting in a chirality imbalance or parity violation in a local domain. This would give rise to an imbalance ($\Delta n = \frac{N_{\text{L}} - N_{\text{R}}}{\langle N_{\text{L}} + N_{\text{R}} \rangle} \neq 0$) between left- and right-handed $\Lambda$ ($\bar{\Lambda}$) as well as a charge separation along the magnetic field, referred to as the chiral magnetic effect (CME). This charge separation can be characterized by the parity-even azimuthal correlator ($\Delta\gamma$) and parity-odd azimuthal harmonic observable ($\Delta a_{1}$). Measurements of $\Delta P$, $\Delta\gamma$, and $\Delta a_{1}$ have not led to definitive conclusions concerning the CME or the magnetic field, and $\Delta n$ has not been measured previously. Correlations among these observables may reveal new insights. This paper reports measurements of correlation between $\Delta n$ and $\Delta a_{1}$, which is sensitive to chirality fluctuations, and correlation between $\Delta P$ and $\Delta\gamma$ sensitive to magnetic field in Au+Au collisions at 27 GeV. For both measurements, no correlations have been observed beyond statistical fluctuations.
In relativistic heavy-ion collisions, a global spin polarization, $P_\mathrm{H}$, of $\Lambda$ and $\bar{\Lambda}$ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing $P_\mathrm{H}$ with decreasing $\sqrt{s_{NN}}$. A splitting between $\Lambda$ and $\bar{\Lambda}$ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of $\sqrt{s_{NN}}=19.6$ and $27$ GeV in the RHIC Beam Energy Scan Phase II using the STAR detector, with an upper limit of $P_{\bar{\Lambda}}-P_{\Lambda}<0.24$% and $P_{\bar{\Lambda}}-P_{\Lambda}<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the na\"ive extraction of the late-stage magnetic field of $B<9.4\cdot10^{12}$ T and $B<1.4\cdot10^{13}$ T at $\sqrt{s_{NN}}=19.6$ and $27$ GeV, respectively, although more thorough derivations are needed. Differential measurements of $P_\mathrm{H}$ were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of $|y|<1$ and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.
We present results on strange and multi-strange particle production in Au+Au collisions at $\sqrt{s_{NN}}=62.4$ GeV as measured with the STAR detector at RHIC. Mid-rapidity transverse momentum spectra and integrated yields of $K^{0}_{S}$, $\Lambda$, $\Xi$, $\Omega$ and their anti-particles are presented for different centrality classes. The particle yields and ratios follow a smooth energy dependence. Chemical freeze-out parameters, temperature, baryon chemical potential and strangeness saturation factor obtained from the particle yields are presented. Intermediate transverse momentum ($p_T$) phenomena are discussed based on the ratio of the measured baryon-to-meson spectra and nuclear modification factor. The centrality dependence of various measurements presented show a similar behavior as seen in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
Correction factors (acceptance × efficiency) for the most central events ( 0−5% for KS0, Λ and Ξ; 0−20% for Ω) at mid-rapidity (|y| < 1) as a function of pT for the different particle species as obtained via embedding. The branching ratio of the measured decay channel is not factored into this plot.
Efficiency corrected pT spectra for the different centrality bins and for the various particles. Note that 7 centrality bins have been used for the KS0 and the Λ while only 6 and 3 have been used for the Ξ and Ω, respectively. Errors are statistical only. The Λ spectra are corrected for the feed-down of the Ξ decay.
Efficiency corrected pT spectra for the different centrality bins and for the various particles. Note that 7 centrality bins have been used for the KS0 and the Λ while only 6 and 3 have been used for the Ξ and Ω, respectively. Errors are statistical only. The Λ spectra are corrected for the feed-down of the Ξ decay.
Efficiency corrected pT spectra for the different centrality bins and for the various particles. Note that 7 centrality bins have been used for the KS0 and the Λ while only 6 and 3 have been used for the Ξ and Ω, respectively. Errors are statistical only. The Λ spectra are corrected for the feed-down of the Ξ decay.
Efficiency corrected pT spectra for the different centrality bins and for the various particles. Note that 7 centrality bins have been used for the KS0 and the Λ while only 6 and 3 have been used for the Ξ and Ω, respectively. Errors are statistical only. The Λ spectra are corrected for the feed-down of the Ξ decay.
Efficiency corrected pT spectra for the different centrality bins and for the various particles. Note that 7 centrality bins have been used for the KS0 and the Λ while only 6 and 3 have been used for the Ξ and Ω, respectively. Errors are statistical only. The Λ spectra are corrected for the feed-down of the Ξ decay.
Efficiency corrected pT spectra for the different centrality bins and for the various particles. Note that 7 centrality bins have been used for the KS0 and the Λ while only 6 and 3 have been used for the Ξ and Ω, respectively. Errors are statistical only. The Λ spectra are corrected for the feed-down of the Ξ decay.
Efficiency corrected pT spectra for the different centrality bins and for the various particles. Note that 7 centrality bins have been used for the KS0 and the Λ while only 6 and 3 have been used for the Ξ and Ω, respectively. Errors are statistical only. The Λ spectra are corrected for the feed-down of the Ξ decay.
Extrapolated average transverse momenta ⟨pT ⟩ as a function of dNch/dy for different particle species in Au+Au collisions at 62.4 GeV. Statistical uncertainties are represented by the error bars at the points while the systematic uncertainties are represented by the gray bars. The π, charged K and p data were extracted from Ref. [14].
KS0 dN/dpT spectra compared to the charged Kaon spectra for the event centrality of 0-5% and 30-40%. The charged Kaons data points are for rapidity range of |y| < 0.1 and were extracted from Ref. [14].
KS0 dN/dpT spectra compared to the charged Kaon spectra for the event centrality of 0-5% and 30-40%. The charged Kaons data points are for rapidity range of |y| < 0.1 and were extracted from Ref. [14].
Strange particle production yields at mid-rapidity in central Au+Au and Pb+Pb collisions versus the center of mass energy √sNN. The top panel shows results for K0S and Λ. The AGS values are from E896 [1] (centrality 0 − 5 %). The SPS values are from NA49 [20] (centrality 0 − 7 %) and the RHIC values are from STAR [4, 15] (centrality 0 − 5 %). For the multi-strange baryons Ξ and Ω (bottom panel), the SPS results are from NA57 [2] (centrality 0 − 11 %) and the RHIC values are from STAR [15, 21] (centrality 0 − 20 %).
Strange particle production yields at mid-rapidity in central Au+Au and Pb+Pb collisions versus the center of mass energy √sNN. The top panel shows results for K0S and Λ. The AGS values are from E896 [1] (centrality 0 − 5 %). The SPS values are from NA49 [20] (centrality 0 − 7 %) and the RHIC values are from STAR [4, 15] (centrality 0 − 5 %). For the multi-strange baryons Ξ and Ω (bottom panel), the SPS results are from NA57 [2] (centrality 0 − 11 %) and the RHIC values are from STAR [15, 21] (centrality 0 − 20 %).
Anti-baryon to baryon yield ratios for strange baryons versus the center of mass energy √sNN. Λ/Λ is shown in the top panel while the multi-strange baryons are on the bottom panel. The data from AGS are not corrected for the weak decay feed-down from the multistrange baryons while the data from SPS and RHIC are corrected. The lines are the results of a thermal model calculation (see text section IV A). The AGS values are from E896 [1] (centrality 0 − 5 %). The SPS values are from NA49 [20] (centrality 0 − 7 %) and the RHIC values are from STAR [4, 15] (centrality 0 − 5 %). For the multi- strange baryons Ξ and Ω (bottom panel), the SPS results are from NA57 [2] (centrality 0 − 11 %) and the RHIC values are from STAR [15, 21] (centrality 0 − 20 %).
Antibaryon-to-baryon yield ratios for strange particles and protons as a function of dNch/dy at √sNN=62.4 and 200 GeV. The p data were extracted from Ref. [14]. The √sNN=200 GeV strange hadron data were extracted from Ref. [15].
Particle-yield ratios as obtained by measurements (black dots) for the most central (0–5%) Au+Au collisions at 62.4 GeV and statistical model predictions (lines). The ratios indicated by the dashed lines (blue) were obtained by using only π, K, and protons, whereas the ratios indicated by the full lines (green) were obtained by also using the hyperons in the fit.
Chemical freeze-out temperature Tch (a) and strangeness saturation factor γs (b) as a function of the mean number of participants.
Chemical freeze-out temperature Tch (a) and strangeness saturation factor γs (b) as a function of the mean number of participants.
Temperature and baryon chemical potential obtained from thermal model fits as a function of √sNN (see Ref. [22]). The dashed lines correspond to the parametrizations given in Ref. [22]. The solid stars show the result for √sNN=62.4 and 200 GeV.
Temperature and baryon chemical potential obtained from thermal model fits as a function of √sNN (see Ref. [22]). The dashed lines correspond to the parametrizations given in Ref. [22]. The solid stars show the result for √sNN=62.4 and 200 GeV.
Ratio of baryon (solid symbols) and antibaryon (open symbols) to π+ as a function of dNch/dy for √sNN=62.4 GeV (left) and √sNN=200 GeV (right). The π and p data were extracted from Ref. [14].
Ratio of baryon (solid symbols) and antibaryon (open symbols) to π+ as a function of dNch/dy for √sNN=62.4 GeV (left) and √sNN=200 GeV (right). The π and p data were extracted from Ref. [14].
Ratio of baryon (solid symbols) and antibaryon (open symbols) to π+ as a function of dNch/dy for √sNN=62.4 GeV (left) and √sNN=200 GeV (right). The π and p data were extracted from Ref. [14].
Ratio of baryon (solid symbols) and antibaryon (open symbols) to π+ as a function of dNch/dy for √sNN=62.4 GeV (left) and √sNN=200 GeV (right). The π and p data were extracted from Ref. [14].
Ratio of baryon (solid symbols) and antibaryon (open symbols) to π− as a function of √sNN. The lines are the results of the thermal model calculation (see text Sec. 4a). The SPS values are from NA49 [20] (centrality 0–7%) and the RHIC values are from STAR [4, 15] (centrality 0–5%). For the multistrange baryons Ξ and Ω (bottom), the SPS results are from NA57 [2] (centrality 0–11%) and the RHIC values are from STAR [15, 21] (centrality 0–20%).
Nuclear modification factor RCP, calculated as the ratio between 0–10% central spectra and 40–80% peripheral spectra, for π, K0S, Λ, and Ξ particles in Au+Au collisions at 62.4 GeV. The π RCP values were extracted from Ref. [10]. The gray band on the right side of the plot shows the uncertainties on the estimation of the number of binary collisions and the gray band on the lower left side indicates the uncertainties on the number of participants.
Nuclear modification factor RCP, calculated as the ratio between 0–5% central spectra and 40–60% peripheral spectra, for Λ and Ξ particles measured in Au + Au collisions at 62.4 GeV. The gray band corresponds to the equivalent RCP curve for the Λ particles measured in Au+Au collisions at 200 GeV [15].
Λ/K0S ratio as a function of transverse momentum for different centrality classes. 0–5% (solid circles), 40–60% (open squares), and 60–80% (solid triangles) in Au+Au collisions at 62.4 GeV.
Maximum value of the Λ/K0S ratio from Au+Au collisions at 62.4 GeV (solid circles) and 200 GeV (open circles) [11] as a function of ⟨Npart⟩ for different centrality classes. The lowest ⟨Npart⟩ point corresponds to p+p collisions at 200 GeV [44]. The maximum of the Λ––/K0S from Au+Au collisions at 62.4 GeV is shown as solid triangles.
Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange, a full understanding of the strong interaction remains a major challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that in relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely $\phi$ and $K^{*0}$, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for $\phi$ is unexpectedly large, while that for $K^{*0}$ is consistent with zero. The observed spin-alignment pattern and magnitude for the $\phi$ cannot be explained by conventional mechanisms, while a model with a connection to strong force fields, i.e. an effective proxy description within the Standard Model and Quantum Chromodynamics, accommodates the current data. This connection, if fully established, will open a potential new avenue for studying the behaviour of strong force fields.
Global spin alignment of $\phi$ and $K^{*0}$ vector mesons in heavy-ion collisions. The measured matrix element $\rho_{00}$ as a function of beam energy for the $\phi$ and $K^{*0}$ vector mesons within the indicated windows of centrality, transverse momentum ($p_T$) and rapidity ($y$). The open symbols indicate ALICE results for Pb+Pb collisions at 2.76 TeV at $p_{T}$ values of 2.0 and 1.4 GeV/c for the $\phi$ and $K^{*0}$ mesons, respectively, corresponding to the $p_{T}$ bin nearest to the mean $p_{T}$ for the 1.0 – 5.0 GeV/$c$ range assumed for each meson in the present analysis. The red solid curve is a fit to data in the range of $\sqrt{s_{NN}} = 19.6$ to 200 GeV, based on a theoretical calculation with a $\phi$-meson field. Parameter sensitivity of $\rho_{00}$ to the $\phi$-meson field is shown in Ref.5. The red dashed line is an extension of the solid curve with the fitted parameter $G_s^{(y)}$. The black dashed line represents $\rho_{00}=1/3.$
Global spin alignment of $\phi$ and $K^{*0}$ vector mesons in heavy-ion collisions. The measured matrix element $\rho_{00}$ as a function of beam energy for the $\phi$ and $K^{*0}$ vector mesons within the indicated windows of centrality, transverse momentum ($p_T$) and rapidity ($y$). The open symbols indicate ALICE results for Pb+Pb collisions at 2.76 TeV at $p_{T}$ values of 2.0 and 1.4 GeV/c for the $\phi$ and $K^{*0}$ mesons, respectively, corresponding to the $p_{T}$ bin nearest to the mean $p_{T}$ for the 1.0 – 5.0 GeV/$c$ range assumed for each meson in the present analysis. The red solid curve is a fit to data in the range of $\sqrt{s_{NN}} = 19.6$ to 200 GeV, based on a theoretical calculation with a $\phi$-meson field. Parameter sensitivity of $\rho_{00}$ to the $\phi$-meson field is shown in Ref.5. The red dashed line is an extension of the solid curve with the fitted parameter $G_s^{(y)}$. The black dashed line represents $\rho_{00}=1/3.$
Example of combinatorial background subtracted invariant mass distributions and the extracted yields as a function of $\cos \theta^*$ for $\phi$ and $K^{*0}$ mesons. \textbf{a)} example of $\phi \rightarrow K^+ + K^-$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{b)} example of $K^{*0} (\overline{K^{*0}}) \rightarrow K^{-} \pi^{+} (K^{+} \pi^{-})$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{c)} extracted yields of $\phi$ as a function of $\cos \theta^*$; \textbf{d)} extracted yields of $K^{*0}$ as a function of $\cos \theta^*$.
Example of combinatorial background subtracted invariant mass distributions and the extracted yields as a function of $\cos \theta^*$ for $\phi$ and $K^{*0}$ mesons. \textbf{a)} example of $\phi \rightarrow K^+ + K^-$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{b)} example of $K^{*0} (\overline{K^{*0}}) \rightarrow K^{-} \pi^{+} (K^{+} \pi^{-})$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{c)} extracted yields of $\phi$ as a function of $\cos \theta^*$; \textbf{d)} extracted yields of $K^{*0}$ as a function of $\cos \theta^*$.
Example of combinatorial background subtracted invariant mass distributions and the extracted yields as a function of $\cos \theta^*$ for $\phi$ and $K^{*0}$ mesons. \textbf{a)} example of $\phi \rightarrow K^+ + K^-$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{b)} example of $K^{*0} (\overline{K^{*0}}) \rightarrow K^{-} \pi^{+} (K^{+} \pi^{-})$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{c)} extracted yields of $\phi$ as a function of $\cos \theta^*$; \textbf{d)} extracted yields of $K^{*0}$ as a function of $\cos \theta^*$.
Example of combinatorial background subtracted invariant mass distributions and the extracted yields as a function of $\cos \theta^*$ for $\phi$ and $K^{*0}$ mesons. \textbf{a)} example of $\phi \rightarrow K^+ + K^-$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{b)} example of $K^{*0} (\overline{K^{*0}}) \rightarrow K^{-} \pi^{+} (K^{+} \pi^{-})$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{c)} extracted yields of $\phi$ as a function of $\cos \theta^*$; \textbf{d)} extracted yields of $K^{*0}$ as a function of $\cos \theta^*$.
Efficiency corrected $\phi$-meson yields as a function of cos$\theta$* and corresponding fits with Eq.1 in the method section.
Efficiency and acceptance corrected $K^{*0}$-meson yields as a function of cos$\theta$* and corresponding fits with Eq.4 in the method section.
$\phi$-meson $\rho_{00}$ obtained from 1st- and 2nd-order event planes. The red stars (gray squares) show the $\phi$-meson $\rho_{00}$ as a function of beam energy, obtained with the 2nd-order (1st-order) EP.
$\phi$-meson $\rho_{00}$ with respect to different quantization axes. $\phi$-meson $\rho_{00}$ as a function of beam energy, for the out-of-plane direction (stars) and the in-plane direction (diamonds). Curves are fits based on theoretical calculations with a $\phi$-meson field. The corresponding $G_s^{(y)}$ values obtained from the fits are shown in the legend.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $\phi$ for different collision energies. The gray squares and red stars are results obtained with the 1st- and 2nd-order EP, respectively.
$\rho_{00}$ as a function of transverse momentum for $K^{*0}$ for different collision energies.
$\rho_{00}$ as a function of transverse momentum for $K^{*0}$ for different collision energies.
$\rho_{00}$ as a function of transverse momentum for $K^{*0}$ for different collision energies.
$\rho_{00}$ as a function of transverse momentum for $K^{*0}$ for different collision energies.
$\rho_{00}$ as a function of transverse momentum for $K^{*0}$ for different collision energies.
$\rho_{00}$ as a function of transverse momentum for $K^{*0}$ for different collision energies.
$\rho_{00}$ as a function of transverse momentum for $K^{*0}$ for different collision energies.
$\rho_{00}$ as a function of centrality for $\phi$ (upper panels) and $K^{*0}$ (lower panels). The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for the $K^{*0}$ meson, obtained with the 2nd-order EP.}
$\rho_{00}$ as a function of centrality for $\phi$ (upper panels) and $K^{*0}$ (lower panels). The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for the $K^{*0}$ meson, obtained with the 2nd-order EP.}
$\rho_{00}$ as a function of centrality for $\phi$ (upper panels) and $K^{*0}$ (lower panels). The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for the $K^{*0}$ meson, obtained with the 2nd-order EP.}
$\rho_{00}$ as a function of centrality for $\phi$ (upper panels) and $K^{*0}$ (lower panels). The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for the $K^{*0}$ meson, obtained with the 2nd-order EP.}
$\rho_{00}$ as a function of centrality for $\phi$ (upper panels) and $K^{*0}$ (lower panels). The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for the $K^{*0}$ meson, obtained with the 2nd-order EP.}
$\rho_{00}$ as a function of centrality for $\phi$ (upper panels) and $K^{*0}$ (lower panels). The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for the $K^{*0}$ meson, obtained with the 2nd-order EP.}
$\rho_{00}$ as a function of centrality for $\phi$ (upper panels) and $K^{*0}$ (lower panels). The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for the $K^{*0}$ meson, obtained with the 2nd-order EP.}
Global spin alignment measurement of $\phi$ and $K^{*0}$ vector mesons in Au+Au collisions at 0-20\% centrality. The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for $K^{*0}$-meson, obtained with the 2nd-order EP.
Global spin alignment measurement of $\phi$ and $K^{*0}$ vector mesons in Au+Au collisions at 0-20\% centrality. The solid squares and stars are results for the $\phi$ meson, obtained with the 1st- and 2nd-order EP, respectively. The solid circles are results for $K^{*0}$-meson, obtained with the 2nd-order EP.
The extreme temperatures and energy densities generated by ultra-relativistic collisions between heavy nuclei produce a state of matter with surprising fluid properties. Non-central collisions have angular momentum on the order of 1000$\hbar$, and the resulting fluid may have a strong vortical structure that must be understood to properly describe the fluid. It is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have so far been found. Here we present the first measurement of an alignment between the angular momentum of a non-central collision and the spin of emitted particles, revealing that the fluid produced in heavy ion collisions is by far the most vortical system ever observed. We find that $\Lambda$ and $\overline{\Lambda}$ hyperons show a positive polarization of the order of a few percent, consistent with some hydrodynamic predictions. A previous measurement that reported a null result at higher collision energies is seen to be consistent with the trend of our new observations, though with larger statistical uncertainties. These data provide the first experimental access to the vortical structure of the "perfect fluid" created in a heavy ion collision. They should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the Strong Force. Our results extend the recent discovery of hydrodynamic spin alignment to the subatomic realm.
Lambda and AntiLambda polarization as a function of collision energy. A 0.8% error on the alpha value used in the paper is corrected in this table. Systematic error bars include those associated with particle identification (negligible), uncertainty in the value of the hyperon decay parameter (2%) and reaction plane resolution (2%) and detector efficiency corrections (4%). The dominant systematic error comes from statistical fluctuations of the estimated combinatoric background under the (anti-)$\Lambda$ mass peak.
Lambda and AntiLambda polarization as a function of collision energy calculated using the new $\alpha_\Lambda=0.732$ updated on PDG2020. Systematic error bars include those associated with particle identification (negligible), uncertainty in the value of the hyperon decay parameter (2%) and reaction plane resolution (2%) and detector efficiency corrections (4%). The dominant systematic error comes from statistical fluctuations of the estimated combinatoric background under the (anti-)$\Lambda$ mass peak.
Elliptic flow (v_2) values for identified particles at midrapidity in Au + Au collisions measured by the STAR experiment in the Beam Energy Scan at the Relativistic Heavy Ion Collider at sqrt{s_{NN}}= 7.7--62.4 GeV are presented for three centrality classes. The centrality dependence and the data at sqrt{s_{NN}}= 14.5 GeV are new. Except at the lowest beam energies we observe a similar relative v_2 baryon-meson splitting for all centrality classes which is in agreement within 15% with the number-of-constituent quark scaling. The larger v_2 for most particles relative to antiparticles, already observed for minimum bias collisions, shows a clear centrality dependence, with the largest difference for the most central collisions. Also, the results are compared with A Multiphase Transport Model and fit with a Blast Wave model.
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The difference in $v_{2}$ between particles (X) and their corresponding antiparticles $\bar{X}$ (see legend) as a function of $\sqrt{s_{NN}}$ for 10%-40% central Au + Au collisions. The systematic errors are shown by the hooked error bars. The dashed lines in the plot are fits with a power-law function.
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The difference in $v_{2}$ between protons and antiprotons as a function of $\sqrt{s_{NN}}$ for 0%-10%, 10%-40% and 40%-80% central Au + Au collisions. The systematic errors are shown by the hooked error bars. The dashed lines in the plot are fits with a power-law function.
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The relative difference. The systematic errors are shown by the hooked error bars. The dashed lines in the plot are fits with a power-law function.
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The $v_{2}$ difference between protons and antiprotons (and between $\pi^{+}$ and $pi^{-}$) for 10%-40% centrality Au+Au collisions at 7.7, 11.5, 14.5, and 19.6 GeV. The $v_{2}{BBC} results were slightly shifted horizontally.
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We report on K*0 production at mid-rapidity in Au+Au and Cu+Cu collisions at \sqrt{s_{NN}} = 62.4 and 200 GeV collected by the Solenoid Tracker at RHIC (STAR) detector. The K*0 is reconstructed via the hadronic decays K*0 \to K+ pi- and \bar{K*0} \to K-pi+. Transverse momentum, pT, spectra are measured over a range of pT extending from 0.2 GeV/c to 5 GeV/c. The center of mass energy and system size dependence of the rapidity density, dN/dy, and the average transverse momentum, <pT>, are presented. The measured N(K*0)/N(K) and N(\phi)/N(K*0) ratios favor the dominance of re-scattering of decay daughters of K*0 over the hadronic regeneration for the K*0 production. In the intermediate pT region (2.0 < pT < 4.0 GeV/c), the elliptic flow parameter, v2, and the nuclear modification factor, RCP, agree with the expectations from the quark coalescence model of particle production.
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.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum, p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow,v_2, as a function of the transverse momentum,p_T, from 0–80% central Au+Au collisions for various particle species and energies.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected particles re plotted only for the transverse momentum range of 0.2< pT<1.6 GeV/c to emphasize the mass ordering at low p__T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2 (p_T), in 0–80% central Au+Au collisions for selected anti-particles are plotted only for the transverse momentum range of 0.2< p_T<1.6 GeV/c to emphasize the mass ordering at low p_T.
The elliptic flow, v_2, of charged koans as a function of the transverse momentum,p_T,for 0–80% central Au+Au collisions.
The elliptic flow,v_2 of p, $\overline{p}$ as a function of the transverse momentum, p_T,for 0–80% central Au+Au collisions.
The elliptic flow,v_2 of p, $\overline{p}$ as a function of the transverse momentum, p_T,for 0–80% central Au+Au collisions.
The elliptic flow,v_2 of p, $\overline{p}$ as a function of the transverse momentum, p_T,for 0–80% central Au+Au collisions.
The elliptic flow,v_2 of $Omega^{-}$ and $\overline{\Omega^{+}}$ as a function of the transverse momentum, p_T,for 0–80% central Au+Au collisions.
The elliptic flow, v_2, of p and $\overline{p}$ as a function of the transverse momentum, p_T, for 0–10% central Au+Au collisions.
The elliptic flow, v_2, of p and $\overline{p}$ as a function of the transverse momentum, p_T, for 0–10% central Au+Au collisions.
The elliptic flow,v_2, of 0–80% central Au+Au collisions as a function of the reduced transverse mass,$ m_T−m_0 $, for selected particles.
The elliptic flow,v_2, of 0–80% central Au+Au collisions as a function of the reduced transverse mass,$ m_T−m_0 $, for selected particles.
The elliptic flow,v_2, of 0–80% central Au+Au collisions as a function of the reduced transverse mass,$ m_T−m_0 $, for selected particles.
The elliptic flow,v_2, of 0–80% central Au+Au collisions as a function of the reduced transverse mass,$ m_T−m_0 $, for selected anti-particles.
The difference in the v_2 values between a particle X and its corresponding anti-particle $\overline{X}$ as a function of √sNN for 0–80% central Au+Au collisions.
The difference in the v_2 values between a particle X and its corresponding anti-particle $\overline{X}$ as a function of √sNN for 0–80% central Au+Au collisions.
The difference in the v_2 values between a particle X and its corresponding anti-particle $\overline{X}$ as a function of √sNN for 0–80% central Au+Au collisions.
The difference in the v_2 values between a particle X and its corresponding anti-particle $\overline{X}$ as a function of √sNN for 0–80% central Au+Au collisions.
The difference in the v_2 values between a particle X and its corresponding anti-particle $\overline{X}$ as a function of $μ_B$ for 0–80% central Au+Au collisions.
The difference in the v_2 values between a particle X and its corresponding anti-particle $\overline{X}$ as a function of $μ_B$ for 0–80% central Au+Au collisions.
The difference in the v_2 values between a particle X and its corresponding anti-particle $\overline{X}$ as a function of $μ_B$ for 0–80% central Au+Au collisions.
The difference in the v_2 values between a particle X and its corresponding anti-particle $\overline{X}$ as a function of $μ_B$ for 0–80% central Au+Au collisions.
We present STAR measurements of azimuthal anisotropy by means of the two- and four-particle cumulants $v_2$ ($v_2\{2\}$ and $v_2\{4\}$) for Au+Au and Cu+Cu collisions at center of mass energies $\sqrt{s_{_{\mathrm{NN}}}} = 62.4$ and 200 GeV. The difference between $v_2\{2\}^2$ and $v_2\{4\}^2$ is related to $v_{2}$ fluctuations ($\sigma_{v_2}$) and nonflow $(\delta_{2})$. We present an upper limit to $\sigma_{v_2}/v_{2}$. Following the assumption that eccentricity fluctuations $\sigma_{\epsilon}$ dominate $v_2$ fluctuations $\frac{\sigma_{v_2}}{v_2} \approx \frac{\sigma_{\epsilon}}{\epsilon}$ we deduce the nonflow implied for several models of eccentricity fluctuations that would be required for consistency with $v_2\{2\}$ and $v_2\{4\}$. We also present results on the ratio of $v_2$ to eccentricity.
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