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Register a new userAn experimental review on elliptic flow of strange and multi-strange hadrons in relativistic heavy ion collisions
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Shusu Shi
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Strange hadrons, especially multi-strange hadrons are good probes for the early partonic stage of heavy ion collisions due to their small hadronic cross sections. In this paper, I give a brief review on the elliptic flow measurements of strange and multi-strange hadrons in relativistic heavy ion collisions at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC).
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We present recent results of the mid-rapidity elliptic flow ($v_2$) for multi-strange hadrons and the $phi$ meson as a function of centrality in Au + Au collisions at the center of mass energy $sqrt{s_{NN}}$ = 200 GeV. The transverse momentum dependence of $phi$ and $Omega$ $v_2$ is similar to that of pion and proton, indicating that the heavier strange ($s$) quark flows as strongly as the lighter up ($u$) and down ($d$) quarks. These observations constitute a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. In addition, the mass ordering of $v_2$ breaks between the $phi$ and proton at low transverse momenta in the 0-30% centrality bin, possibly due to the effect of late hadronic interactions on the proton $v_2$.
We present an elaborate version of the hadron resonance gas model with the combined treatment of separate chemical freeze-outs for strange and non-strange hadrons and with an additional $gamma_{s}$ factor which accounts for the remaining strange particle non-equilibration. Within suggested approach the parameters of two chemical freeze-outs are connected by the conservation laws of entropy, baryonic charge, third isospin projection and strangeness. The developed model enables us to perform a high-quality fit of the hadron multiplicity ratios measured at AGS, SPS and RHIC with $chi^2/dof simeq 0.93$. A special attention is paid to a successful description of the Strangeness Horn. The well-known problem of selective suppression of $bar Lambda $ and $bar Xi$ hyperons is also discussed. The main result is that for all collision energies the $gamma_{s}$ factor is about 1 within the error bars, except for the center of mass collision energy 7.6 GeV at which we find about 20% enhancement of strangeness. Also we confirm an existence of strong jumps in pressure, temperature and effective number of degrees of freedom at the stage of strange particle chemical freeze-out, when the center of mass collision energy changes from 4.3 to 4.9 GeV. We argue that these irregularities may signal about the quark-hadron phase transition.
We study effects of eccentricity fluctuations on the elliptic flow coefficient v_2 at mid-rapidity in both Au+Au and Cu+Cu collisions at sqrt{s_NN}=200 GeV by using a hybrid model that combines ideal hydrodynamics for space-time evolution of the quark gluon plasma phase and a hadronic transport model for the hadronic matter. For initial conditions in hydrodynamic simulations, both the Glauber model and the color glass condensate model are employed to demonstrate the effect of initial eccentricity fluctuations originating from the nucleon position inside a colliding nucleus. The effect of eccentricity fluctuations is modest in semicentral Au+Au collisions, but significantly enhances v_2 in Cu+Cu collisions.
We present high precision measurements of elliptic flow near midrapidity ($|y|<1.0$) for multi-strange hadrons and $phi$ meson as a function of centrality and transverse momentum in Au+Au collisions at center of mass energy $sqrt{s_{NN}}=$ 200 GeV. We observe that the transverse momentum dependence of $phi$ and $Omega$ $v_{2}$ is similar to that of $pi$ and $p$, respectively, which may indicate that the heavier strange quark flows as strongly as the lighter up and down quarks. This observation constitutes a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. Number of constituent quark scaling is found to hold within statistical uncertainty for both 0-30$%$ and 30-80$%$ collision centrality. There is an indication of the breakdown of previously observed mass ordering between $phi$ and proton $v_{2}$ at low transverse momentum in the 0-30$%$ centrality range, possibly indicating late hadronic interactions affecting the proton $v_{2}$.
By analyzing the available data on strange hadrons in central Pb+Pb collisions from the NA49 Collaboration at the Super Proton Synchrotron (SPS) and in central Au+Au collisions from the STAR Collaboration at the Relativistic Heavy-Ion Collider (RHIC) in a wide collision energy range from $sqrt{s_{rm NN}}$ = 6.3 GeV to 200 GeV, we find a possible non-monotonic behavior in the ratio $mathcal{O}_text{K-$Xi$-$phi$-$Lambda$}$= $frac{N(K^+)N(Xi^-)}{N(phi)N(Lambda)}$ of $K^+$, $Xi^-$, $phi$, and $Lambda$ yields as a function of $sqrt{s_{rm NN}}$. Based on the quark coalescence model, which can take into account the effect of quark density fluctuations on hadron production, a possible non-monotonic behavior in the dependence of the strange quark density fluctuation on $sqrt{s_{NN}}$ is obtained. This is in contrast to the coalescence model that does not include quark density fluctuations and also to the statistical hadronization model as both fail to describe even qualitatively the collision energy dependence of the ratio $mathcal{O}_text{K-$Xi$-$phi$-$Lambda$}$. Our findings thus suggest that the signal and location of a possible critical endpoint in the QCD phase diagram, which is expected to result in large quark density fluctuations, can be found in the on-going Bean Energy Scan program at RHIC.
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