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Register a new userObservation of charge asymmetry dependence of pion elliptic flow and the possible chiral magnetic wave in heavy-ion collisions
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We present measurements of $pi^-$ and $pi^+$ elliptic flow, $v_2$, at midrapidity in Au+Au collisions at $sqrt{s_{_{rm NN}}} =$ 200, 62.4, 39, 27, 19.6, 11.5 and 7.7 GeV, as a function of event-by-event charge asymmetry, $A_{ch}$, based on data from the STAR experiment at RHIC. We find that $pi^-$ ($pi^+$) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at $sqrt{s_{_{rm NN}}} = text{27 GeV}$ and higher. At $sqrt{s_{_{rm NN}}} = text{200 GeV}$, the slope of the difference of $v_2$ between $pi^-$ and $pi^+$ as a function of $A_{ch}$ exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies.
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Parity-odd domains, corresponding to non-trivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the orbital momentum of the system created in non-central collisions. To study this effect, we investigate a three particle mixed harmonics azimuthal correlator which is a P-even observable, but directly sensitive to the charge separation effect. We report measurements of this observable using the STAR detector in Au+Au and Cu+Cu collisions at $sqrt{s_{NN}}$=200 and 62~GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. A signal consistent with several of the theoretical expectations is detected in all four data sets. We compare our results to the predictions of existing event generators, and discuss in detail possible contributions from other effects that are not related to parity violation.
Isobaric $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr collisions at $sqrt{s_{_{NN}}}=200$ GeV have been conducted at the Relativistic Heavy Ion Collider to circumvent the large flow-induced background in searching for the chiral magnetic effect (CME), predicted by the topological feature of quantum chromodynamics (QCD). Considering that the background in isobar collisions is approximately twice that in Au+Au collisions (due to the smaller multiplicity) and the CME signal is approximately half (due to the weaker magnetic field), we caution that the CME may not be detectable with the collected isobar data statistics, within $sim$2$sigma$ significance, if the axial charge per entropy density ($n_5/s$) and the QCD vacuum transition probability are system independent. This expectation is generally verified by the Anomalous-Viscous Fluid Dynamics (AVFD) model. While our estimate provides an approximate experimental baseline, theoretical uncertainties on the CME remain large.
The chiral magnetic effect (CME) is a novel transport phenomenon, arising from the interplay between quantum anomalies and strong magnetic fields in chiral systems. In high-energy nuclear collisions, the CME may survive the expansion of the quark-gluon plasma fireball and be detected in experiments. Over the past decade, the experimental searches for the CME have aroused extensive interest at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). The main goal of this article is to investigate three pertinent experimental approaches: the $gamma$ correlator, the $R$ correlator and the signed balance functions. We will exploit both simple Monte Carlo simulations and a realistic event generator (EBE-AVFD) to verify the equivalence in the kernel-component observables among these methods and to ascertain their sensitivities to the CME signal for the isobaric collisions at RHIC.
The charge asymmetry ($A_{rm ch}$) dependence of the $pi^{-}$ and $pi^{+}$ elliptic flow difference, $Delta v_{2}(A_{rm ch})$, has been regarded as a sensitive observable for the possible chiral magnetic wave (CMW) in relativistic heavy ion collisions. In this work, we first demonstrate that, due to non-flow backgrounds, the flow measurements by the Q-cumulant method using all charged particles as reference introduce a trivial linear term to $Delta v_{2}(A_{rm ch})$. The trivial slope can be negative in the triangle flow difference $Delta v_{3}(A_{rm ch})$ if the non-flow is dominated by back-to-back pairs. After eliminating the trivial term, we find that the non-flow between like-sign pairs gives rise to an additional positive slope to $Delta v_{2}(A_{rm ch})$ because of the larger dilution effect to $pi^{+}$ ($pi^{-}$) at positive (negative) $A_{rm ch}$. We further find that the competition between different $pi$ sources can introduce another non-trivial linear-$A_{rm ch}$ term due to their different multiplicity fluctuations and anisotropic flows. We then study the effect of neutral cluster (resonance) decays as a mechanism for local charge conservation on the slope parameter of $Delta v_{2}(A_{rm ch})$. We find that the slope parameter is sensitive to the kinematics of those neutral clusters. Light resonances give positive slopes while heavy resonances give negative slopes. Local charge conservation from continuum cluster mass distribution can give a positive slope parameter comparable to experimental data. Our studies indicate that many non-CMW physics mechanisms can give rise to a $A_{rm ch}$-dependent $Delta v_{2}(A_{rm ch})$ and the interpretation of $Delta v_{2}(A_{rm ch})$ in terms of the CMW is delicate.
We present a comparison of inclusive photon elliptic flow parameter (v_{2}) measured at RHIC and SPS high energy heavy-ion collision experiments to calculations done using the AMPT and UrQMD models. The new results discussed includes the comparison of the model calculations of photon v_{2} to corresponding measurements at the forward rapidities. We observe that the AMPT model which includes partonic interactions and quark coalescence as a mechanism of hadronization is in good agreement with the measurements even at forward rapidities (2.3 < eta < 3.9) at RHIC as was previously observed for measurements at midrapidity. At the top SPS energy the contribution from partonic effects are smaller than that at RHIC energy, based on the comparison of the measured photon v_{2} with those from the AMPT default and UrQMD model calculations. We find that if the measurements in RHIC beam energy scan (BES) and LHC energies would require an energy dependent partonic cross section in the AMPT models, then the observed longitudinal scaling of v_{2} at top RHIC energies (19.6-200 GeV) will be violated. We also discuss the relation between the inclusive photon v_{2} and those of their parent pi^{0}s for the beam energies of 7.7 GeV to 2.76 TeV. The model results show that the transverse momentum (p_{mathrm T}) integrated v_{2} of pi^{0} is larger by about 44% relative to those of the inclusive photons. Finally we present the expectations of inclusive photon v_{2} for the RHIC beam energy scan (BES) program and LHC from the transport models, so that they can be compared to corresponding measurements using the data already collected at RHIC and LHC.
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