Charge-dependent Azimuthal Correlations Research Articles

Search for the Chiral Magnetic Effect with charge-dependent azimuthal correlations in Xe–Xe collisions at [formula omitted] TeV

Charge-dependent two- and three-particle correlations measured in Xe–Xe collisions at sNN=5.44 TeV are presented. Results are obtained for charged particles in the pseudorapidity range |η|<0.8 and transverse momentum interval 0.2≤pT<5.0 GeV/c for different collision centralities. The three-particle correlator γαβ≡〈cos⁡(φα+φβ−2Ψ2)〉, calculated for different combinations of charge sign α and β, is expected to be sensitive to the presence of the Chiral Magnetic Effect (CME). Its magnitude is similar to the one observed in Pb–Pb collisions in contrast to a smaller CME signal in Xe–Xe collisions than in Pb–Pb collisions predicted by Monte Carlo (MC) calculations including a magnetic field induced by the spectator protons. These observations point to a large non-CME contribution to the correlator. Furthermore, the charge dependence of γαβ can be described by a blast wave model calculation that incorporates background effects and by the Anomalous Viscous Fluid Dynamics model with values of the CME signal consistent with zero. The Xe–Xe and Pb–Pb results are combined with the expected CME signal dependence on the system size from the MC calculations including a magnetic field to obtain the fraction of CME contribution in γαβ, fCME. The CME fraction is compatible with zero for the 30% most central events in both systems and then becomes positive. This yields an upper limit of 2% (3%) and 25% (32%) at 95% (99.7%) confidence level for the CME signal contribution to γαβ in the 0–70% Xe–Xe and Pb–Pb collisions, respectively.

Search for the Chiral Magnetic Effect via Charge-Dependent Azimuthal Correlations Relative to Spectator and Participant Planes in Au+Au Collisions at sqrt[s_{NN}]=200 GeV.

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field due to imbalanced chirality of quarks in local parity and charge-parity violating domains in quantum chromodynamics. The experimental measurement of the charge separation is made difficult by the presence of a major background from elliptic azimuthal anisotropy. This background and the CME signal have different sensitivities to the spectator and participant planes, and could thus be determined by measurements with respect to these planes. We report such measurements in Au+Au collisions at a nucleon-nucleon center-of-mass energy of 200GeV at the Relativistic Heavy-Ion Collider. It is found that the charge separation, with the flow background removed, is consistent with zero in peripheral (large impact parameter) collisions. Some indication of finite CME signals is seen in midcentral (intermediate impact parameter) collisions. Significant residual background effects may, however, still be present.

Constraining the Chiral Magnetic Effect with charge-dependent azimuthal correlations in Pb-Pb collisions at sqrt{s_{mathrm{NN}}} = 2.76 and 5.02 TeV

Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at sqrt{s_{mathrm{NN}}} = 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity (η) and transverse momentum (pT) ranges |η| < 0.8 and 0.2 < pT< 5 GeV/c. A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators γ1, 1 = 〈cos(φα + φβ − 2Ψ2)〉 and γ1, − 3 = 〈cos(φα − 3φβ + 2Ψ2)〉. The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured γ1,1. Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form γ1, 2 = 〈cos(φα + 2φβ − 3Ψ3)〉, which are insensitive to the CME signal, verify again that background contributions dominate the measurement of γ1,1.

Sensitivity analysis for observables of the chiral magnetic effect using a multiphase transport model

Because the traditional observable of charge-dependent azimuthal correlator $\gamma$ contains both contributions from the chiral magnetic effect (CME) and its background, a new observable of $R_{\Psi_{m}}$ has been recently proposed which is expected to be able to distinguish the CME from the background. In this study, we apply two methods to calculate $R_{\Psi_{m}}$ using a multiphase transport model without or with introducing a percentage of CME-induced charge separation. We demonstrate that the shape of final $R_{\Psi_{2}}$ distribution is flat for the case without the CME, but concave for that with an amount of the CME, because the initial CME signal survives from strong final state interactions. By comparing the responses of $R_{\Psi_{2}}$ and $\gamma$ to the strength of the initial CME, we observe that two observables show different nonlinear sensitivities to the CME. We find that the shape of $R_{\Psi_{2}}$ has an advantage in measuring a small amount of the CME, although it requires large event statistics.

Back-to-back relative-excess observable to identify the chiral magnetic effect

$\textbf{Background:}$ The chiral magnetic effect (CME) is extensively studied in heavy-ion collisions at RHIC and LHC. In the commonly used reaction plane (RP) dependent, charge dependent azimuthal correlator ($\Delta\gamma$), both the close and back-to-back pairs are included. Many backgrounds contribute to the close pairs (e.g. resonance decays, jet correlations), whereas the back-to-back pairs are relatively free of those backgrounds. $\textbf{Purpose:}$ In order to reduce those backgrounds, we propose a new observable which only focuses on the back-to-back pairs, namely, the relative back-to-back opposite-sign (OS) over same-sign (SS) pair excess ($r_{\text{BB}}$) as a function of the pair azimuthal orientation with respect to the RP ($\varphi_{\text{BB}}$). $\textbf{Methods:}$ We use analytical calculations and toy model simulations to demonstrate the sensitivity of $r_{\text{BB}}(\varphi_{\text{BB}})$ to the CME and its insensitivity to backgrounds. $\textbf{Results:}$ With finite CME, the $\varphi_{\text{BB}}$ distribution of $r_{\text{BB}}$ shows a clear characteristic modulation. Its sensitivity to background is significantly reduced compared to the previous $\Delta\gamma$ observable. The simulation results are consistent with our analytical calculations. $\textbf{Conclusions:}$ Our studies demonstrate that the $r_{\text{BB}}(\varphi_{\text{BB}})$ observable is sensitive to the CME signal and rather insensitive to the resonance backgrounds.

Multiparticle and charge-dependent azimuthal correlations in heavy-ion collisions at the Relativistic Heavy-Ion Collider

We study multi-particle azimuthal correlations in relativistic heavy-ion collisions at a center of mass energy of 200 GeV. We use the IP-Glasma model to initialize the viscous hydrodynamic simulation MUSIC and employ the UrQMD transport model for the low temperature region of the collisions. In addition, we study effects of local charge and global momentum conservation among the sampled particles. With the exception of the lowest order three particle correlator $C_{112}$, our framework provides a good description of the existing charge inclusive azimuthal correlation data for Au+Au and U+U collisions at RHIC. We also present results for charge dependent two and three particle correlators in Au+Au and U+U collisions, and make predictions for isobar (Ru+Ru & Zr+Zr) collisions to provide a much needed baseline for the search for the Chiral Magnetic Effect at RHIC.

Isolating the chiral magnetic effect from backgrounds by pair invariant mass

Topological gluon configurations in quantum chromodynamics induce quark chirality imbalance in local domains, which can result in the chiral magnetic effect (CME) – an electric charge separation along a strong magnetic field. Experimental searches for the CME in relativistic heavy ion collisions via the charge-dependent azimuthal correlator (Delta gamma ) suffer from large backgrounds arising from particle correlations (e.g. due to resonance decays) coupled with the elliptic anisotropy. We propose differential measurements of the Delta gamma as a function of the pair invariant mass (m_mathrm{inv}), by restricting to high m_mathrm{inv} thus relatively background free, and by studying the m_mathrm{inv} dependence to separate the possible CME signal from backgrounds. We demonstrate by model studies the feasibility and effectiveness of such measurements for the CME search.

Search for the chiral magnetic effect at the LHC with the CMS experiment

Searches for the chiral magnetic effect (CME) using charge-dependent azimuthal correlations with respect to event planes are presented in PbPb collisions at 5.02 TeV and pPb collisions at 5.02 and 8.16 TeV, with the CMS experiment at the LHC. The azimuthal correlations with respect to the second- and third-order event planes are explored as a function of pseudorapidity, transverse momentum, and event multiplicity, which provides new insights into the underlying background correlations. By employing an event-shape engineering technique, a linear dependence of charge-dependent correlations on the second-order anisotropy flow (ν2) is observed, and the upper limits on the ν2-independent fraction, which is directly related to the CME signal, are obtained at 95% confidence level for both pPb and PbPb collisions. These results provide strong constraints on the search for the chiral magnetic effect at LHC energies, and establish new guidelines for searches in future experiments.

Search for the chiral magnetic effect and vorticity-induced polarization effects in nuclear collisions

In relativistic heavy ion collisions, the chiral magnetic effect, known as the “CME”, has been extensively searched for over a decade in order to gain insights into the chiral symmetry restoration in the Quark-Gluon-Plasma, the possible local strong parity violation, and the extremely strong magnetic field. Based on the magnetic field direction and its correlation with the reaction plane, the CME has been mostly measured via a charge-dependent azimuthal correlator with respect to the second-order event plane, where experimental results are not yet conclusive due to their dominant background contaminations. Recent experimental efforts from STAR, ALICE, and CMS Collaboration will be discussed in this paper. Moreover, the total angular momentum from a noncentral collision can induce many important effects via its coupling to the particles' spin, e.g., Λ polarization, which provides essential information about properties of this strongly-interacting fluid-like medium. Besides the discovery of this exotic phenomenon, recent preliminary results from the STAR and ALICE experiment will be reviewed.

Constraints on the chiral magnetic effect using charge-dependent azimuthal correlations in pPb and PbPb collisions at the CERN Large Hadron Collider

Charge-dependent azimuthal correlations of same- and opposite-sign pairs with respect to the second- and third-order event planes have been measured in pPb collisions at sNN=8.16TeV and PbPb collisions at 5.02 TeV with the CMS experiment at the LHC. The measurement is motivated by the search for the charge separation phenomenon predicted by the chiral magnetic effect (CME) in heavy ion collisions. Three- and two-particle azimuthal correlators are extracted as functions of the pseudorapidity difference, the transverse momentum (pT) difference, and the pT average of same- and opposite-charge pairs in various event multiplicity ranges. The data suggest that the charge-dependent three-particle correlators with respect to the second- and third-order event planes share a common origin, predominantly arising from charge-dependent two-particle azimuthal correlations coupled with an anisotropic flow. The CME is expected to lead to a v2-independent three-particle correlation when the magnetic field is fixed. Using an event shape engineering technique, upper limits on the v2-independent fraction of the three-particle correlator are estimated to be 13% for pPb and 7% for PbPb collisions at 95% confidence level. The results of this analysis, both the dominance of two-particle correlations as a source of the three-particle results and the similarities seen between PbPb and pPb, provide stringent constraints on the origin of charge-dependent three-particle azimuthal correlations and challenge their interpretation as arising from a chiral magnetic effect in heavy ion collisions.18 MoreReceived 4 August 2017DOI:https://doi.org/10.1103/PhysRevC.97.044912Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.©2018 CERN, for the CMS CollaborationPhysics Subject Headings (PhySH)Research AreasRelativistic heavy-ion collisionsNuclear Physics

New experimental constrains on chiral magnetic effect using charge-dependent azimuthal correlation in pPb and PbPb collisions at the LHC

Studies of charge-dependent azimuthal correlations for the same- and oppositesign particle pairs are presented in PbPb collisions at 5 TeV and pPb collisions at 5 and 8.16 TeV, with the CMS experiment at the LHC. The azimuthal correlations are evaluated with respect to the second- and also higher-order event planes, as a function of particle pseudorapidity and transverse momentum, and event multiplicity. By employing an event-shape engineering technique, the dependence of correlations on azimuthal anisotropy flow is investigated. Results presented provide new insights to the origin of observed charge-dependent azimuthal correlations, and have important implications to the search for the chiral magnetic effect in heavy ion collisions.

Chiral magnetic effect search in p+Au, d+Au and Au+Au collisions at RHIC

Metastable domains of fluctuating topological charges can change the chirality of quarks and induce local parity violation in quantum chromodynamics. This can lead to observable charge separation along the direction of the strong magnetic field produced by spectator protons in relativistic heavy-ion collisions, a phenomenon called the chiral magnetic effect (CME). A major background source for CME measurements using the charge-dependent azimuthal correlator (Δϒ) is the intrinsic particle correlations (such as resonance decays) coupled with the azimuthal elliptical anisotropy (v2). In heavy-ion collisions, the magnetic field direction and event plane angle are correlated, thus the CME and the v2-induced background are entangled. In this report, we present two studies from STAR to shed further lights on the background issue. (1) The Δϒ should be all background in small system p+Au and d+Au collisions, because the event plane angles are dominated by geometry fluctuations uncorrelated to the magnetic field direction. However, significant Δϒ is observed, comparable to the peripheral Au+Au data, suggesting a background dominance in the latter, and likely also in the mid-central Au+Au collisions where the multiplicity and v2 scaled correlator is similar. (2) A new approach is devised to study Δϒ as a function of the particle pair invariant mass (minv) to identify the resonance backgrounds and hence to extract the possible CME signal. Signal is consistent with zero within uncertainties at high minv. Signal at low minv, extracted from a two-component model assuming smooth mass dependence, is consistent with zero within uncertainties.

Event-shape-engineering study of charge separation in heavy-ion collisions**Supported by a grant (DE-FG02-88ER40424) from U.S. Department of Energy, Office of Nuclear Physics

Recent measurements of charge-dependent azimuthal correlations in high-energy heavy-ion collisions have indicated charge-separation signals perpendicular to the reaction plane, and have been related to the chiral magnetic effect (CME). However, the correlation signal is contaminated with the background caused by the collective motion (flow) of the collision system, and an effective approach is needed to remove the flow background from the correlation. We present a method study with simplified Monte Carlo simulations and a multi-phase transport model, and develop a scheme to reveal the true CME signal via event-shape engineering with the flow vector of the particles of interest.

Charge-dependent azimuthal correlations in pPb collisions with CMS experiment

Charge-dependent azimuthal correlations relative to the event plane in AA collisions have been suggested as providing evidence for the chiral magnetic effect (CME) caused by local strong parity violation. However, the observation of the CME remains inconclusive because of several possible sources of background correlations that may account for part or all of the observed signals. This talk will present the first application of three-particle, charge-dependent azimuthal correlation analysis in proton-nucleus collisions, using pPb data collected with the CMS experiment at the LHC at sNN=5.02 TeV. The differences found in comparing same and opposite sign correlations are studied as a function of event multiplicity and the pseudorapidity gap between two of the particles detected in the CMS tracker detector. After selecting events with comparable charge-particle multiplicities, the results for pPb collisions are found to be similar to those for PbPb collisions collected at the same collision energy. With a reduced magnetic field strength and a random field orientation in high multiplicity pPb events, the CME contribution to any charge separation signal is expected to be much smaller than found in peripheral PbPb events. These results pose a challenge for the interpretation of charge-dependent azimuthal correlations in heavy ion collisions in terms of the chiral magnetic effect.

Observation of Charge-Dependent Azimuthal Correlations in p-Pb Collisions and Its Implication for the Search for the Chiral Magnetic Effect.

Charge-dependent azimuthal particle correlations with respect to the second-order event plane in p-Pb and PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02TeV have been studied with the CMS experiment at the LHC. The measurement is performed with a three-particle correlation technique, using two particles with the same or opposite charge within the pseudorapidity range |η|<2.4, and a third particle measured in the hadron forward calorimeters (4.4<|η|<5). The observed differences between the same and opposite sign correlations, as functions of multiplicity and η gap between the two charged particles, are of similar magnitude in p-Pb and PbPb collisions at the same multiplicities. These results pose a challenge for the interpretation of charge-dependent azimuthal correlations in heavy ion collisions in terms of the chiral magnetic effect.

Charge-dependent azimuthal correlations of secondary particles in heavy ion collisions

The P/CP symmetry breaking in quantum chromodynamics (QCD) could be realized via transitions between local fluctuations of gauge fields. Azimuthal correlations which characterize the asymmetry of the emitted charged particles with respect to the reaction plane in non-central nucleus-nucleus collisions are the promising tools for experimental study of local P/CP violation in the strong interactions. The preliminary estimations of correlators within the model of chiral magnetic effect are presented for types of nuclei and collision energies corresponded to RHIC and the LHC beams for two various nuclear densities, namely, for approach of the hard sphere and for the two-component Fermi model. Besides of the correlator estimations for the symmetric collisions, the preliminary results for magnetic field in asymmetric Cu + Au collisions are also shown.

Formula omitted]-odd pion azimuthal charge correlations in heavy ion collisions

We argue that the large instanton induced Pauli form factor in polarized proton–proton scattering may cause, through topological fluctuations, substantial charge-dependent azimuthal correlations for π± production in peripheral heavy ion collisions both at RHIC and LHC, thanks to the large induced magnetic field. Our results compare favorably to the measured pion azimuthal correlations by the STAR and ALICE Collaborations.

Charge-dependent azimuthal correlations from AuAu to UU collisions

We study the charge-dependent azimuthal correlations in relativistic heavy ion collisions, as motivated by the search for the Chiral Magnetic Effect (CME) and the investigation of related background contributions. In particular we aim to understand how these correlations induced by various proposed effects evolve from collisions with AuAu system to that with UU system. To do that, we quantify the generation of magnetic field in UU collisions at RHIC energy and its azimuthal correlation with the matter geometry using event-by-event simulations. Taking the experimental data for charge-dependent azimuthal correlations from AuAu collisions and extrapolating to UU with reasonable assumptions, we examine the resulting correlations to be expected in UU collisions and compare them with recent STAR measurements. Based on such analysis we discuss the viability for explaining the data with a combination of the CME-like and flow-induced contributions.

Λ(KS0)–h± and Λ–p azimuthal correlations with respect to event plane and search for chiral magnetic and vortical effects

Abstract Parity-odd domains are theorized to form inside the quark–gluon plasma (QGP) created in high-energy heavy-ion collisions. The Chiral Magnetic Effect (CME) will lead to charge separation with respect to the reaction plane. Previous measurements from the RHIC and the LHC using charge-dependent two-particle azimuthal correlations are consistent with charge separations across the reaction plane, possibly induced by the CME. Such charge separation is expected to be absent if a neutral hadron is used for the correlations. The Λ ( Λ ¯ ) and K S 0 are employed to test the hypothesis based on the CME expectation in Au+Au collisions at s N N = 200 and 39 GeV by the STAR experiment. We find no significant charge separation for Λ ( Λ ¯ ) – h ± and K S 0 – h ± azimuthal correlations. In addition, theoretical calculation predicts that the Chiral Vortical Effect (CVE) leads to a baryon number separation with respect to the reaction plane. Measurements of Λ ( Λ ¯ ) – p ( p ¯ ) azimuthal correlation are presented to search for the CVE.

Measurement of charge multiplicity asymmetry correlations in high-energy nucleus-nucleus collisions atsNN=200GeV

A study is reported of the same- and opposite-sign charge-dependent azimuthal correlations with respect to the event plane in Au+Au collisions at 200 GeV. The charge multiplicity asymmetries between the up/down and left/right hemispheres relative to the event plane are utilized. The contributions from statistical fluctuations and detector effects were subtracted from the (co-)variance of the observed charge multiplicity asymmetries. In the mid- to most-central collisions, the same- (opposite-) sign pairs are preferentially emitted in back-to-back (aligned on the same-side) directions. The charge separation across the event plane, measured by the difference, $\Delta$, between the like- and unlike-sign up/down $-$ left/right correlations, is largest near the event plane. The difference is found to be proportional to the event-by-event final-state particle ellipticity (via the observed second-order harmonic $v^{\rm obs}_{2}$), where $\Delta=(1.3\pm1.4({\rm stat})^{+4.0}_{-1.0}({\rm syst}))\times10^{-5}+(3.2\pm0.2({\rm stat})^{+0.4}_{-0.3}({\rm syst}))\times10^{-3}v^{\rm obs}_{2}$ for 20-40% Au+Au collisions. The implications for the proposed chiral magnetic effect are discussed.