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Register a new userIsolating the chiral magnetic effect from backgrounds by pair invariant mass
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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 ($Deltagamma$) 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 $Deltagamma$ as a function of the pair invariant mass ($m_{rm inv}$), by restricting to high $m_{rm inv}$ thus relatively background free, and by studying the $m_{rm inv}$ 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.
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Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME) -- an electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable ($Deltagamma$) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy ($v_{2}$). We report here the first differential measurements of the correlator as a function of the pair invariant mass ($m_{rm inv}$) in 20-50% centrality Au+Au collisions at $sqrt{s_{_{rm NN}}}$= 200 GeV by the STAR experiment at RHIC. Strong resonance background contributions to $Deltagamma$ are observed. At large $m_{rm inv}$ where this background is significantly reduced, the $Deltagamma$ value is found to be also significantly smaller. An event shape engineering technique is deployed to determine the $v_{2}$ background shape as a function of $m_{rm inv}$. A $v_{2}$-independent signal, possibly indicating a $m_{rm inv}$-integrated CME contribution, is extracted to be $Deltagamma_{rm signal}$ = (0.03 $pm$ 0.06 $pm$ 0.08) $times10^{-4}$, or $(2pm4pm5)%$ of the inclusive $Deltagamma(m_{rm inv}>0.4$ GeV/$c^2$)$=(1.58 pm 0.02 pm 0.02) times10^{-4}$. This presents an upper limit of $0.23times10^{-4}$, or $15%$ of the inclusive result at $95%$ confidence level.
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.
Correlation measurements with respect to the spectator and participant planes in relativistic heavy ion collisions were proposed to extract the chiral magnetic effect (CME) from background dominated azimuthal correlators. This paper investigates the effects of two- and three-particle nonflow correlations on the extracted CME signal fraction, $f_{text{CME}}$. It is found, guided by a multiphase transport (AMPT) model and the heavy ion jet interaction generator (HIJING) together with experimental data, that the nonflow effects amount to approximately $(4pm5)$% and $(-5pm3)$% without and with pseudorapidity gaps, respectively, in 20-50% centrality Au+Au collisions at $sqrt{s_{text{NN}}}= 200 text{ GeV}$.
A new sine observable, $R_{Psi_2}(Delta S)$, has been proposed to measure the chiral magnetic effect (CME) in heavy-ion collisions; $Delta S = left langle sin varphi_+ right rangle - left langle sin varphi_- right rangle$, where $varphi_pm$ are azimuthal angles of positively and negatively charged particles relative to the reaction plane and averages are event-wise, and $R_{Psi_2}(Delta S)$ is a normalized event probability distribution. Preliminary STAR data reveal concave $R_{Psi_2}(Delta S)$ distributions in 200 GeV Au+Au collisions. Studies with a multiphase transport (AMPT) and anomalous-viscous Fluid Dynamics (AVFD) models show concave $R_{Psi_2}(Delta S)$ distributions for CME signals and convex ones for typical resonance backgrounds. A recent hydrodynamic study, however, indicates concave shapes for backgrounds as well. To better understand these results, we report a systematic study of the elliptic flow ($v_{2}$) and transverse momentum ($p_{T}$) dependences of resonance backgrounds with toy-model simulations and central limit theorem (CLT) calculations. It is found that the concavity or convexity of $R_{Psi_2}(Delta S)$ depends sensitively on the resonance $v_2$ (which yields different numbers of decay $pi^+pi^-$ pairs in the in-plane and out-of-plane directions) and $p_T$ (which affects the opening angle of the decay $pi^+pi^-$ pair). Qualitatively, low $p_{T}$ resonances decay into large opening-angle pairs and result in more `back-to-back pairs out-of-plane, mimicking a CME signal, or a concave $R_{Psi_2}(Delta S)$. Supplemental studies of $R_{Psi_3}(Delta S)$ in terms of the triangular flow ($v_3$), where only backgrounds exist but any CME would average to zero, are also presented.
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.
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