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تسجيل مستخدم جديدDecipher the $R_{Psi_{m}}$ correlator in search for the chiral magnetic effect in relativistic heavy ion collisions
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Background: The chiral magnetic effect (CME) is extensively studied in heavy-ion collisions at RHIC and the LHC. An azimuthal correlator called $R_{Psi_{m}}$ was proposed to measure the CME. By observing the same $R_{Psi_{2}}$ and $R_{Psi_{3}}$ (convex) distributions from A Multi-Phase Transport (AMPT) model, by contrasting data and model as well as large and small systems and by event shape engineering (ESE), a recent preprint (arXiv:2006.04251v1) from STAR suggests that the $R_{Psi_{m}}$ observable is sensitive to the CME signal and relatively insensitive to backgrounds, and their Au+Au data are inconsistent with known background contributions. Purpose: We examine those claims by studying the robustness of the $R_{Psi_{m}}$ observable using AMPT as well as toy model simulations. We compare $R_{Psi_{m}}$ to the more widely used $Deltagamma$ azimuthal correlator to identify their commonalities and differences. Methods: We use AMPT to simulate Au+Au, p+Au, and d+Au collisions at $sqrt{s_{NN}} = 200 text{ GeV}$, and study the responses of $R_{Psi_{m}}$ to anisotropic flow backgrounds in the model. We also use a toy model to simulate resonance flow background and input CME signal to investigate their effects in $R_{Psi_{2}}$. Additionally we use the toy model to perform an ESE analysis to compare to STAR data as well as predict the degree of sensitivity of $R_{Psi_{2}}$ to isobar collisions with the event statistics taken at RHIC. ...
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The Chiral Magnetic Effect (CME) is a remarkable phenomenon that stems from highly nontrivial interplay of QCD chiral symmetry, axial anomaly, and gluonic topology. It is of fundamental importance to search for the CME in experiments. The heavy ion c
ollisions provide a unique environment where a hot chiral-symmetric quark-gluon plasma is created, gluonic topological fluctuations generate chirality imbalance, and very strong magnetic fields $|vec{bf B}|sim m_pi^2$ are present during the early stage of such collisions. Significant efforts have been made to look for CME signals in heavy ion collision experiments. In this contribution we give a brief overview on the status of such efforts.
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 ma
gnetic 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.
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 azimu
thal 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.
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