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Yicheng Feng

Publications and source records attributed to Yicheng Feng.

47 records · Page 3Linked to original sources
arXiv2022

Estimate of a nonflow baseline for the chiral magnetic effect in isobar collisions at RHIC

Recently, STAR reported the isobar (${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$, ${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) results for the chiral magnetic effect (CME) search. The Ru+Ru to Zr+Zr ratio of the CME-sensitive observable $Δγ$, normalized by elliptic anisotropy ($v_{2}$), is observed to be close to the inverse multiplicity ($N$) ratio. In other words, the ratio of the $NΔγ/v_{2}$ observable is close to the naive background baseline of unity. However, nonflow correlations are expected to cause the baseline to deviate from unity. To further understand the isobar results, we decompose the nonflow contributions to $NΔγ/ v_{2}$ (isobar ratio) into three terms and quantify each term by using the nonflow in $v_{2}$ measurement, published STAR data, and HIJING simulation. From these studies, we estimate a nonflow baseline of the isobar ratio of $N Δγ/ v_{2}$ for the CME and discuss its implications.

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arXiv2021

Investigation of Experimental Observables in Search of the Chiral Magnetic Effect in Heavy-ion Collisions in the STAR experiment

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 $γ$ 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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arXiv2021

Revisit the Chiral Magnetic Effect Expectation in Isobaric Collisions at the Relativistic Heavy Ion Collider

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$2$σ$ 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.

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arXiv2021

Two- and three-particle nonflow contributions to the chiral magnetic effect measurement by spectator and participant planes in relativistic heavy ion collisions

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 $(4\pm5)$% and $(-5\pm3)$% without and with pseudorapidity gaps, respectively, in 20-50% centrality Au+Au collisions at $\sqrt{s_{\text{NN}}}= 200 \text{ GeV}$.

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arXiv2021

HIJING can describe the anisotropy-scaled charge-dependent correlations at the Relativistic Heavy Ion Collider

The experimentally measured charge-depdendent correlations in heavy ion collisions have been suggested as a signature of the chiral magenetic effect (CME). Early model studies could not reproduce the measurement. For example, the Hijing model yielded far smaller magnitude for the charge-dependent correlation than observed in data. This led to the conclusion that the CME had to be invoked to explain the observed correlations in heavy ion collisions. In this paper we show that this conclusion of the CME interpretation is premature. We show that the reason that Hijing predicts a far smaller correlation than data is because the elliptic anisotrpy ($v_{2}$) parameter in Hijing is too small. When properly scaled, the Hijing model can reproduce in entirety the measured correlations. We also employ the AMPT model, which has a large enough $v_2$, to demonstrate that the measured data can be easily accommodated by models without invoking the CME.

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arXiv2021

Decipher the $R_{Ψ_{m}}$ correlator in search for the chiral magnetic effect in relativistic heavy ion collisions

Background: The chiral magnetic effect (CME) is extensively studied in heavy-ion collisions at RHIC and the LHC. An azimuthal correlator called $R_{Ψ_{m}}$ was proposed to measure the CME. By observing the same $R_{Ψ_{2}}$ and $R_{Ψ_{3}}$ (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 $R_{Ψ_{m}}$ 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 $R_{Ψ_{m}}$ observable using AMPT as well as toy model simulations. We compare $R_{Ψ_{m}}$ to the more widely used $Δγ$ 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 $R_{Ψ_{m}}$ 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 $R_{Ψ_{2}}$. 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 $R_{Ψ_{2}}$ to isobar collisions with the event statistics taken at RHIC. ...

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arXiv2020

Importance of non-flow background on the chiral magnetic wave search

An observable sensitive to the chiral magnetic wave (CMW) is the charge asymmetry dependence of the $π^{-}$ and $π^{+}$ anisotropic flow difference, $Δv_{n}(A_{\rm ch})$. We show that, due to non-flow correlations, the flow measurements by the Q-cumulant method using all charged particles as reference introduce a trivial linear term to $Δv_{n}(A_{\rm ch})$. The trivial slope contribution to the triangle flow difference $Δv_{3}(A_{\rm ch})$ can be negative if the non-flow is dominated by back-to-back pairs. This can explain the observed negative $Δv_{3}(A_{\rm ch})$ slope in the preliminary STAR data. We further find that the non-flow correlations give rise to additional backgrounds to the slope of $Δv_{2}(A_{\rm ch})$ from the competition among different pion sources and from the larger multiplicity dilution to $π^{+}$ ($π^{-}$) at positive (negative) $A_{\rm ch}$.

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arXiv2020

Back-to-back relative-excess observable in search for 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 ($Δγ$), 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 ($φ_{\text{BB}}$). $\textbf{Methods:}$ We use analytical calculations and toy model simulations to demonstrate the sensitivity of $r_{\text{BB}}(φ_{\text{BB}})$ to the CME and its insensitivity to backgrounds. $\textbf{Results:}$ With finite CME, the $φ_{\text{BB}}$ distribution of $r_{\text{BB}}$ shows a clear characteristic modulation. Its sensitivity to background is significantly reduced compared to the previous $Δγ$ observable. The simulation results are consistent with our analytical calculations. $\textbf{Conclusions:}$ Our studies demonstrate that the $r_{\text{BB}}(φ_{\text{BB}})$ observable is sensitive to the CME signal and rather insensitive to the resonance backgrounds.

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arXiv2019

Complications in the interpretation of the charge asymmetry dependent $π$ flow for the chiral magnetic wave

The charge asymmetry ($A_{\rm ch}$) dependence of the $π^{-}$ and $π^{+}$ elliptic flow difference, $Δv_{2}(A_{\rm ch})$, 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 $Δv_{2}(A_{\rm ch})$. The trivial slope can be negative in the triangle flow difference $Δv_{3}(A_{\rm ch})$ 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 $Δv_{2}(A_{\rm ch})$ because of the larger dilution effect to $π^{+}$ ($π^{-}$) at positive (negative) $A_{\rm ch}$. We further find that the competition between different $π$ 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 $Δv_{2}(A_{\rm ch})$. 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 $Δv_{2}(A_{\rm ch})$ and the interpretation of $Δv_{2}(A_{\rm ch})$ in terms of the CMW is delicate.

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arXiv2018

Responses of the chiral-magnetic-effect-sensitive sine observable to resonance backgrounds in heavy-ion collisions

A new sine observable, $R_{Ψ_2}(ΔS)$, has been proposed to measure the chiral magnetic effect (CME) in heavy-ion collisions; $ΔS = \left \langle \sin φ_+ \right \rangle - \left \langle \sin φ_- \right \rangle$, where $φ_\pm$ are azimuthal angles of positively and negatively charged particles relative to the reaction plane and averages are event-wise, and $R_{Ψ_2}(ΔS)$ is a normalized event probability distribution. Preliminary STAR data reveal concave $R_{Ψ_2}(ΔS)$ distributions in 200 GeV Au+Au collisions. Studies with a multiphase transport (AMPT) and anomalous-viscous Fluid Dynamics (AVFD) models show concave $R_{Ψ_2}(ΔS)$ 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 $R_{Ψ_2}(ΔS)$ depends sensitively on the resonance $v_2$ (which yields different numbers of decay $π^+π^-$ pairs in the in-plane and out-of-plane directions) and $p_T$ (which affects the opening angle of the decay $π^+π^-$ 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 $R_{Ψ_2}(ΔS)$. Supplemental studies of $R_{Ψ_3}(ΔS)$ 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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