arXiv subjects

Fuqiang Wang

Publications and source records attributed to Fuqiang Wang.

At least 55 records · Page 3Linked to original sources

arXiv2020

Beam energy dependence of cumulants of the net-baryon, net-charge and deuteron multiplicity distributions in Au+Au collisions at $\sqrt{s_{NN}}=3.0-5.0$ GeV

Within the ultra-relativistic quantum molecular dynamics (UrQMD) model, in which the Lorentz-covariant treatment of nuclear mean-field potential is considered, the fluctuations of net-baryon, net-charge and deuterons multiplicity distributions in Au+Au head-on collisions at $\sqrt{s_{NN}}=3.0-5.0$ GeV are calculated. The results show that the nuclear mean-field potential can significantly enhance the magnitude of baryon number fluctuations in narrow rapidity windows, and this enhancement rapidly weakens with increasing beam energy. However, for proton and net-charge number fluctuations, the mean-field effects are less noticeable than that for baryon number. In addition, for net-charge number fluctuations, the negative binomial distribution agrees well with the calculated results at mid-pseudorapidity window. Finally, the event-by-event fluctuations of deuteron number in the coalescence production picture are calculated as well, it is found that its cumulant ratios decrease linearly with increasing the average multiplicity of deuterons per event, i.e., increase with increasing beam energy.

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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

Heavy quark flow as better probes of QGP properties

In earlier studies we have proposed that most parton $v_2$ comes from the anisotropic escape of partons, not from the hydrodynamic flow, even for semi-central Au+Au collisions at $\sqrt {s_{NN}}=200$ GeV. Here we study the flavor dependence of this escape mechanism with a multi-phase transport model. In contrast to naive expectations, we find that the charm $v_2$ is much more sensitive to the hydrodynamic flow than the lighter quark $v_2$, and the fraction of $v_2$ from the escape mechanism decreases strongly with the quark mass for large collision systems. We also find that the light quark collective flow is essential for the charm quark $v_2$. Our finding thus suggests that heavy quark flows are better probes of the quark-gluon-plasma properties than light quark flows.

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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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arXiv2019

Data-driven subtraction of anisotropic flows in jet-like correlation studies in heavy-ion collisions

Measurements of two-particle azimuthal angle correlations are a useful tool to study the distribution of jet energy loss, however, they are complicated because of the significant anisotropic flow background. We devise a data-driven method for subtracting anisotropic flow background in jet-like correlation analysis. We first require a large recoil momentum ($P_x$) within a given pseudo-rapidity ($η$) range from a high-transverse momentum particle to enhance in-acceptance population of away-side jet-like correlations. Then we take the difference of two-particle correlations in the close-region and far-region with respect to the $η$ region of $P_x$ to subtract the anisotropic flow background. We use a toy model which contains only anisotropic flow and PYTHIA8 which have jets to demonstrate the validity of our data-driven method. The results indicate that the data-driven method can subtract anisotropic flow effectively.

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arXiv2019

Experimental searches for the chiral magnetic effect in heavy-ion collisions

The chiral magnetic effect (CME) in quantum chromodynamics (QCD) refers to a charge separation (an electric current) of chirality imbalanced quarks generated along an external strong magnetic field. The chirality imbalance results from interactions of quarks, under the approximate chiral symmetry restoration, with metastable local domains of gluon fields of non-zero topological charges out of QCD vacuum fluctuations. Those local domains violate the $\mathcal{P}$ and $\mathcal{CP}$ invariance, potentially offering a solution to the strong $\mathcal{CP}$ problem in explaining the magnitude of the matter-antimatter asymmetry in today's universe. Relativistic heavy-ion collisions, with the likely creation of the high energy density quark-gluon plasma and restoration of the approximate chiral symmetry, and the possibly long-lived strong magnetic field, provide a unique opportunity to detect the CME. Early measurements of the CME-induced charge separation in heavy-ion collisions are dominated by physics backgrounds. Major efforts have been devoted to eliminate or reduce those backgrounds. We review those efforts, with a somewhat historical perspective, and focus on the recent innovative experimental undertakings in the search for the CME in heavy-ion collisions.

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arXiv2019

Multi-phase transport model predictions of isobaric collisions with nuclear structures from density functional theory

Isobaric $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr collisions were performed at the Relativistic Heavy Ion Collider in 2018. Using the "a multi-phase transport" model with nuclear structures calculated by the density functional theory (DFT), we make predictions for the charged hadron multiplicity distributions and elliptic azimuthal anisotropies in these collisions. Emphases are put on the relative differences between the two collision systems that can decisively discriminate DFT nuclear distributions from the commonly used Woods-Saxon densities.

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arXiv2019

Momentum anisotropy in nuclear collisions from quantum mechanics

We point out that the intrinsic relationship between space and momentum in quantum physics through the uncertainty principle has potential implications for momentum anisotropy in heavy-ion collisions. Using a harmonic oscillator potential we calculate the elliptic anisotropy and find it to be sizeable compared to elliptic flow measurements in nuclear collisions. Our results question the validity of the completely hydrodynamic interpretation of anisotropic flow data, and highlight the importance of including quantum physics in hydrodynamic calculations which has largely been neglected so far.

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arXiv2019

The effect of internal magnetic field on collective flow in heavy ion collisions at intermediate energies

The properties of nuclear matter under extreme conditions of high temperature, density and isospin-asymmetry have attracted wide attentions in recent years. At present, heavy ion reactions in combination with corresponding model simulations are one of the most important ways to investigate this subject. It is known that a strong magnetic field can be created in heavy ion collisions. However, its effect on the motion of charged particles is usually neglected in previous transport model simulations. In this work, within the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, the temporal evolution and spatial distribution of the internal magnetic field are calculated. The magnetic field strength is found to reach about $eB\approx470$ MeV$^{2}$ ($B\approx8\times10^{16}$ G) for Au+Au collisions at $E_{\text{lab}}$=1 GeV/nucleon with impact parameter of 7 fm. The magnetic field in Cu+Au collisions exhibits somewhat different spatial distribution from that in Au+Au collisions. The magnetic field is found to affect the directed flow of pions at forward and backward rapidities to some extent, dependent of the impact parameter and beam energy while the effect on the elliptic flow is small. This suggests that, because $π$ mesons produced in heavy ion collisions at intermediate energies are considered as a sensitive probe for the nuclear symmetry energy, it is necessary to consider the effect of the internal magnetic field.

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arXiv2019

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 ($Δγ$) 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 $Δγ$ 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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arXiv2019

Elliptical flow coalescence to identify the $f_{0}$(980) content

We use a simple coalescence model to generate $f_{0}$(980) particles for three configurations: a ${s\bar{s}}$ meson, a ${u\bar{u}s\bar{s}}$ tetraquark, and a ${K^{+}K^{-}}$ molecule. The phase-space information of the coalescing constituents is taken from a multi-phase transport (AMPT) simulation of heavy-ion collisions. It is shown that the number of constituent quarks scaling of the elliptic flow anisotropy can be used to discern ${s\bar{s}}$ from ${u\bar{u}s\bar{s}}$ and ${K^{+}K^{-}}$ configurations.

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arXiv2018

Charm quarks are more hydrodynamic than light quarks in final-state elliptic flow

We study the charm quark elliptic flow ($v_{2}$) in heavy ion as well as small system collisions by tracking the evolution history of quarks of different flavors within a multi-phase transport model. The charm quark $v_{2}$ is studied as a function of the number of collisions the charm quark suffers with other quarks and then compared to the $v_{2}$ of lighter quarks. We find that the common escape mechanism is at work for both the charm and light quark $v_{2}$. However, contrary to the naive expectation, the hydrodynamics-type flow is found to contribute more to the final state charm $v_{2}$ than light quark $v_{2}$. This could be explained by the smaller average deflection angle the heavier charm quark undergoes in each collision, so that heavy quarks need more scatterings to accumulate a significant $v_{2}$, while lighter quarks can more easily change directions with scatterings with their $v_{2}$ coming more from the escape mechanism. Our finding thus suggests that the charm $v_{2}$ is a better probe for studying the hydrodynamic properties of the quark-gluon plasma.

hep-ph↗

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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arXiv2018

A novel invariant mass method to isolate resonance backgrounds from the chiral magnetic effect

The Chiral Magnetic Effect (CME) refers to charge separation along a strong magnetic field, due to topological charge fluctuations in QCD. Charge correlation ($Δγ$) signals consistent with CME have been first observed almost a decade ago. It has also been known since then that the $Δγ$ is contaminated by a major background from resonance decays coupled with elliptic flow. In this contribution, we propose differential $Δγ$ measurements as function of the pair invariant mass ($m_{\rm inv}$). The $Δγ$ in the high $m_{\rm inv}$ region is essentially free of resonance backgrounds. In the low $m_{\rm inv}$ region, the $Δγ$ backgrounds show resonance peaks. The CME signal, presumably smooth in $m_{\rm inv}$, may thus be extracted from a two-component model fit. We demonstrate the feasibility and effectiveness of this novel method by using the AMPT and toy-model Monte-Carlo simulations. We also discuss an application of the method in data analysis.

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arXiv2018

Importance of isobar density distributions on the chiral magnetic effect search

Under the approximate chiral symmetry restoration, quark interactions with topological gluon fields in quantum chromodynamics can induce chirality imbalance and parity violation in local domains. An electric charge separation ({\sc cs}) could be generated along the direction of a strong magnetic field ({\bf B}), a phenomenon called the chiral magnetic effect ({\sc cme}). {\sc cs} measurements by azimuthal correlators are contaminated by a major background from elliptic flow anisotropy ($v_2$). Isobaric $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr collisions have been proposed to identify the {\sc cme} (expected to differ between the two systems) out of the background (expected to be almost the same). We show, by using the density-functional calculated proton and neutron distributions, that these expectations may not hold as originally anticipated, because the two systems may have sizable differences in eccentricity and $v_2$ and because their difference in {\bf B} may suffer from large uncertainties.

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arXiv2018

Re-examining the premise of isobaric collisions and a novel method to measure the chiral magnetic effect

In this proceeding we will show that the expectations of the isobaric $^{96}_{44}\mathrm{Ru}+^{96}_{44}\mathrm{Ru}$ and $^{96}_{40}\mathrm{Zr}+^{96}_{40}\mathrm{Zr}$ collisions on chiral magnetic effect (CME) search may not hold as originally anticipated due to large uncertainties in the isobaric nuclear structures. We demonstrate this using Woods-Saxon densities and the proton and neutron densities calculated by the density functional theory. Furthermore, a novel method is proposed to gauge background and possible CME contributions in the same system, intrinsically better than the isobaric collisions of two different systems. We illustrate the method with Monte Carlo Glauber and AMPT (A Multi-Phase Transport) simulations.

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arXiv2018

Inversion of stellar spectral radiative properties based on multiple star catalogues

The spectral flux density of stars can indicate their atmospheric physical properties. A detector can obtain any band flux density at the design stage. However, the band flux density is confirmed and fixed in the process of operation because of the restriction of filters. Other band flux densities cannot be obtained through the same detector. In this study, a computational model of stellar spectral flux density is established based on basic physical parameters which are effective temperature and angular parameter. The stochastic particle swarm optimization algorithm is adopted to address this issue with appropriately chosen values of the algorithm parameters. Four star catalogues are studied and consist of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), Wide-field Infrared Survey Explorer (WISE), Midcourse Space Experiment (MSX), and Two Micron All Sky Survey (2MASS). The given flux densities from catalogues are input parameters. Stellar effective temperatures and angular parameters are inversed using the given flux densities according to SPSO algorithm. Then the flux density is calculated according to Planck's law on the basis of stellar effective temperatures and angular parameters. The calculated flux density is compared with the given value from catalogues. It is found that the inversion results are in good agreement for all bands of the MSX and 2MASS catalogues, whereas they do not agree well in some bands of the LAMOST and WISE catalogues. Based on the results, data from the MSX and 2MASS catalogues can be used to calculate the spectral flux density at different wavelengths of given wavelength ranges. The stellar flux density is obtained and can provide data support and an effective reference for detection and recognition of stars.

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