Within a multi-phase transport (AMPT) model with the string melting mechanism and imported initial electric quadrupole moment, the difference between the elliptic flow of positive and negative pions is calculated. The slope parameter $r$ of the linear dependence of $Δv_{2}=v_{2}(π^{-})-v_{2}(π^{+})$ on $A_{ch}=(N^{+}-N^{-})/(N^{+}+N^{-})$ is yielded owing to the parton cascade, which converts the initial electric quadrupole distribution into the final charge-dependent elliptic flow. The slope parameter $r$ is found to be increased by the hadronization given by the coalescence, and decreased by the resonance decays. The slope parameter $r$ is very sensitive to both the initial electric quadrupole percentage and centrality bin and consequently a helpful constraint on the quadrupole moment of the chiral magnetic wave is obtained for Au+Au collisions at the top RHIC energy.
Recently, the STAR collaboration at RHIC has presented experimental evidence for the correlation between the elliptic flow difference of charged pions and charge asymmetry as a possible signal of the chiral magnetic wave. We demonstrate that the STAR results can be understood within the standard viscous hydrodynamics.
In this letter, we study the pion momentum distribution of a coherent source and investigate the influences of coherent emission on the pion transverse-momentum ($p_T$) spectrum and elliptic anisotropy. With a partially coherent source, constructed by a conventional viscous hydrodynamics model (chaotic part) and a parameterized expanding coherent source model, we reproduce the pion $p_T$ spectrum and elliptic anisotropy coefficient $v_2(p_T)$ in the peripheral Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. It is found that the influences of coherent emission on the pion $p_T$ spectrum and $v_2(…
Cross section and single transverse target spin asymmetry for backward pion electroproduction
核子-介子跃迁分布振幅（TDA）是核子-介子态之间非定域三夸克算符的非对角矩阵元，它是在共线因式分解描述的介子从核子反向排斥电生过程中产生的.下面我们将讨论π介子到核子时间分辨模型的问题。我们提出了一个因式分解的四重分布与软π介子定理的输入π N TDAs的。为了满足多项式性质在其完整的形式的频谱表示补充与D-项一样的贡献，从核子交换的u-通道的反应。我们提出了我们的估计后向π介子电产生非极化截面和它的横向目标单自旋不对称性在我们的复合模型为π N TDAs。
We calculate the energy dependence of the asymmetry in the cross sections for circularly polarized photons on an unpolarized deuteron target in $d\vecγ \to np$ in pionless effective field theory. By matching the parity-violating low-energy constants to different sets of corresponding model parameters we obtain estimates for the asymmetry. In addition we calculate two possible figures of merit for the asymmetry in order to assess the preferred photon energy at which to perform a possible future experiment at a high-intensity photon source.
Non-identical particle femtoscopy measures the size of the system emitting particles ("radius") in heavy-ion collisions as well as the difference between mean emission space-time coordinates of two particle species ("emission asymmetry"). The system created in such collisions at the LHC behaves collectively and its dynamics is well described by hydrodynamic models. A significant emission asymmetry between pions and kaons, coming from collective flow, enhanced by contribution from flowing resonances is predicted. We present calculations within the (3+1)D viscous hydrodynamic model coupled to st…
Based on the framework of an extended multiphase transport model with mean-field potentials in both the partonic phase and the hadronic phase, we explain the elliptic flow difference between $π^+$ and $π^-$ in the beam-energy scan program at the relativistic heavy-ion collider by incorporating the vector-isovector potential for quarks and antiquarks with different isospins. It is found that the isospin splitting of pion elliptic flow favors a strong vector-isovector interaction, and thus serves as a probe of the quark matter equation of state as well as the QCD phase structure at finite baryon…
Despite the success of modern nuclear parton distribution functions (nPDFs) in describing nuclear hard-process data, they still suffer from large uncertainties. One of the poorly constrained features is the possible asymmetry in nuclear modifications of valence $u$ and $d$ quarks. We study the possibility of using pion-nucleus Drell-Yan dilepton data as a new constraint in the global analysis of nPDFs. We find that the nuclear cross-section ratios from the NA3, NA10 and E615 experiments can be used without imposing significant new theoretical uncertainties and, in particular, that these datase…
Based on a chiral approach to nuclear matter, we calculate the quartic term in the expansion of the equation of state of isospin-asymmetric nuclear matter. The contributions to the quartic isospin asymmetry energy $A_4(k_f)$ arising from $1π$-exchange and chiral $2π$-exchange in nuclear matter are calculated analytically together with three-body terms involving virtual $Δ(1232)$-isobars. From these interaction terms one obtains at saturation density $ρ_0 = 0.16\,$fm$^{-3}$ the value $A_4(k_{f0})= 1.5\,$MeV, more than three times as large as the kinetic energy part. Moreover, iterated $1π$-exch…
Non-perturbative flavor asymmetry in the nucleon and deuteron: The light-front Hamiltonian effective field theory approach
University of Science and Technology of China（中国科学技术大学）；Hunan University（湖南大学）；University of Pittsburgh（匹兹堡大学）；Anhui Center for Fundamental Sciences in Theoretical Physics（安徽基础科学前沿中心）；Institute of Modern Physics, Chinese Academy of Sciences（中国科学院近代物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Iowa State University（爱荷华州立大学）；Advanced Energy Science and Technology Guangdong Laboratory（广东先进能源科学与技术实验室）
我们比较微观输运模型计算最近的数据在2 - 10 A GeV的金+金和铅（158 A GeV）铅碰撞的各种强子的定向和椭圆流。对于金+金激发函数的过渡，从挤出到面内增强发射的一致描述与平均场电位对应于一个不可压缩性。对于Pb（158 AGeV）Pb系统，质子和π介子的椭圆流都倾向于面内发射，质子的定向流与π介子的定向流相反，表现为反流。强定向横向流动是目前质子和拉姆达在金（6 A GeV）金碰撞以及。对于SPS和AGS能量，数据与计算之间的一致性是显著的。
Constraining the initial temperature and shear viscosity in a hybrid hydrodynamic model of $\sqrt{s_{NN}}$=200 GeV Au+Au collisions using pion spectra, elliptic flow, and femtoscopic radii
We systematically investigate the pion transverse momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in the granular source model of quark-gluon plasma droplets in ultra-relativistic heavy ion collisions. The granular source model can well reproduce the experimental results of the Au-Au collisions at $\sqrt{s_{NN}}=$ 200 GeV and the Pb-Pb collisions at $\sqrt{s_{NN}} =$ 2.76 TeV with different centralities. We examine the parameters of the granular source models with an uniform and Woods-Saxon initial energy distributions in a droplet. The parameters exhibit certain…
Even-by-event hydrodynamical simulations for $\sqrt{s}_{NN}$=200 GeV Au+Au collisions and the correlation between flow coefficients and initial asymmetry measures
We examine the evolution of quark-gluon plasma (QGP) droplets with viscous hydrodynamics and analyze the pion transverse-momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in a granular source model consisting of viscous QGP droplets. The shear viscosity of the QGP droplet speeds up and slows down the droplet evolution in the central and peripheral regions of the droplet, respectively. The effect of the bulk viscosity on the evolution is negligible. Although there are viscous effects on the droplet evolution, the pion momentum spectrum and elliptic flow change littl…
At the early stage of heavy ion collisions, non-trivial topologies of the gauge fields can be created resulting in an imbalance of axial charge density and eventually separation of electric charges along the direction of the magnetic field produced in such collisions. This process is called the chiral magnetic effect (CME). In this work we implement such a charge separation at the partonic level in AMPT for Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV to study its consequence on experimental observables. We present the effects on the pion elliptic flow ($v_2$) and the charged particle balance…
We study the phase diagram and the thermodynamic properties of QCD at nonzero isospin asymmetry at physical quark masses with staggered quarks. In particular, continuum results for the phase boundary between the normal and the pion condensation phases and the chiral/deconfinement transition are presented. Our findings indicate that the pion condensation phase is restricted to $T\lesssim170$~MeV for isospin chemical potentials up to 325~MeV. We also use the data to test the range of validity of the Taylor expansion method and show first results for the equation of state.
We systematically investigate the pion transverse momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in the granular source model for the heavy ion collisions of Au-Au at $\sqrt{s_{NN}}=$ 200 GeV and Pb-Pb at $\sqrt{s_{NN}}=$ 2.76 TeV with different centralities. The granular source model can well reproduce the experimental results of the heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). We examine the parameters involved in the granular source model. The experimental data of the momentum spectrum, elliptic flow,…
The two-photon-exchange (TPE) effects in the process $e^+e^- \rightarrow π^+π^-$ at large momentum transfer are discussed within the perturbative QCD (pQCD). The contributions from the twist-2 and twist-3 distribution amplitudes (DAs) of pion are considered in the estimation. Different with the results under the one-photon-exchange (OPE) approximation, the TPE effects result in an asymmetry of the differential cross section on the scattering angle. The precise measurement of this asymmetry by the further experiment is an precise test of pQCD at large momentum transfer. The time-like electromag…
Based on the ultra-relativistic quantum molecular dynamics (UrQMD) model, the impacts of momentum dependent interaction, symmetry energy and near-threshold $NN\to NΔ$ cross sections on isospin sensitive collective flow and pion observables are investigated. Our results confirm that the elliptic flow of neutrons and charged particles, i.e. $v_2^n$ and $v_2^{ch}$, are sensitive to the strength of momentum dependence interaction and the elliptic flow ratio, i.e., $v_2^n/v_2^{ch}$, is sensitive to the stiffness of symmetry energy. For describing the pion multiplicity near the threshold energy, acc…
Based on the newly updated version of the ultrarelativistic quantum molecular dynamics (UrQMD) model, the pion potentials obtained from the in-medium dispersion relation of the $Δ$-hole model and from the modified phenomenological approach are further introduced. Both the rapidity $y_0$ and transverse-velocity $u_{t0}$ dependence of directed $v_1$ and elliptic $v_2$ flows of $π^{+}$ and $π^{-}$ charged mesons produced from Au+Au collisions at two beam energies of 1.0 GeV/nucleon and 1.5 GeV/nucleon and within a large centrality region of $0<b_0<0.55$ are scanned. Calculations with pion potenti…
The elliptic flow splitting $Δv_2$ between $\bar{u}$ and $u$ quarks as well as between $π^-$ and $π^+$ in midcentral Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV has been studied, based on the framework of an extended multiphase transport model with the partonic evolution described by the chiral kinetic equations of motion. Within the available statistics, the slope of $Δv_2$ between $\bar{u}$ and $u$ quarks with respect to the electric charge asymmetry $A_{ch}$ from the linear fit is found to be negative, due to the correlation between the velocity and the coordinate in the initial parton phas…
The elliptic, $v_2$, triangular, $v_3$, and quadrangular, $v_4$, azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range $|η|<0.8$ at different collision centralities and as a function of transverse momentum, $p_{\rm T}$, out to $p_{\rm T}=20$ GeV/$c$. The observed non-zero elliptic and triangular flow depends only w…
We study beam-transverse and normal single-spin asymmetry for electron-proton inelastic scattering in the resonance region for the case where a pion is observed in the final state. There is no beam single-spin asymmetry if the observed particle has single-photon exchange as its only interaction, but this is naturally circumvented when one observes a strongly interacting final state particle. The asymmetries are not large as there is an electron mass divided by momentum transfer suppression. We present results for the cases where the final electron and a pion are both observed, with asymmetries…
The QCD phase diagram is studied in the presence of an isospin asymmetry using continuum extrapolated staggered quarks with physical masses. In particular, we investigate the phase boundary between the normal and the pion condensation phases and the chiral/deconfinement transition. The simulations are performed with a small explicit breaking parameter in order to avoid the accumulation of zero modes and thereby stabilize the algorithm. The limit of vanishing explicit breaking is obtained by means of an extrapolation, which is facilitated by a novel improvement program employing the singular va…
