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Register a new userCharge-dependent transverse momentum and its impact on the search for the chiral magnetic wave
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The chiral magnetic wave (CMW) is sought using the charge asymmetry ($A_{rm ch}$) dependence of anisotropic flow in heavy-ion collisions. The charge dependent transverse momentum ($p_{rm T}$), however, could play a role as a background. With the string fragmentation models, including PYTHIA, we demonstrate the origin of the $A_{rm ch}-p_{rm T}$ correlation and its connection with the local charge conservation (LCC). The impact of $A_{rm ch}-p_{rm T}$ and its behavior in varied kinematic windows are also discussed. This study provides more insights for the search for the CMW and comprehending the collective motion of the quark-gluon plasma.
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The charge asymmetry ($A_{rm ch}$) dependence of the $pi^{-}$ and $pi^{+}$ elliptic flow difference, $Delta v_{2}(A_{rm ch})$, 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 $Delta v_{2}(A_{rm ch})$. The trivial slope can be negative in the triangle flow difference $Delta v_{3}(A_{rm ch})$ 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 $Delta v_{2}(A_{rm ch})$ because of the larger dilution effect to $pi^{+}$ ($pi^{-}$) at positive (negative) $A_{rm ch}$. We further find that the competition between different $pi$ 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 $Delta v_{2}(A_{rm ch})$. 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 $Delta v_{2}(A_{rm ch})$ and the interpretation of $Delta v_{2}(A_{rm ch})$ in terms of the CMW is delicate.
The charge asymmetry (Ach) dependence of anisotropic flow serves as an important tool to search for the chiral magnetic wave (CMW) in heavy-ion collisions. However, the background effect, such as the local charge conservation (LCC) entwined with collective flow, has not yet been unambiguously eliminated in the measurement. With the help of two models, the AMPT with initial quadrupole moment and the blast wave (BW) incorporating LCC, we discuss the features of the LCC-induced and the CMW-induced correlations between Ach and the flow. More importantly, we first propose to use the Event Shape Engineering (ESE) technique to distinguish the background and the signal for the CMW study. This method would be highly desirable in the experimental search for the CMW and provides more insights for understanding the charge-dependent collective motion of the quark-gluon plasma.
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.
In order to describe the hadronization of polarized quarks, we discuss an extension of the quark-jet model to transverse momentum dependent fragmentation functions. The description is based on a product ansatz, where each factor in the product represents one of the transverse momentum dependent splitting functions, which can be calculated by using effective quark theories. The resulting integral equations and sum rules are discussed in detail for the case of inclusive pion production. In particular, we demonstrate that the 3-dimensional momentum sum rules are satisfied naturally in this transverse momentum dependent quark-jet model. Our results are well suited for numerical calculations in effective quark theories, and can be implemented in Monte-Carlo simulations of polarized quark hadronization processes.
$textbf{Background:}$ 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 ($Deltagamma$), 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 ($varphi_{text{BB}}$). $textbf{Methods:}$ We use analytical calculations and toy model simulations to demonstrate the sensitivity of $r_{text{BB}}(varphi_{text{BB}})$ to the CME and its insensitivity to backgrounds. $textbf{Results:}$ With finite CME, the $varphi_{text{BB}}$ distribution of $r_{text{BB}}$ shows a clear characteristic modulation. Its sensitivity to background is significantly reduced compared to the previous $Deltagamma$ observable. The simulation results are consistent with our analytical calculations. $textbf{Conclusions:}$ Our studies demonstrate that the $r_{text{BB}}(varphi_{text{BB}})$ observable is sensitive to the CME signal and rather insensitive to the resonance backgrounds.
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