Event Shape Engineering Research Articles

Event shape engineering via Glauber MC model

In our exploration of event shape engineering, the Glauber model served as a foundational tool for under- standing the anisotropic geometry of the Quark-Gluon Plasma (QGP). Utilizing the TGlauberMC-3.2 model within ROOT, we systematically analyzed one million events. From the 2 & Npart plot, our data revealed an average maximum dN value of 12.00 with associated parameters: 2 = 0.91, 2 =2.74, 3 =0.91 and Npart = 14.00. These findings illuminate the distinct configurations that yield the most pronounced anisotropic geometries of the QGP, providing insights into optimizing event shape configurations.

New measurements of inclusive jet suppression and jet ν2 in Pb–Pb collisions at √SNN = 5.02 TeV with ALICE

We report measurements of the inclusive charged-particle jet yield in central Pb–Pb collisions at √SNN = 5.02 TeV. Uncorrelated background is suppressed by a novel mixed-event technique, enabling extension of the jet RAA measurement down to pT = 13.5 GeV/c, with kinematic overlap with RHIC jet measurements. We also present measurements of inclusive charged-particle jet v2 in semi-central Pb–Pb collisions at √SNN = 5.02 TeV, and azimuthal dependence of jet yield suppression for event topologies selected using eventshape engineering.

Search for anomalous chiral effects in heavy-ion collisions with ALICE

The interplay between the chiral anomaly and the strong magnetic or vortical fields created in noncentral heavy-ion collisions can lead to various anomalous chiral effects in the quark–gluon plasma, including the chiral magnetic effect (CME), the chiral magnetic wave (CMW), and the chiral vortical effect (CVE). In this proceeding, recent ALICE measurements of these effects are summarized. Utilizing Event Shape Engineering, fractions of CME and CMW signals extracted in Pb–Pb collisions at √SNN = 5.02 TeV are consistent with zero within uncertainties. The CVE is studied using azimuthal correlations between baryon pair Λ–p with Λ–h and h–h as reference. These measurements provide new insights into the experimental search for anomalous chiral effects in heavy-ion collisions.

Probing the chiral magnetic wave with charge-dependent flow measurements in Pb-Pb collisions at the LHC

The Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision sNN\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s_{\ extrm{NN}}} $$\\end{document} = 5.02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic (v2) and triangular (v3) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope r3Norm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {r}_3^{\ extrm{Norm}} $$\\end{document} is found to be larger than zero and to have a magnitude similar to r2Norm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {r}_2^{\ extrm{Norm}} $$\\end{document}, thus pointing to a large background contribution for these measurements. Furthermore, r2Norm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {r}_2^{\ extrm{Norm}} $$\\end{document} can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW (fCMW) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for fCMW, and in the 10–60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level.

Probing initial geometrical anisotropy and final azimuthal anisotropy in heavy-ion collisions at Large Hadron Collider energies through event-shape engineering

Anisotropic flow is accredited to have effects from the initial state geometry and fluctuations in the nuclear overlap region. The elliptic flow (${v}_{2}$) and triangular flow (${v}_{3}$) coefficients of the final state particles are expected to have influenced by eccentricity (${ϵ}_{2}$) and triangularity (${ϵ}_{3}$) of the participants, respectively. In this work, we study ${v}_{2}$, ${v}_{3}$, ${ϵ}_{2}$, ${ϵ}_{3}$, and the correlations among them with respect to event topology in the framework of a multiphase transport model (AMPT). We use transverse spherocity and reduced flow vector as event shape classifiers in this study. Transverse spherocity has the unique ability to separate events based on geometrical shapes, i.e., jetty and isotropic, which pertain to pQCD and non-pQCD domains of particle production in high-energy physics, respectively. We use the two-particle correlation method to study different anisotropic flow coefficients. We confront transverse spherocity with a more widely used event shape classifier--reduced flow vector (${q}_{n}$) and they are found to have significant (anti)correlations among them. We observe significant spherocity dependence on ${v}_{2}$, ${v}_{3}$, and ${ϵ}_{2}$. This work also addresses transverse momentum dependent crossing points between ${v}_{2}$ and ${v}_{3}$, which varies for different centrality and spherocity percentiles.

Search for the Chiral Magnetic Wave in Pb–Pb collisions using the ALICE detector

The interplay of the chiral anomaly and the strong magnetic field (∼1015 T) created in heavy-ion collisions could give rise to a collective excitation in the quark–gluon plasma called the Chiral Magnetic Wave (CMW). This effect can be experimentally sought by the charge asymmetry (Ach) dependence of elliptic flow v2. However, non-CMW mechanisms such as local charge conservation (LCC) coupled with collective flow can also lead to a similar dependence of v2 on Ach. The triangular flow (v3) measurement serves as a reference as it is not expected to be affected by the CMW. The v2 and v3 of charged hadrons as a function of Ach measured in Pb–Pb collisions at √SNN=2.76 and 5.02 TeV are presented. In addition, the event-shape engineering (ESE) technique is adopted for the first time to quantitatively disentangle the CMW signal and the LCC background. The results indicate that the background effects dominate the CMW measurements.

Hard probe path lengths and event-shape engineering of the quark-gluon plasma

As particles traverse the quark-gluon plasma (QGP) formed during a heavy ion collision they undergo energy loss depending on the distance traveled. We study several temperature- and velocity-weighted path length distributions of non-interacting particles as they traverse the plasma using the Trajectum heavy ion code, including those of back-to-back path lengths. We use event-shape engineering (ESE) in combination with in-plane versus out-of-plane selection to accurately control these path lengths. Lastly, we show how soft observables depend on the different ESE classes.

Event-shape engineering analysis of D meson in ultrarelativistic heavy-ion collisions

We describe the propagation of charm quarks in the quark-gluon plasma (QGP) by means of an event-by-event transport approach. In our calculations the non-perturbative interaction between heavy quarks and light partons has been taken into account through a quasi-particle approach with thermal light quark masses tuned to reproduce lQCD thermodynamics. We found that the flow observables v_2 and v_3 of D mesons are comparable with the experimental measurements for Pb + Pb collisions at 5.02 TeV in different ranges of centrality selections. The results are analyzed with event-shape engineering technique. The comparison of the anisotropic flow coefficients v_n with experimental data show a quite good agreement with experimental data for different flow vector q_2 selections, which confirms the strong coupling between charm quarks and light quarks in the QCD matter. Furthermore, we present here a novel study of the event-by-event correlations between flow harmonics of D mesons and soft hadrons at LHC energy with the event-shape engineering technique that can put further constraints on heavy quark transport coefficients toward a solid comparison between the phenomenological determination and the lattice QCD calculations.

Utilization of event shape in search of the chiral magnetic effect in heavy-ion collisions

The search for the chiral magnetic effect (CME) has been a subject of great interest in the field of high-energy heavy-ion collision physics, and various observables have been proposed to probe the CME. Experimental observables are often contaminated with background contributions arising from collective motions (specifically elliptic flow) of the collision system. We present a method study of event-shape engineering (ESE) that projects the CME-sensitive $\gamma_{112}$ correlator and its variations ($\gamma_{132}$ and $\gamma_{123}$) to a class of events with minimal flow. We discuss the realization of the zero-flow mode, the sensitivity on the CME signal, and the corresponding statistical significance for Au+Au, Ru+Ru, and Zr+Zr collisions at $\sqrt{s_{\rm NN}} = 200$ GeV with a multiphase transport (AMPT) model, as well as a new event generator, Event-By-Event Anomalous-Viscous Fluid Dynamics (EBE-AVFD).

Interpreting the charge-dependent flow and constraining the chiral magnetic wave with event shape engineering

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.

Manipulating strong electromagnetic fields with the average transverse momentum of relativistic nuclear collisions

We show that an event-shape engineering based on the mean transverse momentum of charged hadrons, [p_t], provides an optimal handle on the strength of the magnetic field created in central heavy-ion collisions at high energy. This is established through quantitative evaluations of the correlation existing between the event-by-event magnetic field produced by the spectator protons in 5.02 TeV Pb + Pb collisions and the event-by-event [p_t] at a given collision centrality. We argue that the event selection based on [p_t] provides a better handle on the magnetic field than the more traditional selection based on the event ellipticities. Advantages brought by this new method for the experimental search of the chiral magnetic effect are discussed.

Deciphering the RΨm 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 $R_{\Psi_{m}}$ 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 $\Delta\gamma$ 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. ...

Event shape engineering and multiplicity dependent study of identified particle production in proton + proton collisions at = 13 TeV using PYTHIA8

Small system collectivity observed at the LHC energies along with enhancement of strangeness makes high-multiplicity proton + proton (pp) collisions very interesting in order to look for QGP-like features, usually found in heavy-ion collisions. It may be interesting to perform a double differential study of different observables in pp collisions in terms of charged particle multiplicity and event shape in order to understand the new dimensions in high-multiplicity pp physics. We study the correlation between the number of multi-partonic interactions (nMPI), event shape (transverse spherocity) and charged particle multiplicity classes. For the first time, we report the simulation results on the spherocity and charged particle multiplicity dependent study of (π + + π −), (K+ + K−), (p + ), K∗0, ϕ and () production in pp collisions at = 13 TeV using PYTHIA8. We explore the event shape and charged particle multiplicity dependence of the transverse momentum (p T) spectra, integrated yield, mean transverse momentum (⟨p T⟩) and particle ratios of the identified particles. This study provides a baseline for exploring the event topology and final state multiplicity dependence of identified particle production in the LHC pp collisions.

Transverse-momentum and event-shape dependence of D-meson flow harmonics in Pb–Pb collisions at [formula omitted

The elliptic and triangular flow coefficients v2 and v3 of prompt D0, D+, and D⁎+ mesons were measured at midrapidity (|y|<0.8) in Pb–Pb collisions at the centre-of-mass energy per nucleon pair of sNN=5.02TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays in the transverse momentum interval 1<pT<36GeV/c in central (0–10%) and semi-central (30–50%) collisions. Compared to pions, protons, and J/ψ mesons, the average D-meson vn harmonics are compatible within uncertainties with a mass hierarchy for pT≲3GeV/c, and are similar to those of charged pions for higher pT. The coupling of the charm quark to the light quarks in the underlying medium is further investigated with the application of the event-shape engineering (ESE) technique to the D-meson v2 and pT-differential yields. The D-meson v2 is correlated with average bulk elliptic flow in both central and semi-central collisions. Within the current precision, the ratios of per-event D-meson yields in the ESE-selected and unbiased samples are found to be compatible with unity. All the measurements are found to be reasonably well described by theoretical calculations including the effects of charm-quark transport and the recombination of charm quarks with light quarks in a hydrodynamically expanding medium.

A method to test the coupling strength of the linear and nonlinear contributions to higher-order flow harmonics via event shape engineering

A multi-phase transport model is used to study the efficacy of shape-engineered events to delineate the degree of coupling between the linear and nonlinear contributions to the higher-order flow harmonics v 4 and v 5. The study shows that the nonlinear contributions are strongly shape-dependent while the linear contributions are shape-independent, indicating little if any, coupling between the linear and nonlinear flow coefficients. The experimental verification of such patterns could be an invaluable tool for robust extraction of the linear and mode-coupled flow coefficients, especially for beam energies where the charged particle multiplicity and the event statistics precludes the use of current methods to establish the coupling strength.

Recent results on azimuthal anisotropies of open heavy-flavour particles with ALICE at the LHC

Heavy quarks are unique probes to investigate the properties of the Quark-Gluon Plasma (QGP) formed in ultra-relativistic heavy-ion collisions. The ALICE Collaboration measured the D-meson elliptic flow, ν2, in Pb–Pb collisions at sNN=5.02TeV to study the participation of the low and intermediate transverse momentum (pT) charm quarks in the collective expansion of the system. The charm-quark dynamic in the QGP is further investigated through the Event Shape Engineering (ESE) technique. Furthermore, the effect of unprecedented strong magnetic field produced in heavy-ion collisions is studied by measuring the charged-dependent directed flow (ν1) of D0 mesons for the first time at the LHC. Recent results of azimuthal anisotropy of open heavy-flavour decay electrons and muons in p–Pb collisions at sNN=5.02TeV and sNN=8.16TeV, respectively, are also shown.

Event shape dependence of anisotropic flow for inclusive and identified hadrons in Pb–Pb and Xe–Xe collisions with ALICE

Event Shape Engineering technique has been used to select events corresponding to a specific initial geometry in a given centrality class. For those events, the elliptic (ν2) and triangular (ν3) flow coefficients of inclusive as well as identified particles, including π±, K±, p+p‾, KS0, Λ+Λ‾, and Ξ−+Ξ‾+ are measured in Pb–Pb collisions at sNN=5.02TeV recorded by ALICE. In addition, the ν2 of π±, K±, p+p‾, KS0, and Λ+Λ‾ from Xe–Xe collisions at sNN=5.44TeV is presented. The results obtained using the scalar product method are reported as a function of transverse momentum, pT, within the pseudorapidity (rapidity) range |η| < 0.8 (|y| < 0.5) for various centrality classes. The flow coefficients exhibit a particle mass dependence for pT < 2–3 GeV/c and type grouping (i.e., mesons and baryons) at a higher pT region. The ratios between νn for shape selected and unbiased events are almost flat up to pT = 10 GeV/c, independent of particle species.

Measurement of D-meson production and flow in Pb–Pb collisions with ALICE at the LHC

Open-charmed mesons are unique tools to study the properties of the Quark–Gluon Plasma (QGP) formed in ultra-relativistic nucleus–nucleus collisions. The nuclear modification factor ([Formula: see text]) and elliptic flow ([Formula: see text]) of [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] mesons were measured by the ALICE Collaboration in Pb–Pb collisions at [Formula: see text][Formula: see text]TeV. The D-meson [Formula: see text] provides information on the charm-quark interactions with the medium and the charm-quark energy loss. The D-meson elliptic flow at low transverse momentum ([Formula: see text]) gives insight into the participation of charm quarks in the collective expansion of the system and their possible in-medium thermalization. At high [Formula: see text], the [Formula: see text] is sensitive to the path-length dependence of parton energy loss. The role of the recombination mechanism is investigated measuring the [Formula: see text]-differential yield ratios between D-meson species with and without strange-quark content. Finally, the coupling of charm quarks to light quarks of the underlying medium is examined applying the Event-Shape Engineering (ESE) technique to the nonstrange D-meson elliptic flow.

Examining the event-shape-dependent modifications to charged-particle transverse momentum spectra and elliptic flow in p -Pb collisions at energies available at the CERN Large Hadron Collider

Purported signatures of collective dynamics in small systems like proton-proton (pp) or proton-nucleus (p-A) collisions still lack unambiguous understanding. Despite the qualitative and/or quantitative agreement of the data to hydrodynamic models, it has remained unclear whether the harmonic flows in small systems relate to the common physical picture of hydrodynamic collectivity driven by the initial geometry. In the present work, we aim to address this issue by invoking a novel concept of Event Shape Engineering (ESE), which has been leveraged to get some control of the initial geometry in high-energy heavy-ion collisions. We utilise ESE by constructing a reference flow vector, $q_{2}$ that allows to characterise an event based on it's ellipticity. Applying this technique on a data set, simulated from a 3+1D viscous hydrodynamic model EPOS3, we study the event-shape dependent modifications to some of the bulk properties like, inclusive transverse momentum ($p_{T}$) spectra and $p_{T}$-differential $v_{2}$ for p-Pb collisions at 5.02 TeV. Selecting events on the basis of different magnitudes of reference flow vector $q_{2}$, we observe a hint of event-shape induced modifications of $v_{2}$ as a function of $p_{T}$ but, the inclusive $p_{T}$-spectra of charged particles seem to be insensitive to this event-shape selection.

New estimator for symmetry plane correlations in anisotropic flow analyses

Correlations of symmetry planes are important observables used to quantify anisotropic flow phenomenon and constrain independently the properties of strongly interacting nuclear matter produced in the collisions of heavy ions at the highest energies. In this paper, we point out current problems of measuring correlations between symmetry planes and elaborate on why the available analysis techniques have a large systematic bias. To overcome this problem, we introduce a new approach to approximate multi-harmonic flow fluctuations via a two-dimensional Gaussian distribution. Employing this approximation, we introduce a new estimator, dubbed Gaussian Estimator (GE), to extract pure correlation between symmetry planes. We validate GE by using the realistic event-generator iEBE-VISHNU and demonstrate that it outperforms all existing estimators. Based on event-shape engineering, we propose an experimental procedure to improve GE accuracy even further.