Event Plane Research Articles

Performance of Forward Hadron Calorimeter at MPD/NICA

Forward Hadron Calorimeter is a part of MPD experiment setup at NICA beam facility. FHCal structure and purpose are presented in this proceedings. Methods of collision centrality and event plane reconstruction are discussed. Simulation, beam test results and production status are presented.

Gradient Tomography of Jet Quenching in Heavy-Ion Collisions.

Transverse momentum broadening and energy loss of a propagating parton are dictated by the space-time profile of the jet transport coefficient q[over ^] in a dense QCD medium. The spatial gradient of q[over ^] perpendicular to the propagation direction can lead to a drift and asymmetry in parton transverse momentum distribution. Such an asymmetry depends on both the spatial position along the transverse gradient and path length of a propagating parton as shown by numerical solutions of the Boltzmann transport in the simplified form of a drift-diffusion equation. In high-energy heavy-ion collisions, this asymmetry with respect to a plane defined by the beam and trigger particle (photon, hadron, or jet) with a given orientation relative to the event plane is shown to be closely related to the transverse position of the initial jet production in full event-by-event simulations within the linear Boltzmann transport model. Such a gradient tomography can be used to localize the initial jet production position for more detailed study of jet quenching and properties of the quark-gluon plasma along a given propagation path in heavy-ion collisions.

A sensitivity study of the primary correlators used to characterize chiral-magnetically-driven charge separation

A Multi-Phase Transport (AMPT) model is used to study the detection sensitivity of two of the primary correlators – Δγ and RΨ2 – employed to characterize charge separation induced by the Chiral Magnetic Effect (CME). The study, performed relative to several event planes for different input “CME signals”, indicates a detection threshold for the fraction fCME=ΔγCME/Δγ, which renders the Δγ-correlator insensitive to values of the Fourier dipole coefficient a1≲2.5%, that is larger than the purported signal (signal difference) for ion-ion(isobaric) collisions. By contrast, the RΨ2 correlator indicates concave-shaped distributions with inverse widths (σRΨ2−1) that are linearly proportional to a1, and independent of the character of the event plane used for their extraction. The sensitivity of the RΨ2 correlator to minimal CME-driven charge separation in the presence of realistic backgrounds, could aid better characterization of the CME in heavy-ion collisions.

Investigation of the Elliptic Flow Fluctuations of the Identified Particles Using the a Multi-Phase Transport Model

A Multi-Phase Transport (AMPT) model is used to study the elliptic flow fluctuations of identified particles using participant and spectator event planes. The elliptic flow measured using the first order spectator event plane is expected to give the elliptic flow relative to the true reaction plane which suppresses the flow fluctuations. However, the elliptic flow measured using the second-order participant plane is expected to capture the elliptic flow fluctuations. Our study shows that the first order spectator event plane could be used to study the elliptic flow fluctuations of the identified particles in the AMPT model. The elliptic flow fluctuations magnitude shows weak particle species dependence and transverse momentum dependence. Such observation will have important implications for understanding the source of the elliptic flow fluctuations.

Investigation of the linear and mode-coupled flow harmonics in Au+Au collisions at [formula omitted] = 200 GeV

Flow harmonics (vn) of the Fourier expansion for the azimuthal distributions of hadrons are commonly employed to quantify the azimuthal anisotropy of particle production relative to the collision symmetry planes. While lower order Fourier coefficients (v2 and v3) are more directly related to the corresponding eccentricities of the initial state, the higher-order flow harmonics (vn>3) can be induced by a mode-coupled response to the lower-order anisotropies, in addition to a linear response to the same-order anisotropies. These higher-order flow harmonics and their linear and mode-coupled contributions can be used to more precisely constrain the initial conditions and the transport properties of the medium in theoretical models. The multiparticle azimuthal cumulant method is used to measure the linear and mode-coupled contributions in the higher-order anisotropic flow, the mode-coupled response coefficients, and the correlations of the event plane angles for charged particles as functions of centrality and transverse momentum in Au+Au collisions at nucleon-nucleon center-of-mass energy sNN= 200 GeV. The results are compared to similar LHC measurements as well as to several viscous hydrodynamic calculations with varying initial conditions.

Jet-hadron correlations measured relative to the second order event plane in Pb-Pb collisions at sNN=2.76TeV

The Quark Gluon Plasma (QGP) produced in ultra relativistic heavy-ion collisions at the Large Hadron Collider (LHC) can be studied by measuring the modifications of jets formed by hard scattered partons which interact with the medium. We studied these modifications via angular correlations of jets with charged hadrons for jets with momenta 20 < $p_{\rm{T}}^{\rm{jet}}$ < 40 GeV/$c$ as a function of the associated particle momentum. The reaction plane fit (RPF) method is used in this analysis to remove the flow modulated background. The analysis of angular correlations for different orientations of the jet relative to the second order event plane allows for the study of the path length dependence of medium modifications to jets. We present the dependence of azimuthal angular correlations of charged hadrons with respect to the angle of the axis of a reconstructed jet relative to the event plane in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. The dependence of particle yields associated with jets on the angle of the jet with respect to the event plane is presented. Correlations at different angles relative to the event plane are compared through ratios and differences of the yield. No dependence of the results on the angle of the jet with respect to the event plane is observed within uncertainties, which is consistent with no significant path length dependence of the medium modifications for this observable.

Azimuthally-Differential Pion Femtoscopy in Cu + Au and Au + Au Collisions at $$\sqrt {{{s}_{{NN}}}} $$ = 200 GeV in the STAR Experiment

Anisotropic flow is sensitive to the properties of the systems created in heavy-ion collisions. Azimuthally-sensitive femtoscopy with respect to the first-order event plane is coupled with the directed flow, and probes the space-time structure of the particle-emitting source. The knowledge of the source tilt can give a helpful experimental handle on the origin of the anisotropic flow. In addition, the tilt angle dependence on different collision systems at the same energy can provide constraints on theoretical models. In the experiment, this information can be extracted by measuring pion femtoscopic radii as a function of the pair emission angle with respect to the first-order event plane. In this work, we present comparisons between the results of the radius oscillations of the pion-emitting sources at $$\sqrt {{{s}_{{NN}}}} $$ = 200 GeV in symmetric (Au + Au) and asymmetric (Cu + Au) collisions measured with the STAR experiment, and those estimated in the UrQMD model.

The STAR event plane detector

The Event Plane Detector (EPD) is an upgrade detector to the STAR experiment at RHIC, designed to measure the pattern of forward-going charged particles emitted in a high-energy collision between heavy nuclei. It consists of two highly-segmented disks of 1.2-cm-thick scintillator embedded with wavelength-shifting fiber, coupled to silicon photomultipliers and custom electronics. We describe the general design of the device, its construction, and performance on the bench and in the experiment.

A beam–beam monitoring detector for the MPD experiment at NICA

The Multi-Purpose Detector (MPD) is to be installed at the Nuclotron Ion Collider fAcility (NICA) of the Joint Institute for Nuclear Research (JINR). Its main goal is to study the phase diagram of the strongly interacting matter produced in heavy-ion collisions. These studies, while providing insight into the physics of heavy-ion collisions, are relevant for improving our understanding of the evolution of the early Universe and the formation of neutron stars. In order to extend the MPD trigger capabilities, we propose to include a high granularity beam–beam monitoring detector (BE–BE) to provide a level-0 trigger signal with an expected time resolution of 30 ps. This new detector will improve the determination of the reaction plane by the MPD experiment, a key measurement for flow studies that provides physics insight into the early stages of the reaction.In this work, we use simulated Au+Au collisions at NICA energies to show the potential of such a detector to determine the event plane resolution, providing further redundancy to the detectors originally considered for this purpose namely, the Fast Forward Detector (FFD) and the Hadron Calorimeter (HCAL). We also show our results for the time resolution studies of two prototype cells carried out at the T10 beam line at the CERN PS complex.

Performance studies for collective flow measurements with CBM at FAIR

We present recent results from CBM performance studies for measurements of the directed (v1) flow of π+, Ʌ and . For the performance studies we use the CBMROOT environment for Monte-Carlo simulations and event reconstruction. The Kalman Filter Particle Finder (KFParticleFinder) package is used for hyperon reconstruction via their weak decays, and the Projectile Spectator Detector (PSD) for event plane determination. A status of the fast simulator implementation for the PSD calorimeter response, which is required for high statistics simulation, is also presented.

Study of Very Forward Neutrons with the CMS Zero Degree Calorimeter

Forward neutrons are studied in proton-lead collisions at the CMS experiment at the CERN LHC. They provide information on the centrality and event plane of collisions and provide an opportunity to study nuclear breakup. At the CMS experiment they are detected by the Zero Degree Calorimeters (ZDCs) in the | η | &gt; 8.5 pseudorapidity range. The ZDCs are quartz fiber Cherenkov calorimeters using tungsten as absorber. Test beam data and events with a single spectator neutron are used for the calibration of these detectors. A Fourier-based method is used correct for the effect of multiple pPb collisions. The corrected ZDC energy distribution is used to calculate centrality percentiles and unfold the neutron multiplicity distribution.

Hydrodynamical response of plane correlation in Pb + Pb collisions at s NN = 2.76TeV

In high energy heavy-ion collisions, the final anisotropic flow coefficients and their corresponding event–plane correlations are considered as the medium evolutional response to the initial geometrical eccentricities and their corresponding participant–plane correlations. We formulate a systematic theoretical analysis to study the hydrodynamical responses concerning higher-order effects in [Formula: see text] collisions at [Formula: see text][Formula: see text]TeV by using Monte Carlo (MC) Glauber model. To further understand the transformations of the initial participant–plane correlation, we construct a new set of events which randomize the directions of the initial participant–planes of the original events. Our results indicate that the final strong event–plane correlations are mainly transformed from the large initial eccentricities, rather than the strong participant–plane correlations. However, the large flow coefficients and the discrepancies between the flow coefficients calculated by the single-shot and event-by-event simulations in peripheral collisions are relevant to those strong initial participant–plane correlations.

Polarization of Λ (Λ[over ¯]) Hyperons along the Beam Direction in Au+Au Collisions at sqrt[s_{NN}]=200 GeV.

The Λ (Λ[over ¯]) hyperon polarization along the beam direction has been measured in Au+Au collisions at sqrt[s_{NN}]=200 GeV, for the first time in heavy-ion collisions. The polarization dependence on the hyperons' emission angle relative to the elliptic flow plane exhibits a second harmonic sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction, expected due to elliptic flow. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum (p_{T}) dependence at p_{T} greater than 1 GeV/c. The magnitude of the signal is about 5 times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, the kinematic vorticity calculations in the blast-wave model tuned to reproduce particle spectra, elliptic flow, and the azimuthal dependence of the Gaussian source radii measured with the Hanbury Brown-Twiss intensity interferometry technique reproduce well the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the polarization signal.

Measurement of two-particle correlations with respect to second- and third-order event planes in Au + Au collisions at sNN=200 GeV

We present measurements of azimuthal correlations of charged hadron pairs in $\sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions for the trigger and associated particle transverse-momentum ranges of $1<p_T^t<10$~GeV/$c$ and $0.5<p_T^a<10$~GeV/$c$. After subtraction of an underlying event using a model that includes higher-order azimuthal anisotropy $v_2$, $v_3$, and $v_4$, the away-side yield of the highest trigger-\pt ($p_T^t>4$~GeV/$c$) correlations is suppressed compared to that of correlations measured in $p$$+$$p$ collisions. At the lowest associated particle $p_T$ ($0.5<p_T^a<1$ GeV/$c$), the away-side shape and yield are modified relative to those in $p$$+$$p$ collisions. These observations are consistent with the scenario of radiative-jet energy loss. For the low-$p_T$ trigger correlations ($2<p_T^t<4$ GeV/$c$), a finite away-side yield exists and we explore the dependence of the shape of the away-side within the context of an underlying-event model. Correlations are also studied differentially versus event-plane angle $\Psi_2$ and $\Psi_3$. The angular correlations show an asymmetry when selecting the sign of the difference between the trigger-particle azimuthal angle and the $\Psi_2$ event plane. This asymmetry and the measured suppression of the pair yield out of plane is consistent with a path-length-dependent energy loss. No $\Psi_3$ dependence can be resolved within experimental uncertainties.

Search for the chiral magnetic effect at the LHC with the CMS experiment

Searches for the chiral magnetic effect (CME) using charge-dependent azimuthal correlations with respect to event planes are presented in PbPb collisions at 5.02 TeV and pPb collisions at 5.02 and 8.16 TeV, with the CMS experiment at the LHC. The azimuthal correlations with respect to the second- and third-order event planes are explored as a function of pseudorapidity, transverse momentum, and event multiplicity, which provides new insights into the underlying background correlations. By employing an event-shape engineering technique, a linear dependence of charge-dependent correlations on the second-order anisotropy flow (ν2) is observed, and the upper limits on the ν2-independent fraction, which is directly related to the CME signal, are obtained at 95% confidence level for both pPb and PbPb collisions. These results provide strong constraints on the search for the chiral magnetic effect at LHC energies, and establish new guidelines for searches in future experiments.

Event Plane Dependence of Di-hadron Correlations with Event Shape Engineering at the STAR Experiment

In high-energy-nuclear collisions, di-hadron correlations are used as a probe to study energy-loss mechanisms of jets in the Quark-Gluon Plasma (QGP). In order to understand the interplay between jet-medium interaction and medium expansion, we measured di-hadron correlations where events were classified by the trigger-hadron's angle with respect to the event plane. We further constrained the collision geometry using event-shape engineering based on the magnitude of the reduced flow vector q2. This measurement provides new and unique constraints for the dependence of energy loss of jets on in-medium path length and the role of flow.

ϕ Meson and K*0 Global Spin Alignment at STAR

The spin alignment of ϕ and K*0 vector mesons is sensitive to different hadronization scenarios and the vorticity of the colliding system. We present STAR's measurement of global spin alignment of ϕ and K*0 for Au + Au collisions at sNN=7.7 (ϕ meson only), 11.5, 14.5 (K*0 meson only) 19.6, 27, 39 and 200 (ϕ meson only) GeV. The alignment is quantified by the diagonal spin density matrix element, ρ00, with respect to the normal of the event plane. The ρ00 measurement based on the 1st- and 2nd-order event planes are presented. The results are shown as a function of transverse momentum, collision centrality and beam energy The implications of our results on the vorticity and the hadronization scenarios are discussed.

Spin alignment measurements using vector mesons with ALICE detector at the LHC

We present new measurements related to spin alignment of K*0 vector mesons at mid-rapidity for Pb–Pb collisions at sNN=2.76and5.02TeV. The spin alignment measurements are carried out with respect to production plane and 2nd order event plane. At low pT the spin density matrix element ρ00 for K*0 is found to have values slightly below 1/3, while it is consistent with 1/3, i.e. no spin alignment, at high pT. Similar values of ρ00 are observed with respect to both production plane and event plane. Within statistical and systematic uncertainties, ρ00 values are also found to be independent of sNN. ρ00 also shows centrality dependence with maximum deviation from 1/3 for mid-central collisions with respect to both the kinematic planes. The measurements for K*0 in pp collisions at s=13TeV and for K0S (a spin 0 hadron) in 20-40% central Pb–Pb collisions at sNN=2.76TeV are consistent with no spin alignment.

Global and local polarization of Λ hyperons in Au+Au collisions at 200 GeV from STAR

New results on global polarization of Λ hyperons in Au+Au collisions at sNN=200GeV reveal non-zero signal on the order of a few tenths of a percent. Compared with lower energy results, the current measurement shows that the polarization decreases at higher collision energy, the trend being well reproduced by theoretical calculations. The polarization is found to be larger in more peripheral collisions, as well as in the in-plane than in the out-of-plane direction. The signal seems to depend on the asymmetry between positive and negative charge in each event, which may indicate an influence of the axial current induced by the magnetic field. In addition, the first measurement of a local polarization along the beam direction was performed. The results show a quadrupole modulation relative to the second-order event plane, as expected from the elliptic flow.

Search for the chiral magnetic effect and vorticity-induced polarization effects in nuclear collisions

In relativistic heavy ion collisions, the chiral magnetic effect, known as the “CME”, has been extensively searched for over a decade in order to gain insights into the chiral symmetry restoration in the Quark-Gluon-Plasma, the possible local strong parity violation, and the extremely strong magnetic field. Based on the magnetic field direction and its correlation with the reaction plane, the CME has been mostly measured via a charge-dependent azimuthal correlator with respect to the second-order event plane, where experimental results are not yet conclusive due to their dominant background contaminations. Recent experimental efforts from STAR, ALICE, and CMS Collaboration will be discussed in this paper. Moreover, the total angular momentum from a noncentral collision can induce many important effects via its coupling to the particles' spin, e.g., Λ polarization, which provides essential information about properties of this strongly-interacting fluid-like medium. Besides the discovery of this exotic phenomenon, recent preliminary results from the STAR and ALICE experiment will be reviewed.