Non-central Heavy-ion Collisions Research Articles

Observation of Global Hyperon Polarization in Ultrarelativistic Heavy Ion Collisions

Non-central heavy-ion collisions provide a system with non-zero total angular momentum which can be transferred, in part, to the fireball via baryon stopping. It has been predicted that a net spin of emitted particles aligned with the system angular momentum may emerge through coupling with the bulk material. Due to its parity violating decay the ⋀ baryon is self-analyzing, which allows us to associate the daughter proton decay direction with ⋀ particle spin. Ultimately this allows us to use them as a probe of net-particle spin. In preliminary STAR measurements of the net ⋀ baryon polarization from Au+Au collisions at 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV we find that both ⋀ and ⊼ particles are polarized in the direction of the system angular momentum. Including previously published STAR results we see a polarization of about 2% for mid-central collisions when √Snn < 100GeV.

P-odd effects in heavy ion collisions at NICA

Experimental manifestation of P-odd effects related to the vorticity and hydrodynamic helicity in non-central heavy ion collisions at MPD@NICA and BM@N detectors is discussed. For the NICA and FAIR energy range characterized by the large baryonic charge of the forming medium the effect should manifest itself in the specific neutron asymmetries. The polarization of strange particles probing the vorticity and helicity is also discussed.

Hydrodynamic helicity and strange hyperon polarization in heavy-ion collisions

We study the P-odd effects related to the vorticity of the medium formed in noncentral heavy ion collisions. Using the kinetic Quark-Gluon String Model, we perform numerical simulations of the vorticity and hydrodynamical helicity for various atomic numbers, energies and centralities. We observe vortical structures typically occupying a relatively small fraction of the fireball volume. In the course of numerical simulations a noticeable hydrodynamic helicity was observed manifesting a specific mirror behaviour with respect to the reaction plane. The effect is maximal at the NICA and FAIR energy range.

Vorticity and hydrodynamic helicity in heavy-ion collisions in the hadron-string dynamics model

The hydrodynamic helicity separation effect in noncentral heavy-ion collisions is investigated using the hadron-string dynamics (HSD) model. Computer simulations are done to calculate velocity and hydrodynamic helicity on a mesh in a small volume around the center of the reaction. The time dependence of hydrodynamic helicity is observed for various impact parameters and different calculation methods. Comparison with a similar earlier work is carried out. A new quantity related to jet handedness is used to probe for $p$-odd effects in the final state.

Magnetohydrodynamics and charged currents in heavy ion collisions

The hot QCD matter produced in any heavy ion collision with a nonzero impact parameter is produced within a strong magnetic field. We study the imprint the magnetic fields produced in non-central heavy ion collisions leave on the azimuthal distributions and correlations of the produced charged hadrons. The magnetic field is time-dependent and the medium is expanding, which leads to the induction of charged currents due to the combination of Faraday and Hall effects. We find that these currents result in a charge-dependent directed flow $v_1$ that is odd in rapidity and odd under charge exchange. It can be detected by measuring correlations between the directed flow of charged hadrons at different rapidities, $\langle v_1^\pm (y_1) v_1^\pm (y_2) \rangle$.

Effects of magnetic fields on the quark–gluon plasma

Abstract In this talk, the response of the thermal QCD medium to external (electro)magnetic fields is studied using continuum extrapolated lattice results at physical quark masses. The magnetic susceptibility of QCD is calculated, revealing a strong paramagnetic response at high temperatures. This paramagnetism is shown to result in an anisotropic squeezing of the quark–gluon plasma in non-central heavy-ion collisions, implying a sizeable contribution to the elliptic flow. Another aspect is the magnetic response of topologically non-trivial domains to the magnetic field. We quantify this effect on the lattice and compare the results to a simple model estimate.

Azimuthally differential pion femtoscopy in Pb–Pb collisions at 2.76 TeV with ALICE at the LHC

Femtoscopy of non-central heavy ion collisions provides access to information on the geometry of the effective pion-emitting source. In particular, the source shape can be studied by measuring femtoscopic radii as a function of the pion emission angle relative to the collision symmetry planes. We present the results of azimuthally differential femtoscopy of Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV at the LHC relative to the second harmonic event plane. We observe a clear oscillation of the extracted radii as a function of the emission angle. We find that $R_{side}$ and $R_{out}$ oscillate out of phase for all centralities and pion transverse momenta. The relative amplitude of $R_{side}$ oscillation decreases in more central collisions, but remains positive, which indicates that the source remains out-of-plane extended qualitatively similar to what was observed at RHIC energies. We compare our results to existing hydrodynamical and transport model calculations.

D mesons in non-central heavy-ion collisions: Fluctuating vs. averaged initial conditions

The suppression of D mesons in non-central heavy-ion collisions is investigated. The anisotropy in collisions at finite impact parameter leads to an ordering of all-angle, in- and out-of-plane nuclear modification factors due to the different in-medium path lengths. Within our MC@sHQ+EPOS model of heavy-quark propagation in the QGP we demonstrate that fluctuating initial conditions lead to an effective reduction of the energy loss of heavy quarks, which is seen in a larger nuclear modification factor at intermediate and high transverse momenta. The elliptic flow at small transverse momenta is reduced.

Geometrical aspects of jet quenching in JEWEL

In this publication the performance of the Monte Carlo event generator Jewel in non-central heavy-ion collisions is investigated. Jewel is a consistent perturbative framework for jet evolution in the presence of a dense medium. It yields a satisfactory description of a variety of jet observables in central collisions at the LHC, although so far with a simplistic model of the medium. Here, it is demonstrated that also jet measurements in non-central collisions, and in particular the dependence of the jet suppression on the angle relative to the reaction plane, are reproduced by the same model.

Landau hydrodynamics for non-central heavy-ion collisions and longitudinal scaling of elliptic flow

Landau solution of hydrodynamics is generalized for the non-central high-energy nuclear collisions. The multiparticle production after hydrodynamic expansion from the transversely asymmetric initial state shows elliptic flow formation. Moreover, obtained solution does reproduce the observed longitudinal scaling of the elliptic flow for the different collision energies at LHC and RHIC. It will be argued, that the analyticity of the solution allows us to to fit experimental data in order to obtain initial conditions.

Magnetic susceptibility and equation of state ofNf=2+1QCD with physical quark masses

We determine the free energy of strongly interacting matter as a function of an applied constant and uniform magnetic field. We consider N_f = 2+1 QCD with physical quark masses, discretized on a lattice by stout improved staggered fermions and a tree level improved Symanzik pure gauge action, and explore three different lattice spacings. For magnetic fields of the order of those produced in non-central heavy ion collisions (eB ~ 0.1 GeV^2) strongly interacting matter behaves like a medium with a linear response, and is paramagnetic both above and below the deconfinement transition, with a susceptibility which steeply rises in the deconfined phase. We compute the equation of state, showing that the relative increase in the pressure due to the magnetic field gets larger around the transition, and of the order of 10 % for eB ~ 0.1 GeV^2.

Paramagnetic Squeezing of QCD Matter

We determine the magnetization of quantum chromodynamics for several temperatures around and above the transition between the hadronic and the quark-gluon phases of strongly interacting matter. We obtain a paramagnetic response that increases in strength with the temperature. We argue that due to this paramagnetism, chunks of quark-gluon plasma produced in noncentral heavy ion collisions should become squeezed perpendicular to the magnetic field. This anisotropy will then contribute to the elliptic flow v2 observed in such collisions, in addition to the pressure gradient that is usually taken into account. We present a simple estimate for the magnitude of this new effect and a rough comparison to the effect due to the initial collision geometry. We conclude that the paramagnetic effect might have a significant impact on the value of v2.

Fluctuations of charge separation perpendicular to the event plane and local parity violation insNN=200GeV Au + Au collisions at the BNL Relativistic Heavy Ion Collider

Previous experimental results based on data (∼15×106 events) collected by the STAR detector at the BNL Relativistic Heavy Ion Collider suggest event-by-event charge-separation fluctuations perpendicular to the event plane in noncentral heavy-ion collisions. Here we present the correlator previously used split into its two component parts to reveal correlations parallel and perpendicular to the event plane. The results are from a high-statistics 200-GeV Au + Au collisions data set (57×106 events) collected by the STAR experiment. We explicitly count units of charge separation from which we find clear evidence for more charge-separation fluctuations perpendicular than parallel to the event plane. We also employ a modified correlator to study the possible P-even background in same- and opposite-charge correlations, and find that the P-even background may largely be explained by momentum conservation and collective motion.2 MoreReceived 12 August 2013DOI:https://doi.org/10.1103/PhysRevC.88.064911©2013 American Physical Society

Helicity separation in heavy-ion collisions

We study the $P$-odd effects related to the vorticity of the medium formed in noncentral heavy-ion collisions. Using the kinetic quark-gluon strings model, we perform numerical simulations of the vorticity and hydrodynamical helicity for various atomic numbers, energies, and centralities. We observed vortical structures typically occupy the relatively small fraction of the fireball volume. In the course of numerical simulations, the noticeable hydrodynamical helicity was observed to manifest specific mirror behavior with respect to the reaction plane. The effect is maximal at the Nuclotron based Ion Collider fAcility and Facility for Antiproton and Ion Research in Europe energy range.

Heavy quarkonium production in a strong magnetic field

It is well known that in noncentral heavy-ion collisions a transient strong magnetic field is generated in the direction perpendicular to the reaction plane. The maximal strength of this field is estimated to be $eB \sim m^2_{\pi} \sim 0.02 \text{GeV}^2$ at the RHIC and $eB \sim 15 m^2_{\pi} \sim 0.3 \text{GeV}^2$ at the LHC. We investigate the effects of a strong magnetic field on $B$ and $D$ mesons, focusing on the changes of the energy levels and the masses of the bound states. Using the Color Evaporation Model we discuss the possible changes in the production of $J/\psi$ and $\Upsilon$.

Quark polarization in a viscous quark-gluon plasma

Quarks produced in the early stage of noncentral heavy-ion collisions could develop a global spin polarization along the opposite direction of the reaction plane due to the spin-orbital coupling via parton interaction in a medium that has finite longitudinal flow shear along the direction of the impact parameter. We study how such polarization evolves via multiple scattering in a viscous quark-gluon plasma with an initial laminar flow. The final polarization is found to be sensitive to the viscosity and the initial shear of local longitudinal flow.

Anisotropic flow of charged particles at TeV measured with the ALICE detector

Measurements of anisotropic flow in heavy-ion collisions provide evidence for the creation of strongly interacting matter which appears to behave as an almost ideal fluid. Anisotropic flow signals the presence of multiple interactions and is very sensitive to the initial spatial anisotropy of the overlap region in non-central heavy-ion collisions. In this paper, we report measurements of elliptic v2, triangular v3, quadrangular v4 and pentagonal v5 flow. These measurements have been performed with 2- and multi-particle correlation techniques.

Charge-dependent azimuthal correlations in Pb–Pb collisions at TeV

Separation of charges along the extreme magnetic field created in non-central relativistic heavy-ion collisions is predicted to be a signature of local parity violation in strong interactions. We report on results for charge-dependent two-particle azimuthal correlations with respect to the reaction plane for Pb–Pb collisions at TeV recorded in 2010 with ALICE at the LHC. The results are compared with measurements at RHIC energies and against currently available model predictions for LHC. Systematic studies of possible background effects including comparison with conventional (parity-even) correlations simulated with Monte Carlo event generators of heavy-ion collisions are also presented.

Search for local strong parity violation in STAR using multiple observables

Parity-odd domains from QCD are predicted to cause charge separation of quarks across the reaction plane created in non-central relativistic heavy-ion collisions—the chiral magnetic effect. Here, we present several measurements to search for charge separation across the reaction plane at STAR. We use three different observables: a three-point correlator, a multiplicity asymmetry correlator as well as a reaction-plane-dependent balance function. The correlations are studied differentially and are presented for several Au+Au collision energies: 200, 62.4, 39, 11.5 and 7.7 GeV. We will discuss the sensitivities of these latest measurements to possible parity odd signals and parity even backgrounds.

Energy dependence of the freeze-out eccentricity from the azimuthal dependence of HBT at STAR

Non-central heavy-ion collisions create an out-of-plane-extended participant zone that expands toward a more round state as the system evolves. The recent RHIC beam energy scan at of 7.7, 11.5 and 39 GeV provides an opportunity to explore the energy dependence of the freeze-out eccentricity. The new low-energy data from STAR complement high statistics data sets at of 62.4 and 200 GeV. Hanbury Brown–Twiss (HBT) interferometry allows us to determine the size of pion-emitting source regions. The dependence of the HBT radius parameters on the azimuthal angle relative to the reaction plane has been extracted. These dependences can be related to the freeze-out eccentricity. The new results from STAR are consistent with a monotonically decreasing freeze-out eccentricity and constrain any minimum, suggested by previously available data, to lie in the range between 11.5 and 39 GeV. Of several models, UrQMD appears to best predict the STAR and AGS data.