Non-central Collisions Research Articles

Fluctuations and correlations of hot QCD matter in an external magnetic field

Effects of an external magnetic field on the fluctuations of quark number, fluctuations and correlations of conserved charges, including baryon number, electric charge and strangeness, are studied in the 2+1 flavor Polyakov--Nambu--Jona-Lasinio model. We find that magnetic field increases fluctuations and correlations in the regime of chiral crossover. It makes the transition of quadratic fluctuations more abrupt, and the peak structure of quartic fluctuations more pronounced. Our calculations indicate that $\chi_{4}^{B}/\chi_{2}^{B}$, $\chi_{4}^{Q}/\chi_{2}^{Q}$, and $\chi_{11}^{BQ}$ are very sensitive to the external magnetic field and maybe can be used for probes for the strong magnetic field produced in the early stage of noncentral collisions.

Global flow of glasma in high energy nuclear collisions

We discuss the energy flow of the classical gluon fields created in collisions of heavy nuclei at collider energies. We show how the Yang–Mills analog of Faradayʼs Law and Gaussʼ Law predicts the initial gluon flux tubes to expand or bend. The resulting transverse and longitudinal structure of the Poynting vector field has a rich phenomenology. Besides the well-known radial and elliptic flow in transverse direction, classical quantum chromodynamics predicts a rapidity-odd transverse flow that tilts the fireball for non-central collisions, and it implies a characteristic flow pattern for collisions of non-symmetric systems A+B. The rapidity-odd transverse flow translates into a directed particle flow v1 which has been observed at RHIC and LHC. The global flow fields in heavy ion collisions could be a powerful check for the validity of classical Yang–Mills dynamics in high energy collisions.

Elliptic flow of J/ψ at forward rapidity in Pb–Pb collisions at 2.76 TeV with the ALICE experiment

We present the elliptic flow of inclusive J/$\psi$ measured in the $\mu^{+}\mu^{-}$ channel at forward rapidity ($2.5<y<4.0$), down to zero transverse momentum, in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=2.76$ TeV with the ALICE muon spectrometer. The $p_{\mathrm T}$ dependence of J/$\psi$ $v_2$ in non-central (20%-60%) Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=2.76$ TeV is compared with existing measurements at RHIC and theoretical calculations. The centrality dependence of the $p_{\mathrm{T}}$-integrated elliptic flow, as well as the $p_{\mathrm{T}}$ dependence in several finer centrality classes is presented.

Heavy-flavor suppression and azimuthal anisotropy in Pb–Pb collisions at sNN=2.76TeV with the ALICE detector

Heavy-flavor hadron production studies in Pb–Pb collisions at s NN = 2.76 TeV with the 2010 and 2011 data samples are presented. The measurements are performed with the ALICE detector in various decay channels and in a wide kinematic range. Heavy-flavor hadrons exhibit a suppression in the most central Pb–Pb collisions that amounts to a factor 3–5 for p T ∼ 8 – 10 GeV / c . The second harmonic of the azimuthal distribution Fourier decomposition, v 2 , is non-zero for non-central collisions at intermediate p T ( ∼ 3 GeV / c ).

Holographic thermalization with radial flow

Recently, a lot of effort has been put into describing the thermalization of the quark-gluon plasma using the gauge/gravity duality. In this context, we here present a full numerical solution of the early far-from-equilibrium formation of the plasma, which is expanding radially in the transverse plane and is boost invariant along the collision axis. This can model the early stage of a head-on relativistic heavy ion collision. The resulting momentum distribution quickly reaches local equilibrium, after which it can be evolved using ordinary hydrodynamics. We comment on general implications for these hydrodynamic simulations, both for central and noncentral collisions, and including fluctuations in the initial state.

Suppression of the High-pTCharged-HadronRAAat the LHC

We present a parameter-free postdiction of the high-${p}_{T}$ charged-hadron nuclear modification factor (${R}_{AA}$) in two centralities, measured by the CMS Collaboration in Pb-Pb collisions at the LHC. The evolution of the bulk medium is modeled using viscous fluid dynamics, with parameters adjusted to describe the soft hadron yields and elliptic flow. Assuming the dominance of radiative energy loss, we compute the medium modification of the ${R}_{AA}$ using a perturbative QCD-based formalism, the higher twist scheme. The transverse momentum diffusion coefficient $\stackrel{^}{q}$ is assumed to scale with the entropy density and is normalized by fitting the ${R}_{AA}$ in the most central Au-Au collisions at the Relativistic Heavy-Ion Collider. This setup is validated in noncentral Au-Au collisions at the Relativistic Heavy-Ion Collider and then extrapolated to Pb-Pb collisions at the LHC, keeping the relation between $\stackrel{^}{q}$ and entropy density unchanged. We obtain a satisfactory description of the CMS ${R}_{AA}$ over the ${p}_{T}$ range from 10 to 100 GeV.

Spectator fragmentation in nucleus-nucleus collisions: Phase space approach

Dynamics of spectator matter break-up in non-central collisions of 197Au+ 197Au at 1000 MeV/A are explored within the framework of a quantum molecular dynamics (QMD) model. The phase space of nucleons bound in intermediate mass fragments are studied using two clustering subroutines. Backtracking the origin of fragments to the time of the initial contact between the colliding nuclei and applying the simulated annealing clusterization algorithm (SACA) indicates a significant yield of projectile-like and target-like fragments. The simplest clusterization approach based on the spatial correlation technique, however, predicted a much lesser production probability of fragments in the spectator zone.

A systematic study of the sensitivity of triangular flow to the initial state fluctuations in relativistic heavy-ion collisions

Experimental data from the Relativistic Heavy-Ion Collider suggest that the quark gluon plasma behaves almost like an ideal fluid. Due to its short lifetime, many QGP properties can only be inferred indirectly through a comparison of the final state measurements with transport model calculations. Among the current phenomena of interest are the interdependences between two collective flow phenomena, elliptic and triangular flow. The former is mostly related to the initial geometry and collective expansion of the system whereas the latter is sensitive to the fluctuations of the initial state. For our investigation we use a hybrid transport model based on the ultra-relativistic quantum molecular dynamics (UrQMD) transport approach using an ideal hydrodynamic expansion for the hot and dense stage. Using UrQMD initial conditions for an Au–Au collision, particles resulting from a collision are mapped into an energy density distribution that is evolved event-by-event with a hydrodynamic calculation. By averaging these distributions over different numbers of events, we have studied how the granularity/smoothness of the distribution affects the initial eccentricity, the initial triangularity, and the resulting flow components. The average elliptic flow in non-central collisions is not sensitive to the granularity, while triangular flow is. The triangularity might thus provide a good measure of the amount of initial state fluctuations that is necessary to reproduce the experimental data.

Mapping the hydrodynamic response to the initial geometry in heavy-ion collisions

We investigate how the initial geometry of a heavy-ion collision is transformed into final flow observables by solving event-by-event ideal hydrodynamics with realistic fluctuating initial conditions. We study quantitatively to what extent anisotropic flow (${v}_{n}$) is determined by the initial eccentricity ${\ensuremath{\varepsilon}}_{n}$ for a set of realistic simulations, and we discuss which definition of ${\ensuremath{\varepsilon}}_{n}$ gives the best estimator of ${v}_{n}$. We find that the common practice of using an ${r}^{2}$ weight in the definition of ${\ensuremath{\varepsilon}}_{n}$ in general results in a poorer predictor of ${v}_{n}$ than when using ${r}^{n}$ weight, for $n&gt;2$. We similarly study the importance of additional properties of the initial state. For example, we show that in order to correctly predict ${v}_{4}$ and ${v}_{5}$ for noncentral collisions, one must take into account nonlinear terms proportional to ${\ensuremath{\varepsilon}}_{2}^{2}$ and ${\ensuremath{\varepsilon}}_{2}{\ensuremath{\varepsilon}}_{3}$, respectively. We find that it makes no difference whether one calculates the eccentricities over a range of rapidity or in a single slice at $z=0$, nor is it important whether one uses an energy or entropy density weight. This knowledge will be important for making a more direct link between experimental observables and hydrodynamic initial conditions, the latter being poorly constrained at present.

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.

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.

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.

Longitudinal scaling of elliptic flow in Landau hydrodynamics

This study presents generalization of the Landau hydrodynamic solution for multiparticle production applied to non-central relativistic heavy ion collisions. Obtained results shows longitudinal scaling of elliptic flow $v_2$ as a function of rapidity shifted by beam rapidity ($y-y_{beam}$) for different energies ($\sqrt{s_{NN}}=62.4$ GeV and 200 GeV) and for different systems (Au-Au and Cu-Cu). It is argued, that the elliptic flow and its longitudinal scaling is due to the initial transverse energy density distribution and initial longitudinal thickness effect.

Correspondence between Hanbury-Brown–Twiss radii and the emission zone in noncentral heavy ion collisions

In noncentral collisions between ultrarelativistic heavy ions, the freeze-out distribution is anisotropic, and its major longitudinal axis may be tilted away from the beam direction. The shape and orientation of this distribution are particularly interesting, as they provide a snapshot of the evolving source and reflect the space-time aspect of anisotropic flow. Experimentally, this information is extracted by measuring pion Hanbury-Brown--Twiss (HBT) radii as a function of angle with respect to the reaction plane. The connection between measured radius oscillations and the underlying geometry is necessarily model-dependent; many existing formulas are strictly valid only for Gaussian sources with no collective flow. With a realistic transport model of the collision, which generates flow and non-Gaussian sources, we find that these formulas approximately reflect the anisotropy of the freeze-out distribution.

Using CMBR tools for flow anisotropies in relativistic heavy-ion collisions

Abstract Recently we have shown that there are crucial similarities in the physics of cosmic microwave background radiation (CMBR) and relativistic heavy-ion collision experiments (RHICE). We have argued that following CMB analysis technique, a plot of the root mean square value of the flow coefficient v ˜ n r m s calculated in lab fixed frame will give important information about the initial state fluctuations and also can be used to directly probe elliptic flow in non-central collisions. Here we demonstrate how following the analysis techniques used in CMBR can give important information about the initial state and evolution of the plasma formed in heavy ion collisions. We further explore the similarities in the physics of CMB and RHICE by studying the effect of large magnetic field on flow coefficients.

Probe for the strong parity violation effects at RHIC with three particle correlations

In non-central relativistic heavy ion collisions, \P-odd domains, which might be created in the process of the collision, are predicted to lead to charge separation along the system orbital momentum \cite{Kharzeev:2004ey}. An observable, \P-even, but directly sensitive to the charge separation effect, has been proposed in \cite{Voloshin:2004vk} and is based on 3-particle mixed harmonics azimuthal correlations. We report the STAR measurements using this observable for Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$=200 and 62 GeV. The results are reported as function of collision centrality, particle separation in rapidity, and particle transverse momentum. Effects that are not related to parity violation but might contribute to the signal are discussed.

Shape analysis of strongly interacting systems: the heavy ion case

Collisions between nuclei at ultrarelativistic energies produce a colour-deconfined plasma that expands explosively and rapidly reverts to the colour-confined (hadronic) state. In non-central collisions, the zone of hot matter is transversely anisotropic and may be ‘tilted’ relative to the direction of the incoming beams. As the matter cools and expands into the vacuum, the evolution of the system shape depends sensitively on the dynamical response of the plasma under extreme conditions. Two-pion intensity interferometry performed relative to the impact parameter can be used to measure the approximate final shape of the system when pions decouple from the system. We use several transport models to illustrate the dependence of the final shape on the QCD equation of state and late-stage hadronic rescattering. The dependence of the final shape on collision energy may reveal non-trivial structures in the QCD phase diagram. Indeed, the few measurements published to date show an intriguing behaviour in an energy region under intense experimental and theoretical scrutiny, as signatures of a first-order phase transition may appear there. We discuss strong parallels between shape studies in heavy-ion collisions and those in two other strongly coupled systems.

Heavy quarks at RHIC and LHC within a partonic transport model

Production and space-time evolution of heavy quarks in central and non-central heavy-ion collisions at RHIC and LHC are studied with the partonic transport model Boltzmann Approach of MultiParton Scatterings (BAMPS). In addition to the initially created heavy quarks in hard parton scatterings during nucleon-nucleon collisions, secondary heavy quark production in the quark-gluon plasma is investigated and the sensitivity on various parameters is estimated. In BAMPS heavy quarks scatter with particles of the medium via elastic collisions, whose cross section is calculated with the running coupling and a more precise implementation of Debye screening. In this framework, we compute the elliptic flow and nuclear modification factor of heavy quarks and compare it to the experimental data.