Elliptic Flow In Pb-Pb Collisions Research Articles

Long-range and short-range dihadron correlations in central PbPb collisions at (s_NN )=5.02 TeV and centrality 40-50%

This paper reports the first measurement of charged particle elliptic flow in Pb-Pb collisions at (s_NN ) = 5.02T eV with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed on more than 20,000 events in 40-50% centrality class on both long-range 2 < || < 4 and short-range || < 1 regions. The two-particle correlation method is used over long-range region, in the transverse momentum range of 1 < p_T < 1.5GeV , and the elliptic flow signals, given by v_n =(V_n ) , are v_2 = 0.17 0.01,v_3 = 0.07 0.01,v_4 = 0.04 0.01,v_5 = 0.03 0.01. Since V_1 is obtained as -0.0013, it does not have a square root value. In addition, for n=2-4, it is found that v_n increases as p_T increases. The Q-vectors method is also used to determine the values of v_2. The magnitudes of the Q-vectors are fitted to a function. The values of v_2 produced by the two methods are compared and discussed. At the end, the vectors are used to analyse high-energy jet particles.

Beauty production in heavy–ion collisions with ALICE at the LHC

In this contribution, the measurement of the centrality dependence of the nuclear modification factor (RAA) of non-prompt D0 meson in Pb-Pb collisions at √SNN = 5.02 TeV is presented. This measurement provides important constraints to the in-medium mass dependence of energy loss and hadroniza-tion of the beauty quark. In addition, the first measurements of non-prompt Ds+ meson production in central and semi-central Pb-Pb collisions at √SNN = 5.02 TeV are discussed. The non-prompt Ds+ meson measurements provide further information on the production and hadronization of Bs0 mesons. Finally, the first measurement of non-prompt D-meson elliptic flow in Pb-Pb collisions at √SNN = 5.02 TeV is shown. This measurement can help to investigate the degree of thermalization of beauty quarks in the hot and dense QCD medium.

A large-N expansion for minimum bias

Despite being the overwhelming majority of events produced in hadron or heavy ion collisions, minimum bias events do not enjoy a robust first-principles theoretical description as their dynamics are dominated by low-energy quantum chromodynamics. In this paper, we present a novel expansion scheme of the cross section for minimum bias events that exploits an ergodic hypothesis for particles in the events and events in an ensemble of data. We identify power counting rules and symmetries of minimum bias from which the form of the squared matrix element can be expanded in symmetric polynomials of the phase space coordinates. This expansion is entirely defined in terms of observable quantities, in contrast to models of heavy ion collisions that rely on unmeasurable quantities like the number of nucleons participating in the collision, or tunes of parton shower parameters to describe the underlying event in proton collisions. The expansion parameter that we identify from our power counting is the number of detected particles N and as N → ∞ the variance of the squared matrix element about its mean, constant value on phase space vanishes. With this expansion, we show that the transverse momentum distribution of particles takes a universal form that only depends on a single parameter, has a fractional dispersion relation, and agrees with data in its realm of validity. We show that the constraint of positivity of the squared matrix element requires that all azimuthal correlations vanish in the N → ∞ limit at fixed center-of-mass energy, as observed in data. The approach we follow allows for a unified treatment of small and large system collective behavior, being equally applicable to describe, e.g., elliptic flow in PbPb collisions and the “ridge” in pp collisions. We also briefly comment on power counting and symmetries for minimum bias events in other collider environments and show that a possible ridge in e+e− collisions is highly suppressed as a consequence of its symmetries.

Direct photon production in pp, p-Pb and Pb-Pb collisions: results from ALICE

Direct photon measurements provide a tool for testing QCD predictions in pp collisions and valuable information about properties of the quarkgluon matter created in nucleus-nucleus collisions. Comparing direct photon production in pp, p-Pb and Pb-Pb collisions, one can check the scaling with the number of binary collisions expected at high transverse momentum and get insight into the hot and cold hadronic matter properties with soft photons. The elliptic flow of direct photons is another unique possibility to trace the collective flow formation and space-time evolution of the fireball. We present the latest measurements of direct photon production in pp, p-Pb and Pb-Pb collisions and of the elliptic flow in Pb-Pb collisions at the LHC energies. Comparison to recent theory predictions will be also discussed.

Quarkonium in A—A collisions with the ALICE experiment

Quarkonia, i.e. bound states of heavy quarks (charm/bottom), provide remarkable probes of the hot and strongly-interacting medium, the quark gluon plasma (QGP), which is created during heavy-ion collisions. Heavy quarks are produced during initial hard-scattering processes prior to the QGP formation and their number is conserved throughout the partonic and hadronic phases of the collision. At LHC, experimental observations of quarkonium states in A-A collisions are reproduced through two antagonist mechanisms: the sequential suppression and quarkonium production by (re)combination of deconfined quarks. Recent measurements of a significant J/ψ elliptic flow (v2) in Pb-Pb collisions at TeV suggest a participation of the charm quarks to the collectivity of the medium. However, quarkonium interactions in the medium are not yet fully understood and several points should still be clarified. The ALICE experiment at the LHC measures quarkonia at mid-rapidity in the dielectron decay channel and at forward rapidity in the dimuon one, both down to zero transverse momentum. Latest measurements of quarkonium nuclear modification factor and elliptic flow in Pb-Pb collisions will be presented and compared to lower energy results and theoretical predictions.

Quarkonium production in Pb-Pb collisions at √SNN = 5.02 TeV with ALICE

Ultra-relativistic heavy-ion collisions at the Large Hadron Collider provide a unique opportunity to study the properties of matter at extreme energy densities where a phase transition from the hadronic matter to a deconfined medium of quarks and gluons, the Quark-Gluon Plasma (QGP) is predicted. Among the prominent probes of the QGP, heavy quarks play a crucial role since they are created during the initial stages of the collision, before the QGP formation, and their number is conserved throughout the partonic and hadronic phases of the collision. The azimuthal anisotropy of charmonium production, quantified using the second harmonic Fourier coefficient (referred to as elliptic flow), provides important information on the magnitude and dynamics of charmonium production. Measurements of the quarkonium nuclear modification factor at forward rapidity and J/ψ elliptic flow in Pb-Pb collisions as a function of centrality, transverse momentum and rapidity will be presented and compared to different collision energy results and available theoretical calculations.

J/ψ Elliptic Flow in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

We report a precise measurement of the J/ψ elliptic flow in Pb-Pb collisions at sqrt[s_{NN}]=5.02 TeV with the ALICE detector at the LHC. The J/ψ mesons are reconstructed at midrapidity (|y|<0.9) in the dielectron decay channel and at forward rapidity (2.5<y<4.0) in the dimuon channel, both down to zero transverse momentum. At forward rapidity, the elliptic flow v_{2} of the J/ψ is studied as a function of the transverse momentum and centrality. A positive v_{2} is observed in the transverse momentum range 2<p_{T}<8 GeV/c in the three centrality classes studied and confirms with higher statistics our earlier results at sqrt[s_{NN}]=2.76 TeV in semicentral collisions. At midrapidity, the J/ψ v_{2} is investigated as a function of the transverse momentum in semicentral collisions and found to be in agreement with the measurements at forward rapidity. These results are compared to transport model calculations. The comparison supports the idea that at low p_{T} the elliptic flow of the J/ψ originates from the thermalization of charm quarks in the deconfined medium but suggests that additional mechanisms might be missing in the models.

Measurement of deuteron spectra and elliptic flow in Pb-Pb collisions at root s(NN)=2.76 TeV at the LHC

Measurement of deuteron spectra and elliptic flow in Pb-Pb collisions at root s(NN)=2.76 TeV at the LHC

Open heavy-flavour measurements in p-Pb and Pb-Pb collisions with ALICE at the LHC

Heavy flavours are sensitive probes of the hot and dense QCD medium formed in high-energy heavy-ion collisions. Measurements of their production in p-Pb collisions are crucial for the interpretation of heavy-ion results, by investigating the cold nuclear matter effects. The open heavy-flavour production studied with ALICE at the LHC in p-Pb collisions at and in Pb-Pb collisions at are presented. Emphasis is given to the recent measurements of D0 production cross section down to pT=0, the nuclear modification factor of heavy-flavour hadron decay electrons in p-Pb collisions, the nuclear modification factor of D-meson, and heavy-flavour hadron decay electron elliptic flow in Pb-Pb collisions, as a function of centrality.

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions atsNN=2.76TeV

We report on results obtained with the Event Shape Engineering technique applied to Pb-Pb collisions at $\sqrt{s_\rm{NN}}=2.76$ TeV. By selecting events in the same centrality interval, but with very different average flow, different initial state conditions can be studied. We find the effect of the event-shape selection on the elliptic flow coefficient $v_2$ to be almost independent of transverse momentum $p_\rm{T}$, as expected if this effect is due to fluctuations in the initial geometry of the system. Charged hadron, pion, kaon, and proton transverse momentum distributions are found to be harder in events with higher-than-average elliptic flow, indicating an interplay between radial and elliptic flow.

Results on open-charm production in pp, p-Pb and Pb-Pb collisions with ALICE at the LHC

ALICE (A Large Ion Collider Experiment) is designed to study the strongly-interacting medium created in heavy-ion collisions at LHC energies, the Quark-Gluon Plasma (QGP). Charm and beauty quarks are powerful probes to study the QGP in heavy-ion collisions: produced in hard partonic scattering processes on a short time scale, they are expected to traverse the QCD medium, interacting with its constituents and losing energy through radiative and collisional processes. In ALICE, open-charm production is studied through the reconstruction of the hadronic decays of D0, D+, D*+ and Ds+ mesons at mid-rapidity. The high precision tracking, good vertexing capabilities and excellent particle identification offered by ALICE allow for the measurement of particles containing heavy quarks (particularly D mesons) in a wide transverse-momentum range in pp, p-Pb and Pb-Pb collisions. A review of the main results on D-meson production in pp collisions at = 7 TeV, Pb-Pb collisions at = 2.76 TeV and the most recent results in p-Pb collisions at = 5.02 TeV will be presented. In particular, the pT-differential yields and cross sections in the three collision systems, the nuclear modification factors RAA and RpPb in Pb-Pb and p-Pb collisions, and the elliptic flow in Pb-Pb collisions will be discussed. The D-meson yield in pp and p-Pb collisions will also be shown as a function of charged-particle multiplicity.

D meson elliptic flow in Pb-Pb collisions at = 2.76 TeV measured with ALICE

These proceedings present the D0, D+ and D*+ meson v2 measured by ALICE in Pb-Pb collisions at = 2.76 TeV with various methods (the standard event plane, the scalar product and the Q-cumulant) and in different centrality classes.

Measurement of Direct-Photon Elliptic Flow in Pb-Pb Collisions at = 2.76 TeV

We present the first measurement of the direct-photon elliptic flow vγ,dir2 in Pb-Pb collisions at = 2.76 TeV with data taken by the ALICE experiment at the LHC. The measurement provides evidence for a non-zero vγ,dir2 for 1 < pT < 3GeV/c with a magnitude similar to the observed charged pion . In order to explain the large inverse slope parameter Teff of the low pT direct-photon spectrum observed at LHC and RHIC, recent hydrodynamical descriptions of the direct-photon production include a substantial portion of thermal photons from the hot plasma phase. As a consequence of the early production time, vγ,dir2 is expected to be small compared to hadrons. A large vγ,dir2 might lend support for a significant direct-photon emission from late stages of the system evolution where hadron flow has developed.

Measurement of electrons from semi-leptonic heavy-flavour hadron decays in proton-proton and Pb–Pb collisions with ALICE at the LHC

Heavy quarks are produced in early partonic hard scatterings and experience the full history of a heavy-ion collision. Consequently heavy quarks are a good tool to study properties of the hot and dense medium. In proton-proton collisions, the study of the production of heavy quarks provides an important test for perturbative QCD and serves as a reference for heavy ion collisions. Heavy-flavour measurements in the semi-electronic channel are performed with ALICE at midrapidity. We present the measurement of the pT-differential cross section in proton-proton collisions at = 7 TeV, and of the nuclear modification factor and elliptic flow in Pb-Pb collisions at = 2.76 TeV.

Multi-strange baryon elliptic flow in Pb-Pb collisions at $\sqrt {s_{NN} } $ = 2.76 TeV measured with the ALICE detector

Abstract We present the results on elliptic flow with multi-strange baryons produced in Pb-Pb collisions at $\sqrt {s_{NN} } $ = 2.76 TeV. The analysis is performed with the ALICE detector at LHC. Multi-strange baryons are reconstructed via their decay topologies and the v 2 measurements are analyzed with the two-particle scalar product method. The p T differential v 2 values are compared to the viscous hydrodynamical (VISH2+1) model calculation and to the STAR measurements in Au-Au collisions at $\sqrt {s_{NN} } $ = 200 GeV. We found that the model describes ≡ and Ωv 2 measurements within uncertainties. The differential v 2 of ≡ and Ω is similar to the STAR measurements at 200 GeV in Au-Au collisions.

Elliptic flow at energies available at the CERN Large Hadron Collider: Comparing heavy-ion data to viscous hydrodynamic predictions

I compare the first viscous hydrodynamic prediction for integrated elliptic flow in Pb-Pb collisions at the LHC with the first data released by the ALICE collaboration. These new data are found to be consistent with hydrodynamic extrapolations of RHIC data with no change in medium parameters (e.g., average viscosity). I also discuss how, in general, a precise comparison of data to theoretical calculations requires an understanding of some subtleties of the measurement -- most notably the cut on transverse momentum of the particles used and the differing sensitivities to flow fluctuations and non-flow effects of the various measurement methods.

Elliptic Flow of Charged Particles in Pb-Pb Collisions atsNN=2.76 TeV

We report the first measurement of charged particle elliptic flow in Pb-Pb collisions at sqrt[S(NN)] =2.76 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed in the central pseudorapidity region (|η|<0.8) and transverse momentum range 0.2<p t<5.0 GeV/c. The elliptic flow signal v₂, measured using the 4-particle correlation method, averaged over transverse momentum and pseudorapidity is 0.087 ± 0.002(stat) ± 0.003(syst) in the 40%-50% centrality class. The differential elliptic flow v₂ p t reaches a maximum of 0.2 near p t =3 GeV/c. Compared to RHIC Au-Au collisions at sqrt[S(NN)] 200 GeV, the elliptic flow increases by about 30%. Some hydrodynamic model predictions which include viscous corrections are in agreement with the observed increase.