Azimuthal anisotropy of electrons from heavy flavor decays in \sqrt{s_{NN}} =200 GeV Au-Au collisions at PHENIX

The PHENIX experiment has measured the azimuthal anisotropy parameter v₂, the second harmonic of the azimuthal distribution, for electrons at mid-rapidity (|η|<0.35) as a function of transverse momentum (0.5<p_T (GeV/c)<5.0) in Au+Au collisions at  =200 GeV. From the result we have calculated the non-photonic electron v₂, which is expected to reflect charm quark azimuthal anisotropy, by subtracting the v₂ of electrons from other sources such as photon conversions and Dalitz decays.     

Out-of-plane emission of nuclear matter in Au+Au collisions between 100 and 800 AMeV

We present new experimental results concerning the azimuthal distributions of proton-likes, light and intermediate mass fragments at midrapidity for Au(100–800 A MeV) +Au collisions measured with the FOPI phase-I detector at GSI in Darmstadt. The azimuthal distributions are investigated as a function of the collision centrality, the incident energy, the fragment charge and transverse momentum. The azimuthal anisotropy is maximum for impact parameters around 7 fm. Intermediate mass fragments present a stronger out-of-plane emission signal than light fragments and a saturation is reached for Z ⩾ 4. The azimuthal anisotropy increases with the fragment transverse momentum and decreases as the incident energy increases. The azimuthal anisotropy of Z = 2 particles investigated as a function of the scaled fragment transverse momentum follows an universal curve for bombarding energies between 250–800 A MeV. A signature for a transition from in-plane to out-of-plane emission is evidenced at the lowest beam energies.     

Screening of instantons in the quark gas

The grand partition function of quark matter is developed about an arbitrary classical gauge field configuration in a systematic weak coupling expansion. In the presence of a finite density massless quark gas the instanton induced effective quark interaction is modified by a factor exp[?2N_F(ω?)²], i.e. the baryon number chemical potential μ acts as an intrinsic infrared cutoff on the instanton scale size ?. The equation of state of the quark matter is also briefly discussed.     

Elliptic flow of particles in heavy-ion collisions and possibility of its observation at the LHC energies

The azimuthal anisotropy of particle distributions is one of the important observables that characterize collective effects at the initial stage of the formation and expansion of quark-gluon matter in collisions of relativistic nuclei. It is expected that an increase in energy from RHIC to LHC values will be accompanied by the growth of the azimuthal-anisotropy coefficient υ ₂ and the growth of nonflow effects (contribution of minijets and other correlations between particles). In the present study, methods for determining the azimuthal-anisotropy coefficient υ ₂ are analyzed on the basis of events simulated with the aid of the HYDJET generator for Pb + Pb collisions at the LHC energies, the contribution of nonflow effects being estimated. The accuracy of measuring the coefficient υ ₂ as a function the transverse momentum and pseudorapidity under experimental conditions at LHC is assessed.     

Spectra,elliptic flow and azimuthally sensitive HBT radii from the Buda-Lund model for $ \sqrt{{s_{NN}}}$ = 200 GeV Au + Au collisions

We present calculations of elliptic flow and azimuthal dependence of correlation radii in the ellipsoidally symmetric generalization of the Buda-Lund hydrodynamic model of hadron production in high-energy nuclear collisions. We compare them to data from RHIC by simultaneous fits to azimuthally integrated invariant spectra of pions, kaons and protons-antiprotons measured by PHENIX in Au + Au reactions at center-of-mass energy of 200 AGeV. STAR data were used for azimuthally sensitive two-particle correlation function radii and for the transverse momentum dependence of the elliptic flow parameter v ₂. We have found that the transverse flow is faster in the reaction plane than out of plane, which results in a reaction zone that gets slightly more elongated in-plane than out of plane. The model parameters extracted from the fits are shown and discussed.     

Measurement of the azimuthal anisotropy coefficient <Emphasis Type="Italic">v</Emphasis><Subscript>2</Subscript> for the emission of α particles in nucleus-nucleus collisions at energies 1.88–10.6 GeV/nucleon

The azimuthal anisotropy of the emission of α particles in collisions of the ²²Ne, ²⁴Mg, ⁵⁶Fe, and ¹⁹⁷Au nuclei with photoemulsion nuclei has been measured at projectile energies E_pr = 1.88–10.6 GeV/nucleon. The results are compared with similar measurements for protons. It has been found that the ratio of the azimuthal anisotropy coefficients v₂ for α particles and protons is equal to 6 ± 2 at low energies E_pr ≈ 2 GeV/nucleon, whereas these coefficients coincide with each other for energies E_pr ≥ 4 GeV/nucleon. This difference may indicate that, at low projectile energies, α particles are formed predominantly at the early stage of a collective flow. Formation of α particles for E_pr ≥ 4 GeV/nucleon likely occurs at the stage of nuclear matter scattering.     

Azimuthal asymmetries in exclusive four-particle “Drell-Yan” events

I study hard collisions between unpolarized protons and antiprotons where a lepton-antilepton pair is detected in coincidence with a final proton-antiproton pair, and no more particles are produced, in the regime 10 GeV2 ≪ s ≪ 1000 GeV2 , M > 4 GeV, q _T < 3 GeV/c. The present work is centered on azimuthal asymmetries. Because of momentum conservation, a Boer-Mulders term in the momentum distribution of a quark implies a balancing effect in the momentum distribution of some spectators. This produces azimuthal asymmetries of the final hadrons. To analyze this, I have organized a parton-level Monte Carlo generator where a standard cos(2f \phi) -asymmetry of the dilepton distribution is produced, thanks to a soft rescattering process between an active quark coming from a hadron and a spectator antidiquark coming from the other hadron. This produces cos(2f \phi) -asymmetries of the final hadron pair. Hadron and lepton asymmetries have the same size.     

Measurement of identified and inclusive photon second-harmonic parameter and implications for direct photon production in [FORMULA: SEE TEXT] Au+Au

The azimuthal distribution of identified pi0 and inclusive photons has been measured in [FORMULA: SEE TEXT] Au+Au collisions with the PHENIX experiment at the Relativistic Heavy-Ion Collider (RHIC). The second-harmonic parameter (nu2) was measured to describe the observed anisotropy of the azimuthal distribution. The measured inclusive photon is consistent with the value expected for the photons from hadron decay and is also consistent with the lack of direct photon signal over the measured pT range 1-6 GeV/c. An attempt is made to extract nu2 of direct photons.     

Correlations among elliptic flow parameter,impact parameter,and multiplicity

The elliptic flow parameter, v₂, in the Fourier expansion of the event-by-event charged-particle multiplicity azimuthal distribution in the momentum space is studied by taking into account the multiplicity fluctuations. The correlations among measurable variables of v₂, charged multiplicity, and impact parameter are investigated using a multiple phase transport model with string melting. It turned out that the charged multiplicity and impact parameter are negatively correlated. This correlation results in the peak structure in v₂ as a function of the participant nucleon number (charged multiplicity, impact parameter) measured in RHIC experiments. It is suggested that v₂ calculated in the event-wise average is about 20% larger than in the particle-wise average, so event-wise average should be used in the theoretical calculations.     

Charged hadron suppression in Au+Au collisions at high p T from STAR

We report high statistics measurements of inclusive charged hadron production in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV and 130 GeV. A large, approximately constant hadron suppression is observed in central Au+Au collisions for 5 ‖p _T ‖ 12 GeV/c. The collision energy dependence of the yields and the centrality and p _T dependence of the suppression provide stringent constraints on theoretical models of suppression. Models incorporating initial-state gluon saturation or partonic energy loss in dense matter are largely consistent with observations. The p _T -dependent suppression expected from models incorporating jet attenuation in cold nuclear matter or absorption of fragmentation hadrons is not observed.     

Direct photon production beyond leading order in QCD

The significance of the quark-quark scattering process (quark+quark→quark+quark+photon) for the production of large-q_T real photons is discussed in the framework of perturbative QCD. To extract the finite contribution of this process to the differential cross section dσ/dy d²q_T (hadron 1+hadron 2→photon+anything) we define the gluon distribution and the quark-to-photon fragmentation function beyond the leading approximation. The calculations are performed consistently in the dimensional regularization scheme. Our numerical estimates show the resulting finite qq→qqγ contribution to be a small (order α_s/2π) correction in comparison with the basic QCD subprocesses.     

Effect of hadronic hard-core radius on the phase transition to an interacting quark-gluon plasma

The effect of volume corrections on the equations of state for the hadron gas by treating nucleons and antinucleons as hard-core particles or bags is studied. Its consequences on the critical values of temperatureT and chemical potential μ _B of the phase transition from a gas of finite sized hadrons to an interacting quark matter are explored.     

Spin physics at HERMES

The primary goal of the HERMES experiment is the study of the spin structure of the nucleon. Results on the measured inclusive and semi-inclusive hadron asymmetries using a polarized positron beam on polarized ³He, hydrogen and deuterium targets are here presented. In the covered kinematic range, 0.023<x _Bj <0.6 and 1 GeV²<Q ²<10GeV² the polarized quark distribution were determined for all up (u+?u) and down (d+?) quarks, and separately for valence and sea quarks. The up quark polarization is positive, and the down quark polarization is negative. The polarization of the sea is consistent with zero in the measured range. A first indication of a positive gluon polarization is presented, based on the measured spin asymmetry in the photo-production of hadron pairs with high transverse momentum p _T . This asymmetry is negative, which is in contrast to the measured positive asymmetry for inclusive experiments.     

J/ψ azimuthal anisotropy relative to the reaction plane in Pb-Pb collisions at 158?GeV per nucleon

The J/ψ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J/ψ mesons at SPS energies. Hence, the measurement of J/ψ elliptic anisotropy, quantified by the Fourier coefficient v ₂ of the J/ψ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at explaining the anomalous J/ψ suppression observed in Pb-Pb collisions. We present the measured J/ψ yields in different bins of azimuthal angle relative to the reaction plane, as well as the resulting values of the Fourier coefficient v ₂ as a function of the collision centrality and of the J/ψ transverse momentum. The reaction plane has been estimated from the azimuthal distribution of the neutral transverse energy detected in an electromagnetic calorimeter. The analysis has been performed on a data sample of about 100 000 events, distributed in five centrality or p _T sub-samples. The extracted v ₂ values are significantly larger than zero for non-central collisions and are seen to increase with p _T.     

Scaling properties of azimuthal anisotropy of mesons and baryons at RHIC

Detailed measurements of the azimuthal anisotropy (v ₂) for identified charged particles are reported as a function of transverse momentum (pT) and centrality for Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV. The measurements indicate clear evidence for eccentricity and particle flavor scaling over a broad range of centralities and transverse rapidity yT, indicating a hydrodynamical origin of the fine structure of azimuthal anisotropy at RHIC. The observed scaling supports the picture of a suddenly hadronizing (recombining) fluid of quarks. An apparent breaking of flavor scaling at relatively large values of yT points to an important change in the mechanism for particle emission.     

Physics with the ALICE experiment

ALICE experiment at LHC collects data in pp collisions at 1497-1 = 0.9, 2.76, and 7 TeV and in PbPb collisions at 2.76 TeV. Highlights of the detector performance and an overview of experimental results measured with ALICE in pp and AA collisions are presented in this paper. Physics with protonproton collisions is focused on hadron spectroscopy at low and moderate p _t . Measurements with lead-lead collisions are shown in comparison with those in pp collisions, and the properties of hot quark matter are discussed.     

Finite corrections to quark fragmentation functions in perturbative QCD

The finite corrections of order α _s in perturbative QCD to the cross-sections for semi-inclusive hadron production from deep inelastic leptonhadron scattering and electron-positron annihilation are calculated. We define the effective quark fragmentation functions viae ⁺ e ^? → hadron +X including the finite terms in order to estimate these corrections for the reactions lepton + hadron → lepton + hadron + anything. Contrary to the leading term the next-to-leading order term does not factorize into parts depending on the target and the fragment, respectively. For the processese +p →e + π^± +X andv +p →μ ^? + π^± +X the finite corrections of order α _s turn out to be at most 20% in the range of momenta covered by present experiments.     

QCD three-jet production in large-p⊥ processes

The contribution of the hard scattering subprocess quark + quark → quark + quark + gluon to two-particle inclusive distributions at large p_⊥ is studied. It is shown to be accessible experimentally, and agrees with recent data on azimuthal distributions in the adequate kinematical region.