Phuong Mai Dinh

Flow analysis from multiparticle azimuthal correlations

We present a new method for analyzing directed and elliptic flow in heavy-ion collisions. Unlike standard methods, it separates the contribution of flow to azimuthal correlations from contributions due to other effects. The separation relies on a cumulant expansion of multiparticle azimuthal correlations, and includes corrections for detector inefficiencies. This new method allows the measurement of the flow of identified particles in narrow phase-space regions, and can be used in every regime, from intermediate to ultrarelativistic energies.

New method for measuring azimuthal distributions in nucleus-nucleus collisions

The methods currently used to measure azimuthal distributions of particles in heavy-ion collisions assume that all azimuthal correlations between particles result from their correlation with the reaction plane. However, other correlations exist, and it is safe to neglect them only if azimuthal anisotropies are much larger than

Transverse energy fluctuations and the pattern of J/psi suppression in Pb-Pb collisions.

1/sqrt{N},

Is the analysis of flow at the CERN Super Proton Synchrotron reliable

with N the total number of particles emitted in the collision. This condition is not satisfied at ultrarelativistic energies. We propose a new method, based on a cumulant expansion of multiparticle azimuthal correlations, which allows measurements of much smaller values of azimuthal anisotropies, down to

Effects of HBT correlations on flow measurements

1/N.

Effects of momentum conservation on the analysis of anisotropic flow

It is simple to implement and can be used to measure both integrated and differential flow. Furthermore, this method automatically eliminates the major systematic errors, which are due to azimuthal asymmetries in the detector acceptance.

Analysis of directed flow from elliptic flow

The NA50 Collaboration has recently observed that the J/psi production rate in Pb-Pb collisions decreases more rapidly as a function of the transverse energy for the most central collisions than for less central ones. We show that this phenomenon can be understood as an effect of transverse energy fluctuations in central collisions. A good fit of the data is obtained using a model which relates J/psi suppression to the local energy density. Our results suggest that the J/psi is completely suppressed at the highest densities achieved in Pb-Pb collisions.

Analysis of directed flow from three-particle correlations

Several heavy ion experiments at SPS have measured azimuthal distributions of particles with respect to the reaction plane. These distributions are deduced from two-particle azimuthal correlations under the assumption that they result solely from correlations with the reaction plane. In this paper, we investigate other sources of azimuthal correlations: transverse momentum conservation, which produces back-to-back correlations, resonance decays, HBT correlations and final state interactions. These correlations increase with impact parameter: most of them vary with the multiplicity N like 1/N. When they are taken into account, the experimental results of the NA49 collaboration at SPS are significantly modified. These correlations might also explain an important fraction of the pion directed flow observed by WA98. Data should be reanalyzed taking into account carefully these non--flow correlations.

Multiparticle azimuthal correlations

Abstract The methods currently used to measure collective flow in nucleus–nucleus collisions assume that the only azimuthal correlations between particles are those arising from their correlation with the reaction plane. However, quantum HBT correlations also produce short range azimuthal correlations between identical particles. This creates apparent azimuthal anisotropies of a few percent when pions are used to estimate the direction of the reaction plane. These should not be misinterpreted as originating from collective flow. In particular, we show that the peculiar behaviour of the directed and elliptic flow of pions observed by NA49 at low pT can be entirely understood in terms of HBT correlations. Such correlations also produce apparent higher Fourier harmonics (of order n≥3) of the azimuthal distribution, with magnitudes of the order of 1%, which should be looked for in the data.

J/ψ suppression in central Pb–Pb collisions

We present a general method for taking into account correlations due to momentum conservation in the analysis of anisotropic flow, either by using the two-particle correlation method or the standard flow-vector method. In the latter, the correlation between the particle and the flow vector is either corrected through a redefinition (shift) of the flow vector, or subtracted explicitly from the observed flow coefficient. In addition, momentum conservation contributes to the reaction plane resolution. Momentum conservation mostly affects the first harmonic in azimuthal distributions, i.e., directed flow. It also modifies higher harmonics, for instance, elliptic flow, when it is measured with respect to a first harmonic event plane such as the one determined with the standard transverse momentum method. Our method is illustrated by application to NA49 data on pion directed flow.