Correlations In Nucleus-nucleus Collisions Research Articles

Integrated Azimuthal Correlations in Nucleus-Nucleus Collisions at CERN SPS

Azimuthal correlations of particles produced in nucleus-nucleus collisions at CERN SPS are discussed. The correlations quantified by the integral measure Phi are shown to be dominated by effects of collective flow.

Jets propagation through a hadron-string medium

Di-jet correlations in nucleus-nucleus collisions are studied within the Hadron-String-Dynamics (HSD) transport approach taking into account the reaction of the medium on the jet energy loss nonperturbatively. A comparison with the STAR and PHENIX data in central Au+Au collisions at the RHIC energy $\sqrt{s}=200$ GeV is performed differentially, i.e. with respect to correlations in azimuthal angle $\Delta \phi$ and pseudorapidity $\Delta \eta$. The HSD results do not show enough suppression for the `away-side' jets in accordance with earlier perturbative studies. Furthermore, the `Mach-cone' structure for the angle distribution in the `away-side' jet as well as `ridge' long rapidity correlations in the `near-side' jet - observed by the STAR and PHOBOS Collaborations - are not seen in the HSD results, thus suggesting a partonic origin.

Forward-backward correlations in nucleus-nucleus collisions: Baseline contributions from geometrical fluctuations

We discuss the effects of initial collision geometry and centrality bin definition on correlation and fluctuation observables in nucleus-nucleus collisions. We focus on the forward-backward correlation coefficient recently measured by the STAR Collaboration in Au+Au collisions at RHIC. Our study is carried out within two models: the Glauber Monte Carlo code with a `toy' wounded nucleon model and the hadron-string dynamics (HSD) transport approach. We show that strong correlations can arise due to averaging over events in one centrality bin. We, furthermore, argue that a study of the dependence of correlations on the centrality bin definition as well as the bin size may distinguish between these `trivial' correlations and correlations arising from `new physics'.

Azimuthally sensitive correlations in nucleus-nucleus collisions

We define a set of model-independent observables which generally characterize the azimuthal dependence of two-particle correlations in nucleus-nucleus collisions. We explain how they can be analyzed, and show to what extent such analyses are model dependent. We discuss specific applications to the anisotropic flow of decaying particles, azimuthally sensitive Hanbury-Brown Twiss interferometry, and correlations between particles at large transverse momentum. A quantitative prediction is made for jet quenching with respect to the reaction plane.

Scaling behaviour and correlations in the string fusion model for heavy ion collisions

The KNO scaling, the scaling of multiplicities, the behaviour of fluctuations in the transverse momentum as a function of the multiplicity and the long range correlations in nucleus-nucleus collisions are studied by a Monte Carlo code based on the string fusion model. It is shown that the fusion of strings produces a strong reduction of the long range correlations at high multiplicities, which could be detected experimentally. On the contrary the KNO scaling, the scaling of multiplicities and the behaviour of fluctuations in the transverse momentum are not modified when string fusion is included.

Large-angle two-particle correlations in nucleus-nucleus collisions at 3.6 GEV/nucl.

For nucleus-nucleus collisions at 3.6 GeV/nucl., we have measured correlations between a charged particle emitted in the range from 10° to 18° and identified a charged fragment (proton, deutron or pion) emitted at a certain angle in the range of 45°–130°. The following projectile-target combinations were studied: C+Al (45°, 60°, 80°, 130°); C+Cu (45°, 60°, 80°); C+Pb (45°, 60°, 80°, 130°); He+Al (45°, 80°, 130°); He+Pb (45°, 80°, 130°). In all the studied reactions, correlated particle emission has been observed.