Fluctuations and correlations in STAR

The study of correlations and fluctuations can provide evidence for the production of the quark-gluon plasma (QGP) in relativistic heavy-ion collisions. Various theories predict that the production of a QGP phase in relativistic heavy-ion collisions could produce significant event-by-event correlations and fluctuations in transverse momentum, multiplicity, etc. Some of the recent results using STAR at RHIC will be presented along with results from other experiments at RHIC. The focus is on forward-backward multiplicity correlations, balance function, charge and transverse-momentum fluctuations, and correlations.     

High transverse momentum correlations in STAR

Azimuthal correlations of charged hadrons with transverse momenta above 2 GeV/c are measured relative to ≥ GeV/c trigger particle in p+p and Au+Au at $\sqrt {s_{NN} } = 200$ GeV in STAR. The correlations exhibit small relative azimuthal angle correlations characteristic of jets. The p+p and peripheral Au+Au data exhibit a peak on the opposite side of the jet indicating the presence of hard scattering in the form of di-jets. The away-side jet disappears for the most central Au+Au collisions, suggesting jet quenching in a dense medium.     

On multiplicity correlations in the STAR data

The STAR data on the multiplicity correlations between narrow pseudorapidity bins in the pp and AuAu collisions are discussed. The PYTHIA 8.145 generator is used for the pp data, and a na?ve superposition model is presented for the AuAu data. It is shown that the PYTHIA generator with default parameter values describes the pp data reasonably well, whereas the superposition model fails to reproduce the centrality dependence seen in the data. Some possible reasons for this failure and a comparison with other models are presented.     

Long range rapidity correlations as seen in the STAR experiment

We analyze long range rapidity correlations observed in the STAR experiment at RHIC. Our goal is to extract properties of the two particle correlation matrix, accounting for the analysis method of the STAR experiment. We find a surprisingly large correlation strength for central collisions of gold nuclei at highest RHIC energies. We argue that such correlations cannot be the result of impact parameter fluctuations.     

High transverse momentum identified particle spectra in 200 GeV collisions from STAR

Significant baryon over meson enhancement was measured at RHIC in the intermediate transverse momentum range of p _T =2?4 GeV/c (“baryon-meson puzzle”). With STAR detector we were able to extend particle identification towards higher transverse momentum offering further insights into the particle production mechanisms at intermediate to high p _T . In this paper we present results on charged pion, proton and anti-proton spectra and ratios at intermediate to high p _T exploiting the relativistic rise of the specific ionization energy loss measured in the STAR Time Projection Chamber. These measurements provide valuable information about the production mechanisms of particles at intermediate p _T in relativistic heavy ion collisions, e.g. coalescence/recombination versus jet fragmentation.     

Many particle correlations in multiple production on nuclei

In the framework of the multiple scattering models we derive the relations between the many particle rapidity correlations on nuclei and on nucleons.     

Light particle correlations in intermediate energy heavy ion collisions

Various light particle coincidence techniques will be reviewed that were employed to study the emission of energetic light particles in nucleus-nucleus collisions at energies above 10 MeV/nucleon. Several reaction mechanisms have been shown to contribute to the emission of energetic light particles, ranging from the sequential statistical decay of excited projectile residues, direct breakup and knock-out reactions, to multistep emission processes that can be rather well described in terms of the concept of local thermal equilibrium.     

Study of neutral-charged particle correlations in high energy collisions

Recent experiments at Serpukhov, NAL, and CERN indicate a strong correlation between neutral and charged pions produced in high energy collisions, in contrast to the trend shown by data at lower energies. This study of the energy and charge dependence of these correlations indicates that they do not depend upon the initial state particles and that they are in reasonable agreement with the critical fluid gas model. These high energy correlation data are also studied in terms of a semi-inclusive Koba-Nielsen-Olesen scaling relation.     

Transverse radial expansion in nuclear collisions and two particle correlations

At the very first stage of an ultra-relativistic nucleus–nucleus collision new particles are produced in individual nucleon–nucleon collisions. In the transverse plane, all particles from a single NN collision are initially located at the same position. The subsequent thermalization and transverse radial expansion of the system create strong position-momentum correlations and lead to characteristic rapidity, transverse momentum, and azimuthal correlations among the produced particles.     

Neutral strange particle correlations in 360 GeV/cpp interactions

Production properties and correlations forK _s ⁰ K _s ⁰ ,K _s ⁰ Λ,K _s ⁰ ¯Λ and Λ¯Λ systems in 360 GeV/cpp interactions are presented. All rapidity gap distributions are observed to peak at Δy=0 and the azimuthal angular distributions between the two particles are consistent with being flat. Experimental results are compared with the quark fusion and Lund models of particle production.     

Identical particle effect in azimuthal correlations at small rapidity separation

The difference of the short range effect observed in azimuthal correlations of like and unlike particle pairs is interpreted as due to an identical and unidentical boson effect, resp., in the decay of clusters produced in the central region of hadron hadron collisions.     

Spatial particle correlations in light nuclei. II four-particle systems

The geometry of configurations of identical particles in a shell is extended from two to four particles. Results are derived for a general $?$ shell and particular attention is paid to the $p$ shell ($?=1$), which is of relevance for the nucleus ${}^8$He. Expressions are given for the angular probability density in terms of the six angles between pairs of position vectors of the particles. The analysis of the $p$ shell reveals the existence of two classes of favored four-particle configurations, with three members each. The transition from one class to the other is governed by the nuclear dynamics and depends on the conflicting tendencies of the short-range nuclear interaction versus the spin–orbit splitting.     

Near-side Δη correlations of high-p<Subscript>t</Subscript> hadrons from STAR

Systematic measurements of pseudorapidity (Δη) and azimuthal (Δϕ) correlations between high-p_t charged hadrons and associated particles from the high statistics 200 GeV Au+Au and Cu+Cu datasets will be presented. In previous measurements differences in the near-side Δη correlation between central Au+Au and the lighter systems, d+Au and p+p, were observed, including a long-range near-side correlation in Au+Au collisions. Studies to characterize the properties of the long-range correlation will be presented. PACS 25.75.Dw; 25.75.Gz     

Three particle inclusive correlations from relativistic heavy ion collisions

The life-time of the first excited member of the ground-state band in¹⁰¹Zr has been determined to 0.9(3) ns through the measurement of γ-γ-delayed coincidences with Ge detectors at the fission-product separator JOSEF. The result shows that this nucleus is strongly deformed with |ß|=0.32(6). This finding provides further important evidence for the fact that the odd-mass nuclei at A ~ 100 with more than 59 neutrons are good rotors.     

Nuclear matter equation of state with single particle correlations

Symmetrical nuclear matter at finite temperature is studied in the framework of the Brueckner-Hartree-Fock approximation extended to include single-particle correlations. A liquid-vapor phase transition is observed, wtih a critical temperature of about 20 MeV, in close similarity with Skyrme force calculations. The inclusion of single-particle correlations introduces a significant temperature dependence in the single-particle potential as well as in the nucleon effective mass. In this scheme the Hughenholtz-Van Hove theorem is well satisfied throughout the range of density and temperature considered.