Repositório RCAAP

Experimental observation of jet quenching in ultra-relativistic heavy-ion collisions is one of the most remarkable discoveries at RHIC. High-pT hadron suppression, disappearance of back-to-back jets, and strong away-side modification at intermediate to low pT have provided us many insights into the matter created at RHIC. Particularly, angular correlations have become a powerful tool to study the QCD matter through its interactions with jets. Di-hadron correlations reveal significant broadening and softening of associated hadrons on the away side of a triggered high-pT particle. Many mechanisms have been proposed to accommodate the data, including the intriguing Mach cone shock waves in a thermalized hydrodynamic matter, which can be discriminated by 3-particle correlations. Here a brief overview of these remarkable experimental measurements on jet quenching is presented and implications of these measurements are discussed.

We report our recent work on conversions between gluon and quark jets as they traverse through a quark-gluon plasma (QGP) and their effects on the nuclear modification factors for quark and gluon jets as well as the ratios of p/pi+ and $\bar{p}$/pi- at high transverse momentum in ultra-relativistic heavy ion collisions.

I give an overview of the presentations at the International Symposium on Multiparticle Dynamics. 2006

I review the prospects for measuring source characteristics from correlations other than those involving identical pions. Correlations generated from Coulomb and strong interactions are shown to provide remarkable resolving power for determining three-dimensional information, in some cases accessing more detail than can be represented by Gaussian fits.

Taking up and extending earlier suggestions, we show how two- and three-dimensional shapes of second-order HBT correlations can be described in a multivariate Edgeworth expansion around Gaussian ellipsoids, with expansion coefficients, identified as the cumulants of pair momentum difference q , acting as shape parameters. Off-diagonal terms dominate both the character and magnitude of shapes. Cumulants can be measured directly and so the shape analysis has no need for fitting.

Shape analyses of two-particle correlation functions are discussed. Two-proton imaging provides information about the volume, the relative contributions between fast and slow emitting sources and the profile of the two-particle source that can be directly compared to microscopic model simulations. In the case of correlations between complex particles, the role played by collective motion and space-momentum correlations needs special considerations. By means of a semi-quantitative Monte Carlo approach, deuteron-alpha correlation functions measured in Xe+Au collisions at E/A=50 MeV are investigated. The comparison reveals the existence of a position-relative momentum correlation that reduces the apparent source size and distorts the line-shape of the correlation function. The developed ideas show how intensity interferometry with complex particles is sensitive not only to the geometry of the system but also to the interplay between collective and thermal motion.

Recent femtoscopic measurements involving the use of an imaging technique and a newly developed moment analysis are presented and discussed. We show that this new paradigm allows robust investigation of reaction dynamics for which the sound speed c s <FONT FACE=Symbol>¹</FONT> 0 during an extended hadronization period. Source functions extracted for charged pions produced in Au+Au and Pb+Pb collisions show non-Gaussian tails for a broad selection of collision energies. The ratio of the RMS radii of these source functions in the out and side directions are found to be greater than 1, suggesting a finite emission time for pions.

The inability of otherwise successful dynamical models to reproduce the "HBT radii" extracted from two-particle correlations measured at the Relativistic Heavy Ion Collider (RHIC) is known as the "RHIC HBT Puzzle". Most comparisons between models and experiment exploit the fact that for Gaussian sources the HBT radii agree with certain combinations of the space-time widths of the source which can be directly computed from the emission function, without having to evaluate, at significant expense, the two-particle correlation function. We here study the validity of this approach for realistic emission function models some of which exhibit significant deviations from simple Gaussian behaviour. By Fourier transforming the emission function we compute the 2-particle correlation function and fit it with a Gaussian to partially mimic the procedure used for measured correlation functions. We describe a novel algorithm to perform this Gaussian fit analytically. We find that for realistic hydrodynamic models the HBT radii extracted from this procedure agree better with the data than the values previously extracted from the space-time widths of the emission function. Although serious discrepancies between the calculated and measured HBT radii remain, we show that a more "apples-to-apples" comparison of models with data can play an important role in any eventually successful theoretical description of RHIC HBT data.

We present femtoscopic results from hydrodynamics-inspired thermal models with single freeze-out. Non-identical particle femtoscopy is studied and compared to results of identical particle correlations. Special emphasis is put on shifts between average space-time emission points of non-identical particles of different masses. They are found to be sensitive to both the spatial shift coming from radial flow, as well as average emission time difference coming from the resonance decays. The Therminator Monte-Carlo program was chosen for this study because it realistically models both of these effects. In order to analyze the results we present and test the methodology of non-identical particle correlations.

Using recent high-statistics STAR data from Au+Au and Cu+Cu collisions at full RHIC energy I discuss strong and Coulomb-induced final state interaction effects on identical (pi-pi) and non-identical (pi-xi) particle correlations. Analysis of pi-xi correlations reveals the strong and Coulomb-induced FSI effects, allowing for the first time to estimate spatial extension of pi and xi sources and the average shift between them. Source imaging techniques provide clean separation of details of the source function and are applied to the one-dimensional relative momentum correlation function of identical pions. For low momentum pions, and/or non-central collisions, a large departure from a single-Gaussian shape is observed.

The emission of pions produced within a dense, strongly-interacting system of matter in the presence of strong radial flow and absorption is described using a relativistic optical model formalism, replacing the attenuated or unattenuated plane waves of earlier emission function approaches with "distorted wave" solutions to a relativistic wave equation including a complex optical potential. The resulting distorted-wave emission function model (DWEF) is used in numerical calculations to fit HBT correlations and the resonance-corrected pion spectrum from central-collision STAR Au+Au pion data at <FONT FACE=Symbol>Ö</FONT>s = 200 GeV. Excellent agreement with the STAR data are obtained. This allows us to predict HBT radii over a range of centralities for both Au+Au and Cu+Cu collisions.

The basics of formalism of femtoscopic and spectroscopic correlations are given, the orthogonal character of these correlations is stressed. The similarity and difference of femtoscopic correlations in multiparticle production and beta-decay is discussed.

We briefly discuss four different possible types of transitions from quark to hadronic matter and their characteristic signatures in terms of correlations. We also highlight the effects arising from mass modification of hadrons in hot and dense hadronic matter, as well as their quantum statistical consequences: the appearance of squeezed quantum states and the associated experimental signatures, i.e., the back-to-back correlations of particle-antiparticle pairs. We briefly review the theoretical results of these squeezed quanta, generated by in-medium modified masses, starting from the first indication of the existence of surprising particle-antiparticle correlations, and ending by considering the effects of chiral dynamics on these correlation patterns. Nevertheless, a prerequisite for such a signature is the experimental verification of its observability. Therefore, the experimental observation of back-to-back correlations in high energy heavy ion reactions would be a unique signature, proving the existence of in-medium mass modification of hadronic states. On the other hand, their disappearance at some threshold centrality or collision energy would indicate that the hadron formation mechanism would have qualitatively changed: asymptotic hadrons above such a threshold are not formed from medium modified hadrons anymore, but rather by new degrees of freedom characterizing the medium. Furthermore, the disappearance of the squeezed BBC could also serve as a signature of a sudden, non-equilibrium hadronization scenario from a supercooled quark-gluon plasma phase.

A huge systematics of femtoscopic measurements have been used over the past 20 years to characterize the system created in heavy ion collisions. These measurements cover two orders of magnitude in energy, and with LHC beams imminent, this range will be extended by more than another order of magnitude. Here, I discuss theoretical expectations of femtoscopy of A+A and p+p collisions at the LHC, based on Boltzmann and hydrodynamic calculations, as well as on naive extrapolation of existing systematics.

Using a multiphase transport (AMPT) model that includes scatterings in both initial partonic and final hadronic matters as well as the transition between these two phases of matter, we make predictions on the rapidity distributions and transverse momentum spectra of various hadrons, their elliptic flows, and two-pion correlation functions in ultra-relativistic heavy ion collisions at the Large Hadron Collider (LHC).

Results of a new two-particle correlation analysis of central Pb+Au collision data at 158 GeV per nucleon are presented. The emphasis is put on pion-proton correlations and on the dependence of the two-pion correlation radii on the azimuthal emission angle with respect to the reaction plane.

The NA49 experiment at CERN SPS has acquired a huge data set of Pb+Pb events over a broad range of energy and centrality during the last several years. This high statistics data set, coupled with a state-of-the-art analysis technique, allows for the first model-independent extraction and energy scan of 3D emission sources for pion pairs at SPS energies. These 3D two-pion emission sources provide new insights into the nature of a long-range source previously reported by PHENIX at RHIC. The new results indicate that the two-pion source function is essentially Gaussian from 20 AGeV to 80 AGeV but it displays significant non-Gaussian tails at 158 AGeV.

Results from two-K0s interferometry in <FONT FACE=Symbol>Ö</FONT>S NN = 200 GeV Au+Au collisions at RHIC are presented. A model that takes into account the strong final state interaction has been used to fit the data. The effect of coupled K0$\bar{K}$0 and K+K- channels was studied. The value of the correlation radius parameter obtained is consistent with the transverse mass (mT) systematics established in pion correlation measurements.

After pointing out the difference between normal and anomalous diffusion, we consider a hadron resonance cascade (HRC) model simulation for particle emission at RHIC and point out that rescattering in an expanding hadron resonance gas leads to a heavy tail in the source distribution. The results are compared to recent PHENIX measurements of the tail of the particle emitting source in Au+Au collisions at RHIC. In this context, we show how can one distinguish experimentally the anomalous diffusion of hadrons from a second order QCD phase transition.

The decoupling surface in relativistic heavy-ion collisions may not be homogeneous. Rather, inhomogeneities should form when a rapid transition from high to low entropy density occurs. We analyze the hadron "chemistry" from high-energy heavy-ion reactions for the presence of such density inhomogeneities. We show that due to the non-linear dependence of the particle densities on the temperature and baryon-chemical potential such inhomogeneities should be visible even in the integrated, inclusive abundances. We analyze experimental data from Pb+Pb collisions at CERN-SPS and Au+Au collisions at BNL-RHIC to determine the amplitude of inhomogeneities and the role of local and global strangeness neutrality.