Boris Tomasik

UNIVERSAL PION FREEZE-OUT PHASE-SPACE DENSITY

Abstract Results on the pion phase-space density at freeze-out in sulphur-nucleus, Pb–Pb and π –p collisions at the CERN SPS are presented. All heavy-ion reactions are consistent with the thermal Bose–Einstein distribution f =[exp( E / T )−1] −1 at T ∼ 120 MeV, modified for radial expansion. π –p data are also consistent with f , but at T ∼ 180 MeV and without radial flow.

Reconstructing the Freeze-out State in Pb+Pb Collisions at 158 A GeV/ c a

For a class of analytical parametrizations of the freeze-out state of relativistic heavy ion collisions, we perform a simultaneous analysis of the single-particle m ⊥-spectra and two-particle Bose-Einstein correlations measured in central Pb+Pb collisions at the CERN SPS. The analysis includes a full model parameter scan with χ2 confidence levels. A comparison of different transverse density profiles for the particle emission region allows for a quantitative discussion of possible model dependencies of the results. Our fit results suggest a low thermal freeze-out temperature T≈95±15 MeV and a large average transverse flow velocity (bar v_ bot approx 0.55 pm 0.07). Moreover, the fit favours a box-shaped transverse density profile over a Gaussian one. We discuss the origins and the consequences of these results in detail. In order to reproduce the measured pion multiplicity our model requires a positive pion chemical potential. A study of the pion phase-space density indicates μπ≈60 MeV for T=100 MeV.

Lifetimes and sizes from two particle correlation functions

Abstract We discuss the Yano-Koonin-Podgoretskii (YKP) parametrization of the two-particle correlation function for azimuthally symmetric expanding sources. We derive model-independent expressions for the YKP fit parameters and discuss their physical interpretation. We use them to evaluate the YKP fit parameters and their momentum dependence for a simple model for the emission function and propose new strategies for extracting the source lifetime. Longitudinal expansion of the source can be seen directly in the rapidity dependence of the Yano-Koonin velocity.

Strangeness Balance in HADES Experiments and the

� /�/K + ratio proves to be significantly smaller compared to the measured value (8 times lower than the experimental median value and 3 times lower than the lower error bar). Various scenarios

\Xi^-

We propose an application of the Kolmogorov-Smirnov test for rapidity distributions of individual events in ultrarelativistic heavy-ion collisions. The test is particularly suited to recognizing nonstatistical differences between the events. Thus when applied to a narrow centrality class it could indicate differences between events that would not be expected if all events evolved according to the same scenario. In particular, as an example we assume here a possible fragmentation of the fireball into smaller pieces at the quark/hadron phase transition. Quantitative studies are performed with a Monte Carlo model capable of simulating such a distribution of hadrons. We conclude that the Kolmogorov-Smirnov test is a very powerful tool for the identification of the fragmentation process.

Enhancement

In nuclear collisions at highest accessible LHC energies, often more than one dijet pairs deposit momentum into the deconfined expanding medium. With the help of 3+1 dimensional relativistic hydrodynamic simulation we show that this leads to measurable contribution to the anisotropy of collective transverse expansion. Hard partons generate streams in plasma which merge if they come close to each other. This mechanism correlates the resulting contribution to flow anisotropy with the fireball geometry and causes an increase of the elliptic flow in non-central collisions.

Kolmogorov-Smirnov test and its use for the identification of fireball fragmentation

Analyzing the m?-spectrum and two-particle correlations of negative hadrons from 158 AGeV/c Pb+Pb collisions at slightly forward rapidities we find a (thermal) freeze-out temperature of about 100 MeV and transverse flow with ¯? � 0.55c. The M?-dependence of the correlation radii prefers a box-like transverse density profile over a Gaussian. From an analysis of the pion phase-space density we find µ� � 60MeV at thermal freeze-out.

Anisotropic flow of the fireball fed by hard partons

Abstract The preliminary CERN SPS NA 49 Pb+Pb 158 GeV / A one- and two-particle h − -spectra at mid-rapidity are consistent with a source of temperature T ≈ 130 MeV, lifetime τ 0 ≈ 9fm/c, transverse flow η f ≈ 0.35, and a transverse geometric size which is twice as large as the cold Pb nucleus.

Dynamics and Sizes of the Fireball at Freeze-out

Abstract The dimension of the region emitting particles in nuclear collisions can be determined by Hanbury-Brown and Twiss (HBT) correlations of identical particles. In thiss note we demonstrate that this dimension as determined by the HBT correlation of bremsstrahlung photons is larger than the actual one due to the photon formation length. Qualitatively r2 → r2 + a/ω2 where the last term is due to the formation length being proportional to 1/ω, where ω is the energy of the photon. This phenomenon permits to “see” experimentally the photon formation length.

Reconstructing the source in heavy ion collisions from particle interferometry

Abstract Hanbury-Brown and Twiss correlations of bremsstrahlung photons produced in hadronic collisions at very high energies show intermittent features. The phenomenon is due to large logitudinal dimensions of the space-time evolution of hadronic collisions and to the fact that the rapidity of a bremsstrahlung photon need not be close to the rapidity of the charged particle radiating the photon.