Urs Achim Wiedemann

Particle interferometry for relativistic heavy-ion collisions

Abstract In this report we give a detailed account on Hanbury Brown/Twiss (HBT) particle interferometric methods for relativistic heavy-ion collisions. These exploit identical two-particle correlations to gain access to the space-time geometry and dynamics of the final freeze-out stage. The connection between the measured correlations in momentum space and the phase-space structure of the particle emitter is established, both with and without final state interactions. Suitable Gaussian parametrizations for the two-particle correlation function are derived and the physical interpretation of their parameters is explained. After reviewing various model studies, we show how a combined analysis of single- and two-particle spectra allows to reconstruct the final state of relativistic heavy-ion collisions.

Transverse momentum dependence of the Landau-Pomeranchuk-Migdal effect

We study the transverse momentum dependence of the Landau-Pomeranchuk-Migdal effect in QED, starting from the high energy expansion of the solution of the Dirac equation in the presence of an external field. The angular integrated energy loss formula differs from an earlier expression of Zakharov by taking finite kinematical boundaries into account. In an expansion in powers of the opacity of the medium, we derive explicit expressions for the radiation cross section associated with N = 1, 2 and 3 scatterings. We verify the Bethe-Heitler and the factorization limit, and we calculate corrections to the factorization limit proportional to the square of the target size. A closed form expression valid to arbitrary orders in the opacity is derived in the dipole approximation. The resulting radiation spectrum is non-analytic in the coupling constant which is traced back to the transverse momentum broadening of a hard parton undergoing multiple small angle Moliere scattering. In extending the results to QCD, we test a previously used dipole prescription by comparing to direct pQCD results for N = 1 and 2. For N = 1, the QCD dipole prescription reproduces exactly the Bertsch-Gunion radiation spectrum. For N = 2, we find a sizeable correction which reduces to a multiplicative factor 17/8 at large separation.

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.

Dynamics and Sizes of the Fireball at Freeze-out

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.

Space-time analysis: HBT at SPS and RHIC

Abstract In this review talk, I first summarize the conclusions which can be drawn from a particle interferometric analysis of CERN SPS data and then I address related questions which are relevant for the upcoming experiments at RHIC.

Coulomb Final State Interactions for Gaussian Wave Packets

Abstract Two-particle like-sign and unlike-sign correlations including Coulomb final state interactions are calculated for Gaussian wave packets emitted from a Gaussian source. We show that the width of the wave packets can be fully absorbed into the spatial and momentum space widths of an effective emission function for plane wave states, and that Coulomb final state interaction effects are sensitive only to the latter, but not to the wave packet width itself. Results from analytical and numerical calculations are compared with recently published work by other authors.

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 ƒ = [exp( E/T ) − 1] −1 at T ∼ 120 MeV, modified for radial expansion. π-p data are also consistent with ƒ, but at T ∼ 180 MeV and without radial flow.