arXiv subjects

Ulrich Heinz

Publications and source records attributed to Ulrich Heinz.

At least 109 records · Page 6Linked to original sources

arXiv1999

Reconstructing the Freeze-out State in Pb+Pb Collisions at 158 AGeV/c

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_t-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 chi^2 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 approximately 95 +- 15 MeV and a large average transverse flow velocity of about 0.55c +- 0.07c. 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 μ_πapproximately 60 MeV for T = 100 MeV.

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arXiv1999

Dynamics and Sizes of the Fireball at Freeze-out

Analyzing the m_t-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 approximately 0.55c. The M_t-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 pion chemical potential of approximately 60 MeV at thermal freeze-out.

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arXiv1999

Primordial hadrosynthesis in the Little Bang

The present status of soft hadron production in high energy heavy-ion collisions is summarized. In spite of strong evidence for extensive dynamical evolution and collective expansion of the fireball before freeze-out I argue that its chemical composition is hardly changed by hadronic final state interactions. The measured hadron yields thus reflect the primordial conditions at hadronization. The observed production pattern is consistent with statistical hadronization at the Hagedorn temperature from a state of uncorrelated, color deconfined quarks and antiquarks, but requires non-trivial chemical evolution of the fireball in a prehadronic (presumably QGP) stage before hadron formation.

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arXiv1999

Kinetic Freeze-out and Radial Flow in 11.6 A GeV Au+Au Collisions

We study the kinetic freeze-out conditions of hadrons in Au+Au collisions at 11.6 A GeV/c using different parametrizations of an expanding thermal fireball. We take into account the available double differential momentum spectra of a variety of particle species, covering a large fraction of the total momentum space. The overall fit to the data is very good and indicates a relatively low kinetic freeze-out temperature of about 90 MeV with an average transverse expansion velocity at midrapidity of about 0.5 c.

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arXiv1999

Anisotropic flow from AGS to LHC energies

Within hydrodynamics we study the effects of the initial spatial anisotropy in non-central heavy-ion collisions on the momentum distributions of the emitted hadrons. We show that the elliptic flow measured at midrapidity in 158 A GeV/c Pb+Pb collisions can be quantitatively reproduced by hydrodynamic expansion, indicating early thermalization in the collision. We predict the excitation functions of the 2nd and 4th harmonic flow coefficients from AGS to LHC energies and discuss their sensitivity to the quark-hadron phase transition.

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arXiv1999

Two-Particle Correlations in Relativistic Heavy-Ion Collisions

Two-particle momentum correlations between pairs of identical particles produced in relativistic heavy-ion reactions can be analyzed to extract the space-time structure of the collision fireball. We review recent progress in the application of this method, based on newly developed theoretical tools and new high-quality data from heavy-ion collision experiments. Implications for our understanding of the collision dynamics and for the search for the quark-gluon plasma are discussed.

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arXiv1999

TFT98: Proceedings of the 5th International Workshop on Thermal Field Theories and Their Applications

This contains the Proceedings of the 5th International Workshop on Thermal Field Theories and Their Applications (TFT98) which took place on 10-14 August 1998 in Regensburg, Germany. The contents are in html and postscript format. The html files contain clickable links to the postscript files of the papers on the LANL e-print archive. All contributions underwent strict peer review before being accepted for this volume. Comments and suggestions for improvements should be sent to ulrich.heinz@cern.ch.

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arXiv1999

Coalescence and flow in ultra-relativistic heavy ion collisions

Using a density matrix approach to describe the process of coalescence, we calculate the coalescence probabilities and invariant momentum spectra for deuterons and antideuterons. We evaluate our expressions with a hydrodynamically motivated parametrization for the source at freeze-out which implements rapid collective expansion of the collision zone formed in heavy ion collisions. We find that the coalescence process is governed by the same lengths of homogeneity which can be extracted from HBT interferometry. They appear in the absolute cluster yield via an effective volume factor as well as in a quantum mechanical correction factor which accounts for the internal structure of the deuteron cluster. Our analysis provides a new interpretation for the parameters in the popular phenomenological coalescence model and for the effective overlap volume in Hagedorn's model for cluster production in pp collisions. Using source parameters extracted from a recent HBT analysis of two-pion correlations, we successfully describe deuteron and antideuteron production data from Pb+Pb collisions at the CERN SPS as measured by the NA44 and NA52 collaborations. We also confirm the recent finding by Polleri et al. that the different measured slopes of nucleon and deuteron transverse mass spectra require a transverse density profile of the source which is closer to a box than to a Gaussian shape.

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arXiv1999

Hunting down the quark-gluon plasma in relativistic heavy-ion collisions

The present status of the heavy-ion program to search for quark-gluon plasma is reviewed. The goal of this program is to recreate the Big Bang in the laboratory, by generating small chunks of exploding quark-gluon plasma (``The Little Bang''). I argue that the analogues of the three pillars of Big Bang Theory (Hubble flow, microwave background radiation, and primordial nucleosynthesis) have now been firmly established in heavy-ion collisions at SPS energies: there is convincing evidence for strong radial flow, thermal hadron emission, and primordial hadrosynthesis from a color-deconfined initial stage. Direct observation of the quark-gluon plasma phase via its electromagnetic radiation will be possible in planned collider experiments at higher energies.

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arXiv1999

Particle Interferometry for Relativistic Heavy-Ion Collisions

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.

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arXiv1999

Potentials and Limits of Bose-Einstein Interferometry

We complete the introduction of Yano-Koonin-Podgoretskii (YKP) parametrization of the correlation function by deriving the corrections to the correlation radii which arise from releasing the popular smoothness approximation and approximation of setting the pair momentum on-shell. We investigate the definition range of this parametrization and find kinematic regions in which the YKP parametrization is inapplicable. These problems disappear if the newly proposed Modified Yano-Koonin-Podgoretskii parametrization is used. We then focus on the physical interpretation of the correlation radii obtained in the different parametrizations. While the extraction of the longitudinal source expansion from the YK rapidity is found to be rather robust against variations of the source density profiles, the extracted emission duration is quite sensitive to such variations and cannot be reliably extracted except for the case of extremely long source lifetimes.

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arXiv1999

Strange Messages: Chemical and Thermal Freeze-out in Nuclear Collisions

Thermal models are commonly used to interpret heavy-ion data on particle yields and spectra and to extract the conditions of chemical and thermal freeze-out in heavy-ion collisions. I discuss the usefulness and limitations of such thermal model analyses and review the experimental and theoretical evidence for thermalization in nuclear collisions. The crucial role of correlating strangeness production data with single particle spectra and two-particle correlation measurements is pointed out. A consistent dynamical picture for the heavy-ion data from the CERN SPS involves an initial prehadronic stage with deconfined color and with an appreciable isotropic pressure component. This requires an early onset of thermalization.

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arXiv1999

Universal Pion Freeze-out Phase-Space Density

Results on the pion freeze-out phase-space density in sulphur-nucleus, Pb-Pb and pion-proton collisions at CERN-SPS are presented. All heavy-ion reactions are consistent with the thermal Bose-Einstein distrtibution f=1/(exp(E/T)-1) at T~120 MeV, modified for expansion. Pion-proton data are also consistent with f, but at T~180 MeV.

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arXiv1998

A Generalized Fluctuation-Dissipation Theorem for Nonlinear Response Functions

A nonlinear generalization of the Fluctuation-Dissipation Theorem (FDT) for the n-point Green functions and the amputated 1PI vertex functions at finite temperature is derived in the framework of the Closed Time Path formalism. We verify that this generalized FDT coincides with known results for n=2 and 3. New explicit relations among the 4-point nonlinear response and correlation (fluctuation) functions are presented.

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arXiv1998

n-point functions at finite temperature

We study $n$-point functions at finite temperature in the closed time path formalism. With the help of two basic column vectors and their dual partners we derive a compact decomposition of the time-ordered $n$-point functions with $2^n$ components in terms of $2^{n-1} -1$ independent retarded/advanced $n$-point functions. This representation greatly simplifies calculations in the real-time formalism.

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arXiv1998

Hanbury Brown - Twiss Interferometry in High Energy Nuclear and Particle Physics

I review recent applications of two-particle intensity interferometry in high energy physics, concentrating on relativistic heavy ion collisions. By measuring hadronic single-particle spectra and two-particle correlations in hadron-hadron or heavy-ion collisions, the size and dynamical state of the collision fireball at freeze-out can be reconstructed. I discuss the relevant theoretical methods and their limitations. By applying the formalism to recent pion correlation data from Pb+Pb collisions at CERN we demonstrate that the collision zone has undergone strong transverse growth before freeze-out (by a factor 2 in each direction), and that it expands both longitudinally and transversally. From the thermal and flow energy density at freeze-out the energy density at the onset of transverse expansion can be estimated from conservation laws. It comfortably exceeds the critical value for the transition to color deconfined matter.

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arXiv1998

Wigner functions in covariant and single-time formulations

We will establish the connection between the Lorentz covariant and so-called single-time formulation for the quark Wigner operator. To this end we will discuss the initial value problem for the Wigner operator of a field theory and give a discussion of the gauge-covariant formulation for the Wigner operator including some new results concerning the chiral limit. We discuss the gradient or semi-classical expansion and the color and spinor decomposition of the equations of motion for the Wigner operator. The single-time formulation will be derived from the covariant formulation by taking energy moments of the equations for the Wigner operator. For external fields we prove that only the lowest energy moments of the quark Wigner operator contain dynamical information.

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arXiv1998

Fine-Tuning Two-Particle Interferometry II: Opacity Effects

We present a model study of single-particle spectra and two-particle Bose-Einstein correlations for opaque sources. We study the transverse mass dependence of the correlation radii R_\perp, R_\parallel and R_0 in the YKP parametrization and find a strong sensitivity of the temporal radius parameter R_0^2 to the source opacity. A simple comparison with the published data from 158 A GeV/c Pb+Pb collisions at CERN indicates that the pion source created in these collisions emits particles from the whole reaction volume and is not opaque. For opaque sources we find certain regions of inapplicability of the YKP parametrization which can be avoided by a slightly different parametrization for the correlator. The physical meaning of the modified parameters is briefly discussed.

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