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Ulrich Heinz

Publications and source records attributed to Ulrich Heinz.

At least 127 records · Page 7Linked to original sources
arXiv1998

Fine-Tuning Two-Particle Interferometry I: Effects from Temperature Gradients in the Source

A comprehensive model study of Bose-Einstein correlation radii in heavy ion collisions is presented. The starting point is a longitudinally and transversally expanding fireball, represented at freeze-out by an azimuthally symmetric emission function. The freeze-out temperature is allowed to feature transverse and temporal gradients. Their effects on the correlation radii are studied. In particular, we evaluate numerically their dependence on the transverse mass of the particle pairs and check a recent suggestion, based on analytical approximations, that for certain reasonable source parameters all three correlation radii satisfy simultaneously a 1/\sqrt{M_\perp} scaling.

nucl-th↗
arXiv1998

Bose-Einstein Correlations from Random Walk Models

We argue that strong final state rescattering among the secondary particles created in relativistic heavy ion collisions is essential to understand the measured Bose-Einstein correlations. The recently suggested ``random walk models'' which contain only initial state scattering are unable to reproduce the measured magnitude and K_\perp-dependence of R_\perp in Pb+Pb collisions and the increase of R_l with increasing size of the collision system.

nucl-th↗
arXiv1998

Resonance contributions to HBT correlation radii

We study the effect of resonance decays on intensity interferometry for heavy ion collisions. Collective expansion of the source leads to a dependence of the two-particle correlation function on the pair momentum K. This opens the possibility to reconstruct the dynamics of the source from the K-dependence of the measured HBT radii. Here we address the question to what extent resonance decays can fake such a flow signal. Within a simple parametrization for the emission function we present a comprehensive analysis of the interplay of flow and resonance decays on the one- and two-particle spectra. We discuss in detail the non-Gaussian features of the correlation function introduced by long-lived resonances and the resulting problems in extracting meaningful HBT radii. We propose to define them in terms of the second order q-moments of the correlator C(q, K). We show that this yields a more reliable characterisation of the correlator in terms of its width and the correlation strength `lambda' than other commonly used fit procedures. The normalized fourth-order q-moments (kurtosis) provide a quantitative measure for the non-Gaussian features of the correlator. At least for the class of models studied here, the kurtosis helps separating effects from expansion flow and resonance decays, and provides the cleanest signal to distinguish between scenarios with and without transverse flow.

nucl-th↗
arXiv1997

Resolving the space-time structure of sonoluminescence by intensity interferometry

We analyze the spatial and temporal resolving power of two-photon intensity interferometry for the light emitting source in single bubble sonoluminescence (SBSL). We show that bubble sizes between several 10 nm and 3 um can be resolved by measuring the transverse correlation function, but that a direct determination of the flash duration via the longitudinal correlation function works only for SBSL pulses which are shorter than 0.1 ps. Larger pulse lengths can be determined indirectly from the intercept of the angular correlator at equal photon frequencies. The dynamics of the bubble is not accessible by two-photon interferometry.

quant-ph↗
arXiv1997

Particle Interferometry: New Theoretical Results

By measuring hadronic single-particle spectra and two-particle correlations in 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 2-3 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.

nucl-th↗
arXiv1997

Massive gluons and quarks and the equation of state obtained from SU(3) lattice QCD

We analyze recent results of SU(3) lattice QCD calculations with a phenomenological parametrization for the quark-gluon plasma equation of state based on a quasi-particle picture with massive quarks and gluons. At high temperature we obtain a good fit to the lattice data using perturbative thermal quark and gluon masses from an improved HTL scheme. At temperatures close to the confinement phase transition the fitted masses increase above the perturbative value, and a non-zero (but small) bag constant is required to fit the lattice data.

hep-ph↗
arXiv1997

Particle Interferometry in Heavy-Ion Collisions

By measuring hadronic single-particle spectra and two-particle correlations in 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 2-3 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.

nucl-th↗
arXiv1996

Gauge Invariance of Resummation Schemes: The QCD Partition Function

We pick up a method originally developed by Cheng and Tsai for vacuum perturbation theory which allows to test the consistency of different sets of Feynman rules on a purely diagrammatic level, making explicit loop calculations superfluous. We generalize it to perturbative calculations in thermal field theory and we show that it can be adapted to check the gauge invariance of physical quantities calculated in improved perturbation schemes. Specifically, we extend this diagrammatic technique to a simple resummation scheme in imaginary time perturbation theory. As an application, we check up to O(g^4) in general covariant gauge the gauge invariance of the result for the QCD partition function which was recently obtained in Feynman gauge.

hep-th↗
arXiv1996

A new class of Hanbury-Brown/Twiss parameters

In heavy ion collisions resonances can create strong non-Gaussian effects in the 2-pion correlation data. Hence, the commonly used Gaussian fit parameters do not fully characterize these correlators. We suggest a different set of HBT parameters which does not presuppose a particular shape of the correlator and allows to extract additional (non-Gaussian) information. Within a simple model for an expanding source including resonance decays it is shown that this additional information provides a clean distinction between scenarios with and without transverse flow.

nucl-th↗
arXiv1996

Equal-Time Hierarchies for Quantum Transport Theory

We investigate in the equal-time formalism the derivation and truncation of infinite hierarchies of equations of motion for the energy moments of the covariant Wigner function. From these hierarchies we then extract kinetic equations for the physical distribution functions which are related to low-order energy moments, and show how to determine the higher order moments in terms of these lowest order ones. We apply the general formalism to scalar and spinor QED with classical background fields and compare with the results derived from the three-dimensional Wigner transformation method.

hep-ph↗
arXiv1996

Photon Spin and the Shape of the Two-Photon Correlation Function

We use the covariant current formalism to derive the general form of the 2-photon correlation function for fully chaotic sources. Motivated by the recent discussion in the literature we concentrate on the effects from the photon spin on the correlator. We show that for locally thermalized expanding sources, like those expected to be created in relativistic heavy ion collisions, the only change relative to 2-pion interferometry is a statistical factor 1/2 for the overall strength of the correlation which results from the experimental averaging over the photon spin.

nucl-th↗
arXiv1996

Hanbury-Brown/Twiss Interferometry for Relativistic Heavy-Ion Collisions: Theoretical Aspects

I discuss two-particle intensity interferometry as a method to extract from measured 1- and 2-particle momentum spectra information on the space-time geometry and dynamics of the particle emitting source. Particular attention is given to the rapid expansion and short lifetime of the sources created in relativistic heavy-ion collisions. Model-independent expressions for the HBT size parameters in terms of the space-time variances of the source are derived, and a new parametrization of the correlation function is suggested which allows to separate the transverse, longitudinal and temporal extension of the source and to measure its transverse and longitudinal expansion velocity. The effects of resonance decays are also discussed.

nucl-th↗
arXiv1996

How to extract physics from HBT radius parameters

I review recent progress in the understanding of the connection between the space-time structure of the particle emitting source and the form of the two-particle correlation function in momentum space. Based on a new scheme for calculating the HBT radius parameters from the emission function, strategies are suggested to separate for rapidly expanding sources the information on the spatial and temporal structure of the source. To this end a new fitting function for the two-particle correlation function is proposed. Its usefulness is demonstrated for a typical expanding model source, and it is shown how the dependence of the resulting fit parameters on the momentum of the particle pair can be used to measure the longitudinal and transverse expansion of the source.

nucl-th↗
arXiv1996

Source Function Determined from HBT Correlations by the Maximum Entropy Principle

We study the reconstruction of the source function in space-time directly from the measured HBT correlation function using the Maximum Entropy Principle. We find that the problem is ill-defined without at least one additional theoretical constraint as input. Using the requirement of a finite source lifetime for the latter we find a new Gaussian parametrization of the source function directly in terms of the measured HBT radius parameters and its lifetime, where the latter is a free parameter which is not directly measurable by HBT. We discuss the implications of our results for the remaining freedom in building source models consistent with a given set of measured HBT radius parameters.

nucl-th↗
arXiv1996

Entropy production by resonance decays

We investigate entropy production for an expanding system of particles and resonances with isospin symmetry -- in our case pions and $ρ$ mesons -- within the framework of relativistic kinetic theory. A cascade code to simulate the kinetic equations is developed and results for entropy production and particle spectra are presented.

hep-ph↗