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Ulrich Heinz

Publications and source records attributed to Ulrich Heinz.

143 records · Page 8Linked to original sources
arXiv1996

Comment on ``Relativistic kinetic equations for electromagnetic, scalar and pseudoscalar interactions''

It is found that the extra quantum constraints to the spinor components of the equal-time Wigner function given in a recent paper by Zhuang and Heinz should vanish identically. We point out here the origin of the error and give an interpretation of the result. However, the principal idea of obtaining a complete equal-time transport theory by energy averaging the covariant theory remains valid. The classical transport equation for the spin density is also found to be incorrect. We give here the correct form of that equation and discuss briefly its structure.

hep-ph↗
arXiv1995

Subthreshold Antiproton Spectra in Relativistic Heavy Ion Collisions

We study the structure of antiproton spectra at extreme subthreshold bombarding energies using a thermodynamic picture. Antiproton production processes and final state interactions are discussed in detail in order to find out what can be learned about these processes from the observed spectra.

hep-ph↗
arXiv1995

Spectral functions for composite fields and viscosity in hot scalar field theory

We derive a spectral representation for the two-point Green function for arbitrary composite field operators in Thermo Field Dynamics (TFD). A simple way for calculating the spectral density within TFD is pointed out and compared with known results from the imaginary time formalism. The method is applied to hot $ϕ^4$ theory. We give a compact derivation of the one-loop contribution to the shear viscosity and show that it is dominated by low-momentum plasmons.

hep-ph↗
arXiv1995

The Plasmon in Hot $ϕ^4$ Theory

We study the 2-loop resummed propagator in hot $g^2ϕ^4$ theory. The propagator has a cut along the whole real axis in the complex energy plane, but for small $g$, the spectral density is sharply peaked around the plasmon. The dispersion relation and the width of the plasmon are calculated at zero {\em and} finite momentum. At large momenta the spectral width vanishes, and the plasmon looses its collectivity and behaves like a non-interacting free particle.

hep-ph↗
arXiv1995

Transverse Momentum Dependence of Hanbury-Brown/Twiss Correlation Radii

The transverse momentum dependence of Hanbury-Brown/Twiss (HBT) interferometry radii for 2-body correlation functions provides experimental access to the collective dynamics in heavy-ion collisions. We present an analytical approximation scheme for these HBT radii which combines the recently derived model-independent expressions with an approximate determination of the saddle point of the emission function. The method is illustrated for a longitudinally boost-invariant hydrodynamical model of a heavy ion collision with freeze-out on a sharp hypersurface. The analytical approximation converges rapidly to the width of the numerically computed correlation function and reproduces correctly its exact transverse momentum dependence. However, higher order corrections within our approximation scheme are essential, and the previously published lowest order results with simple $m_{\perp}$ scaling behaviour are quantitatively and qualitatively unreliable.

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arXiv1995

Extracting Source Parameters from Gaussian Fits to Two-Particle Correlations

Using a quadratic saddle-point approximation, we show how information about a particle-emitting source can be extracted from gaussian fits to two-particle correlation data. Although the formalism is completely general, extraction of the relevant parameters is much simpler for sources within an interesting class of azimuthally symmetric models. After discussing the standard fitting procedure, we introduce a new gaussian fitting procedure which is an azimuthally symmetric generalization of the Yano-Koonin formalism for spherically symmetric sources. This new fitting procedure has the advantage that in addition to being able to measure source parameters in a fixed frame or the longitudinally co-moving system, it can also measure these parameters in the local rest frame of the source.

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arXiv1995

A new cross term in the two-particle Hanbury-Brown-Twiss correlation function

Using two specific models and a model-independent formalism, we show that in addition to the usual quadratic ``side'', ``out'' and ``longitudinal'' terms, a previously neglected ``out-longitudinal'' cross term arises naturally in the exponent of the two-particle correlator. Since its effects can be easily observed, such a term should be included in any experimental fits to correlation data. We also suggest a method of organizing correlation data using rapidity rather than longitudinal momentum differences since in the former every relevant quantity is longitudinally boost invariant.

hep-ph↗
arXiv1995

The Role of Strangeness in Ultrarelativistic Nuclear Collisions

We review the progress in understanding the strange particle yields in nuclear collisions and their role in signalling quark-gluon plasma formation. We report on new insights into the formation mechanisms of strange particles during ultrarelativistic heavy-ion collisions and discuss interesting new details of the strangeness phase diagram. In the main part of the review we show how the measured (multi-)strange particle abundances can be used as a testing ground for chemical equilibration in nuclear collisions, and how the results of such an analysis lead to important constraints on the collision dynamics and space-time evolution of high energy heavy-ion reactions.

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arXiv1995

Model Independent Features of the Two-Particle Correlation Function

The Hanbury-Brown Twiss correlation function for two identical particles is studied for systems with cylindrical symmetry. Its shape for small values of the relative momentum is derived in a model independent way. In addition to the usual quadratic ``side'', ``out'' and ``longitudinal'' terms in the exponent of the correlator, a previously neglected ``out-longitudinal'' cross term is found and discussed. The model-independent expressions for the size parameters of the HBT correlation function are interpreted as lengths of homogeneity of the source, in distinction to its purely geometrical size. They are evaluated analytically and numerically for two specific thermal models featuring collective transverse and longitudinal flow. The analytic expressions derived allow one to establish qualitatively important connections between the space-time features of the source and the shape of the correlation function. New ways of parametrizing the correlation function and a new approach to the measurement of the duration of the emission process are suggested.

hep-ph↗
arXiv1994

Pion Multiplicity Distribution in Proton-Antiproton Annihilation at Rest

The pion multiplicity distribution is widely believed to reflect the statistical aspects of $\bar{p}p$ annihilation at rest. We try to reproduce it in a grand canonical picture with explicit conservation of electric charge, isospin, total angular momentum, and the parity quantum numbers $P$, $C$, and $G$ via the projection operator formalism. Bose statistics is found to be non-negligible, particularly in fixing the interaction volume. The calculated pion multiplicity distribution for $\left\langle n_π \right\rangle = 5$ turns out to depend strongly on the conservation of the angular momentum and connected quantum numbers, as well as on the spin state occupation in S-wave annihilation. However, the empirical Gaussian pion multiplicity distribution cannot be reproduced. This calls in question either the statistical ansatz or the rather old data themselves.

hep-ph↗
arXiv1994

HBT Correlators -- Current Formalism vs. Wigner Function Formulation

We clarify the relationship between the current formalism developed by Gyulassy, Kaufmann and Wilson and the Wigner function formulation suggested by Pratt for the 2-particle correlator in Hanbury-Brown Twiss interferometry. When applied to a hydrodynamical description of the source with a sharp freeze-out hypersurface, our results remove a slight error in the prescription given by Makhlin and Sinyukov which has led to confusion in the literature.

hep-ph↗
arXiv1994

Chiral and Gluon Condensates at Finite Temperature

We investigate the thermal behaviour of gluon and chiral condensates within an effective Lagrangian of pseudoscalar mesons coupled to a scalar glueball. This Lagrangian mimics the scale and chiral symmetries of QCD. (Submitted to Z. Phys. C)

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arXiv1994

Hydrodynamical assessment of 200 AGeV collisions

We are analyzing the hydrodynamics of 200 A GeV S+S collisions using a new approach which tries to quantify the uncertainties arising from the specific implementation of the hydrodynamical model. Based on a previous phenomenological analysis we use the global hydrodynamics model to show that the amount of initial flow, or initial energy density, cannot be determined from the hadronic momentum spectra. We additionally find that almost always a sizeable transverse flow deve- lops, which causes the system to freeze out, thereby limiting the flow velocity in itself. This freeze-out dominance in turn makes a distinction between a plasma and a hadron resonance gas equation of state very difficult, whereas a pure pion gas can easily be ruled out from present data. To complete the picture we also analyze particle multiplicity data, which suggest that chemical equilibrium is not reached with respect to the strange particles. However, the over- population of pions seems to be at most moderate, with a pion chemical potential far away from the Bose divergence.

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arXiv1993

Thermal phenomenology of hadrons from 200 AGeV S+S collisions

We develop a complete and consistent description for the hadron spectra from heavy ion collisions in terms of a few collective variables, in particular temperature, longitudinal and transverse flow. To achieve a meaningful comparison with presently available data, we also include the resonance decays into our picture. To disentangle the influences of transverse flow and resonance decays in the $m_T$-spectra, we analyse in detail the shape of the $m_T$-spectra.

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