arXiv subjects

H. Heiselberg

Publications and source records attributed to H. Heiselberg.

48 records · Page 3Linked to original sources

arXiv1997

The Electrical Conductivity in the Early Universe

We calculate the electrical conductivity in the early universe at temperatures below as well as above the electroweak vacuum scale, $T_c\simeq 100$GeV. Debye and dynamical screening of electric and magnetic interactions leads to a finite conductivity, $σ_{el}\sim T/α\ln(1/α)$, at temperatures well below $T_c$. At temperatures above, $W^\pm$ charge-exchange processes -- analogous to color exchange through gluons in QCD -- effectively stop left-handed charged leptons. However, right-handed leptons can carry current, resulting in $σ_{el}/T$ being only a factor $\sim \cos^4θ_W$ smaller than at temperatures below $T_c$.

astro-ph↗

arXiv1997

Bose-Einstein Correlations from Opaque Sources

Bose-Einstein correlations in relativistic heavy ion collisions are very different for opaque sources than fortransparent ones. The Bose-Einstein radius parameters measured in two-particle correlation functions depend sensitively on the mean free path of the particles. In particular we find that the outward radius for an opaque source is smaller than the sidewards radius for sufficiently short duration of emission. A long duration of emission can compensate the opacity reduction of the longitudinal radius parameter and explain the experimental measurements of very similar side- and outward radius parameters.

nucl-th↗

arXiv1997

Fluctuations and HBT Scales in Relativistic Nuclear Collisions

Bose-Einstein correlations in relativistic heavy ion collisions are examined in a general model containing the essential features of hydrodynamical, cascade as well as other models commonly employed for describing the particle freeze-out. In particular the effects of longitudinal and transverse expansion, emission from surfaces moving in time, the thickness of the emitting layer varying from surface to volume emission and other effects are studied. Model dependences of freeze-out sizes and times are discussed and compared to recent $Pb+Pb$ data at 160A$\cdot$GeV.

nucl-th↗

arXiv1996

Many--Particle Correlations in Relativistic Nuclear Collisions

Many--particle correlations due to Bose-Einstein interference are studied in ultrarelativistic heavy--ion collisions. We calculate the higher order correlation functions from the 2--particle correlation function by assuming that the source is emitting particles incoherently. In particular parametrizations of and relations between longitudinal, sidewards, outwards and invariant radii and corresponding momenta are discussed. The results are especially useful in low statistics measurements of higher order correlation functions. We evaluate the three--pion correlation function recently measured by NA44 and predict the 2--pion--2--kaon correlation function. Finally, many particle Coulomb corrections are discussed.

nucl-th↗

arXiv1996

Interferometry with Resonances and Flow in High-Energy Nuclear Collisions

The effects of resonances and flow on the correlation function for two identical particles are described assuming chaotic sources and classical propagation of particles. Expanding to second order in relative momenta, the source sizes can be calculated directly and understood as contributions from various fluctuations in the source. Specific calculations of source size radii are given assuming Bjorken longitudinal flow with additional transverse expansion. Results are compared to recent $ππ$ and $KK$ correlation data from relativistic nuclear collisions with particular attention to the reduction in the $ππ$ correlation function due to resonances and the decreasing source sizes with increasing transverse momenta of the particles.

hep-ph↗

arXiv1996

Thermal Equilibration in an Expanding Parton Plasma

Thermalization in an expanding parton plasma is studied within the framework of Boltzmann equation in the absence of any mean fields. In particular, we study the time-dependence of the relaxation time to the lowest order in finite temperature QCD and how such time-dependence affects the thermalization of an expanding parton plasma. Because of Debye screening and Landau damping at finite temperature, the relaxation time (or transport rates) is free of infrared divergencies in both longitudinal and transverse interactions. The resultant relaxation time decreases with time in an expanding plasma like $1/τ^β$, with $β<1$. We prove in this case that thermal equilibrium will eventually be established given a long life-time of the system. However, a fixed momentum cut-off in the calculation of the relaxation time gives rise to a much stronger time dependence which will slow down thermal equilibrium. It is also demonstrated that the ``memory effect'' of the initial condition affects the approach to thermal equilibrium and the final entropy production.

hep-ph↗

arXiv1995

Expansion, Thermalization and Entropy Production in High-Energy Nuclear Collisions

The thermalization process is studied in an expanding parton gas using the Boltzmann equation with two types of collision terms. In the relaxation time approximation we determine the criteria under which a time-dependent relaxation time leads to thermalization of the partons. We calculate the entropy production due to collisions for the general time-dependent relaxation time. In a perturbative QCD approach on the other hand, we can estimate the parton collision time and its dependence on expansion time. The effective `out of equilibrium' collision time differs from the standard transport relaxation time, $τ_{\rm tr}\simeq(α_s^2\ln(1/α_s)T)^{-1}$, by a weak time dependence. It is in both cases Debye screening and Landau damping that regulate the singular forward scattering processes. We find that the parton gas does thermalize eventually but only after having undergone a phase of free streaming and gradual equilibration where considerable entropy is produced (``after-burning"). The final entropy and thus particle density depends on the collision time as well as the initial conditions (a ``memory effect"). Results for entropy production are presented based upon various model estimates of early parton production.

hep-ph↗

arXiv1995

Correlations and Fluctuations in High-Energy Nuclear Collisions

Nucleon correlations in the target and projectile nuclei are shown to reduce significantly the fluctuations in multiple nucleon-nucleon collisions, total multiplicity and transverse energy in relativistic heavy-ion collisions, in particular for heavy projectile and target. The interplay between cross-section fluctuations, from color transparency and opacity, and nuclear correlations is calculated and found to be able to account for large fluctuations in transverse energy spectra. Numerical implementation of correlations and cross-section fluctuations in Monte-Carlo codes is discussed.

nucl-th↗

arXiv1995

Quark Matter Droplet Formation in Neutron Stars

The formation rate of quark matter droplets in neutron stars is calculated from a combination of bubble formation rates in cold degenerate and high temperature matter. Nuclear matter calculations of the viscosity and thermal conductivity are applied. Results show that droplets form only in the core of neutron stars shortly after supernova collapse, where pressures and temperatures are high, and for sufficiently small interface tension between nuclear and quark matter. Coulomb energies hinder formation of large droplets whereas the presence of strange hadrons in nuclear matter increase the droplet formation rate.

hep-ph↗

arXiv1994

Color, Spin and Flavor Diffusion in Quark-Gluon Plasmas

In weakly interacting quark-gluon plasmas diffusion of color is found to be much slower than the diffusion of spin and flavor because color is easily exchanged by the gluons in the very singular forward scattering processes. If the infrared divergence is cut off by a magnetic mass, $m_{mag}\sim α_sT$, the color diffusion is $D_{color}\sim (α_s\ln(1/α_s)T)^{-1}$, a factor $α_s$ smaller than spin and flavor diffusion. A similar effect is expected in electroweak plasmas above $M_W$ due to $W^\pm$ exchanges. The color conductivity in quark-gluon plasmas and the electrical conductivity in electroweak plasmas are correspondingly small in relativistic heavy ion collisions and the very early universe.

hep-ph↗

arXiv1994

Viscosities of Quark-Gluon Plasmas

The quark and gluon viscosities are calculated in quark-gluon plasmas to leading orders in the coupling constant by including screening. For weakly interaction QCD and QED plasmas dynamical screening of transverse interactions and Debye screening of longitudinal interactions controls the infrared divergences. For strongly interacting plasmas other screening mechanisms taken from lattice calculations are employed. By solving the Boltzmann equation for quarks and gluons including screening the viscosity is calculated to leading orders in the coupling constant. The leading logarithmic order is calculated exactly by a full variational treatment. The next to leading orders are found to be very important for sizable coupling constants as those relevant for the transport properties relevant for quark-gluon plasmas created in relativistic heavy ion collisions and the early universe.

hep-ph↗

arXiv1994

Transport Properties of Quark and Gluon Plasmas

The kinetic properties of relativistic quark-gluon and electron-photon plasmas are described in the weak coupling limit. The troublesome Rutherford divergence at small scattering angles is screened by Debye screening for the longitudinal or electric part of the interactions. The transverse or magnetic part of the interactions is effectively screened by Landau damping of the virtual photons and gluons transferred in the QED and QCD interactions respectively. Including screening a number of transport coefficients for QCD and QED plasmas can be calculated to leading order in the interaction strength, including rates of momentum and thermal relaxation, electrical conductivity, viscosities, flavor and spin diffusion of both high temperature and degenerate plasmas. Damping of quarks and gluons as well as color diffusion in quark-gluon plasmas is, however, shown not to be sufficiently screened and the rates depends on an infrared cut-off of order the ``magnetic mass", $m_{\rm mag}\sim g^2 T$.

hep-ph↗