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.Comment: additional material included, 8 pages, revtex, epsfig, 2 figure included, manuscript also available at http://www.nbi.dk/~vischer/publications.htm

Time development of a density perturbation in the unstable nuclear matter

We present the solution of the time development of an unstable initial density perturbation in the linearized Vlasov equation, completing the previous analysis in the literature. The additional contributions found are usually damped and can be neglected at large times in the unstable region. The work clarifies also the problem of the normalization of the solution with respect to the initial perturbation of the density.Comment: revision of the discussion, different initial perturbation, 9 pages, 4 figures included, uses epsfi

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.Comment: 9 pages, revtex, epsfig, 2 figure included, manuscript also available at http://www.nbi.dk/~vischer/publications.htm

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, $m_{mag}\sim \alpha_sT$, the color diffusion is $D_{color}\sim (\alpha_s\ln(1/\alpha_s)T)^{-1}$, a factor $\alpha_s$ 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.Comment: 5 pages, no figure

Sound modes at the BCS-BEC crossover

First and second sound speeds are calculated for a uniform superfluid gas of fermi atoms as a function of temperature, density and interaction strength. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature. The sound modes and their dependence on density, scattering length and temperature are calculated in the BCS, molecular BEC and unitarity limits and a smooth crossover is extrapolated. It is found that first and second sound undergo avoided crossing on the BEC side due to mixing. Consequently, they are detectable at crossover both as density and thermal waves in traps.Comment: To appear in Phys. Rev.

Pairing of fermions in atomic traps and nuclei

Pairing gaps for fermionic atoms in harmonic oscillator traps are calculated for a wide range of interaction strengths and particle number, and compared to pairing in nuclei. Especially systems, where the pairing gap exceeds the level spacing but is smaller than the shell splitting $\hbar\omega$, are studied which applies to most trapped Fermi atomic systems as well as to finite nuclei. When solving the gap equation for a large trap with such multi-level pairing, one finds that the matrix elements between nearby harmonic oscillator levels and the quasi-particle energies lead to a double logarithm of the gap, and a pronounced shell structure at magic numbers. It is argued that neutron and proton pairing in nuclei belongs to the class of multi-level pairing, that their shell structure follows naturally and that the gaps scale as $\sim A^{-1/3}$ - all in qualitative agreement with odd-even staggering of nuclear binding energies. Pairing in large systems are related to that in the bulk limit. For large nuclei the neutron and proton superfluid gaps approach the asymptotic value in infinite nuclear matter: $\Delta\simeq 1.1$ MeV.Comment: 11 pages, 5 figure

Color conductivity and ladder summation in hot QCD

The color conductivity is computed at leading logarithmic order using a Kubo formula. We show how to sum an infinite series of planar ladder diagrams, assuming some approximations based on the dominance of soft scattering processes between hard particles in the plasma. The result agrees with the one obtained previously from a kinetical approach.Comment: 15 pages, 4 figures. Explanations enlarged, two figures and some refs added, typos corrected. Final version to be published in Phys.Rev.

Optimization of cool roof and night ventilation in office buildings: A case study in Xiamen, China

Increasing roof albedo (using a “cool” roof) and night ventilation are passive cooling technologies that can reduce the cooling loads in buildings, but existing studies have not comprehensively explored the potential benefits of integrating these two technologies. This study combines an experiment in the summer and transition seasons with an annual simulation so as to evaluate the thermal performance, energy savings and thermal comfort improvement that could be obtained by coupling a cool roof with night ventilation. A holistic approach integrating sensitivity analysis and multi-objective optimization is developed to explore key design parameters (roof albedo, night ventilation air change rate, roof insulation level and internal thermal mass level) and optimal design options for the combined application of the cool roof and night ventilation. The proposed approach is validated and demonstrated through studies on a six-storey office building in Xiamen, a cooling-dominated city in southeast China. Simulations show that combining a cool roof with night ventilation can significantly decrease the annual cooling energy consumption by 27% compared to using a black roof without night ventilation and by 13% compared to using a cool roof without night ventilation. Roof albedo is the most influential parameter for both building energy performance and indoor thermal comfort. Optimal use of the cool roof and night ventilation can reduce the annual cooling energy use by 28% during occupied hours when air-conditioners are on and reduce the uncomfortable time slightly during occupied hours when air-conditioners are off

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.Comment: 12 pages + 6 figures, report LBL-3492