The transition temperature for a dilute, homogeneous, three-dimensional Bose gas has the expansion T c = T 0 {1+c 1 an 1/3 +[c ′ 2 ln(an 1/3 )+c ′′ 2 ]a 2 n 2/3 +O(a 3 n)}, where a is the scattering length, n the number density, and T 0 the ideal gas result. The first-order coefficient c 1 depends on non-perturbative physics. In this paper, we show that the coefficient c ′ 2 can be computed perturbatively. We also show that the remaining second-order coefficient c ′′ 2 depends on nonperturbative physics but can be related, by a perturbative calculation, to quantities that have previously been measured using lattice simulations of three-dimensional O(2) scalar field theory. Making use of those simulation results, we find T c ≃ T 0 {1 + (1.32 ± 0.02) an 1/3 + [19.7518 ln(an 1/3 ) + (75.7 ± 0.4)]a 2 n 2/3 + O(a 3 n)}. in the dilute (or, equivalently, weak-interaction) limit, where c 1 is a numerical constant, a is the scattering length, which parameterizes the low energy 2-particle scattering crosssection, and n is the density of the homogeneous gas. We will assume that the interactions are repulsive (a > 0). A clean argument for (1.1) may be found in Ref. [1], which also shows how the problem of calculating the constant c 1 can be reduced to a problem in threedimensional O(2) field theory. Recent numerical simulations of that theory have obtained the results c 1 = 1.29 ± 0.05 [2] and c 1 = 1.32 ± 0.02 [3,4].In this paper, we shall extend the result for T c (n) to second order in a for a homogeneous Bose gas. This is also the relationship between T c and the central number density for a

We discuss the Yano-Koonin-Podgoretskii (YKP) parametrization of the twoparticle correlation function for azimuthally symmetric expanding sources.We derive model-independent expressions for the YKP fit parameters and discuss their physical interpretation. We use them to evaluate the YKP fit parameters and their momentum dependence for a simple model for the emission function and propose new strategies for extracting the source lifetime.Longitudinal expansion of the source can be seen directly in the rapidity dependence of the Yano-Koonin velocity.PACS numbers: 25.75. Gz, 25.75.Ld, 12.38.Mh Typeset using REVT E X 1

We introduce a new scenario for heavy ion collisions that could solve the lingering problems associated with the so-called HBT puzzle. We postulate that the system starts expansion as the perfect quark-gluon fluid but close to freeze-out it splits into clusters, due to a sharp rise of bulk viscosity in the vicinity of the hadronization transition. We then argue that the characteristic cluster size is determined by the viscosity coefficient and the expansion rate. Typically it is much smaller and at most weakly dependent of the total system volume (hence reaction energy and multiplicity). These clusters maintain the pre-existing outward-going flow, as a spray of droplets, but develop no flow of their own, and hadronize by evaporation. We provide an ansatz for converting the hydrodynamic output into clusters. 25.75.Dw, 25.75.Nq

For some time, the theoretical result for the transition temperature of a dilute three-dimensional Bose gas in an arbitrarily wide harmonic trap has been known to first order in the interaction strength. We extend that result to second order. The first-order result for a gas trapped in a harmonic potential can be computed in mean field theory (in contrast to the first order result for a uniform gas, which cannot). We show that, at second order, perturbation theory suffices for relating the transition temperature to the chemical potential at the transition, but the chemical potential is non-perturbative at the desired order. The necessary information about the chemical potential can be extracted, however, from recent lattice simulations of uniform Bose gases.