Theory of the Weakly Interacting Bose Gas

We review recent advances in the theory of the three-dimensional dilute homogeneous Bose gas at zero and finite temperature. Effective field theory methods are used to formulate a systematic perturbative framework that can be used to calculate the properties of the system at T=0. The perturbative expansion of these properties is essentially an expansion in the gas parameter $\sqrt{na^3}$, where $a$ is the s-wave scattering length and $n$ is the number density. In particular, the leading quantum corrections to the ground state energy density, the condensate depletion, and long-wavelength collective excitations are rederived in and efficient and economical manner. We also discuss nonuniversal effects. These effects are higher-order corrections that depend on properties of the interatomic potential other than the scattering length, such as the effective range. We critically examine various approaches to the dilute Bose gas in equilibrium at finite temperature. These include the Bogoliubov approximation, the Popov approximation, the Hartree-Fock-Bogoliubov approximation, the $\Phi$-derivable approach, optimized perturbation theory, and renormalization group techniques. Finally, we review recent calculations of the critical temperature of the dilute Bose gas, which include 1/N-techniques, lattice simulations, self-consistent calculations, and variational perturbation theory.Comment: 44 pages, 20 Postscript figures. Revised version. Expanded by 7 pages and 4 figs. Updated section on T_c and updated list of references. Discussion on atomic potentials and effective field theory added. Revised version accepted for publication in Review of Modern physic

Dimensional Reduction of the Two-Higgs Doublet Model at High temperature

Dimensional reduction and effective field theory methods are applied to the Two Higgs Doublet Model at finite temperature. A sequence of two effective three-dimensional field theories which are valid on successively longer distance scales is constructed. The resulting Lagrangian can be used to study different aspects of the phase transition in this model as well as the sphaleron rate immediately after the phase transition.Comment: 16 pages, revised versio

Application of Renormalization Group Techniques to a Homogeneous Bose Gas at Finite Temperature

A homogeneous Bose gas is investigated at finite temperature using renormalization group techniques. A non--perturbative flow equation for the effective potential is derived using sharp and smooth cutoff functions. Numerical solutions of these equations show that the system undergoes a second order phase transition in accordance with universality arguments. We obtain the critical exponent $\nu =0.73$ to leading order in the derivative expansion.Comment: 20 pages, 6 Postscript figures, revte

Phase diagram of two-color quark matter at nonzero baryon and isospin density

We investigate the properties of cold dense quark matter composed of two colors and two flavors of light quarks. In particular, we perform the first model calculation of the full phase diagram at nonzero baryon and isospin density, thus matching the model-independent predictions of chiral perturbation theory at low density to the conjectured phase structure at high density. We confirm the presence of the Fulde-Ferrell (FF) phase in the phase diagram and study its dependence on the tunable parameter in the Lagrangian that simulates the effects of the quantum axial anomaly. As a byproduct, we clarify the calculation of the thermodynamic potential in the presence of the FF pairing, which was previously based on an ad hoc subtraction of an unphysical cutoff artifact. Furthermore, we argue that close to the diquark (or pion) Bose-Einstein condensation transition, the system behaves as a dilute Bose gas so that our simple fermionic model in the mean-field approximation is not quantitatively adequate. We suggest that including thermal fluctuations of the order parameter for Bose-Einstein condensation is crucial for understanding available lattice data.Comment: 14 pages, REVTeX4-1, 7 eps figures; v2: minor modifications + references added; version to be published in Phys. Rev.

Ground State of a trapped Bose-Einstein Condensate in Two Dimensions; Beyond the Mean-field Approximation

We consider the ground state of a trapped Bose-Einstein condensate in two dimensions. In the mean-field approximation, the ground state density profile satisfies the Gross-Pitaevskii equation. We compute the leading quantum corrections to the density profile to second order in an expansion around the Thomas-Fermi limit. By summing the ladder diagrams, we are generalizing Schick's result for the ground state energy of a homogeneouns Bose gas to the case of a trapped Bose gas.Comment: 19 pages, 2 figures, revte