Generalizations of the thermal Bogoliubov transformation

The thermal Bogoliubov transformation in thermo field dynamics is generalized in two respects. First, a generalization of the $\alpha$--degree of freedom to tilde non--conserving representations is considered. Secondly, the usual $2\times2$ Bogoliubov matrix is extended to a $4\times4$ matrix including mixing of modes with non--trivial multiparticle correlations. The analysis is carried out for both bosons and fermions.Comment: 20 pages, Latex, Nordita 93/33

Renormalization Group Invariant Constraints among Coupling Constants in a Noncommutative Geometry Model

We study constraints among coupling constants of the standard model obtained in the noncommutative geometry (NCG) method. First, we analyze the evolution of the Higgs boson mass under the renormalization group by adopting the idea of \'Alvarez et al. For this analysis we derive two certain constraints by modifying Connes's way of constructing the standard model. Next, we find renormalization group invariant (RGI) constraints in the NCG method. We also consider the relation between the condition that a constraint among coupling constants of a model becomes RGI and the condition that the model becomes multiplicative renormalizable by using a simple example.Comment: 22 pages, Latex file, 2 figures available upon request to [email protected], important changes are made, This is the last version which will appear in Prog. Theor. Phys. {\bf 100} (1998) as "Constraints among Coupling Constants in Noncommutative Geometry Models

Diagonalization of full finite temperature Green's function by quasi-particles

For thermal systems, standard perturbation theory breaks down because of the absence of stable, observable asymptotic states. We show, how the introduction of {\it statistical} quasi-particles (stable, but not observable) gives rise to a consistent description. Statistical and spectral information can be cleanly separated also for interacting systems.Comment: 9 pages in standard LaTe

Supersymmetry in Thermo Field Dynamics

By considering the enlarged thermal system including the heat bath, it is shown that this system has supersymmetry which is not broken at finite temperature. The super algebra is constructed and the Hamiltonian is expressed as the anti-commutator of two kinds of super charges. With this Hamiltonian and the thermal vacuum $\mid 0(\beta)>$, this supersymmetry is found to be preserved.Comment: 12 pages, Latex fil

The column vector calculus for Thermo Field Dynamics of relativistic quantum fields

A formalism is discussed which simplifies the calculation of Feynman diagrams at finite temperature.Comment: 10 pages, GSI-Preprint 93-32 (1993

Lorentz Invariance And Unitarity Problem In Non-Commutative Field Theory

It is shown that the one-loop two-point amplitude in {\it Lorentz-invariant} non-commutative (NC) $\phi^3$ theory is finite after subtraction in the commutative limit and satisfies the usual cutting rule, thereby eliminating the unitarity problem in Lorentz-non-invariant NC field theory in the approximation considered.Comment: 14 page

String Entanglement and D-branes as Pure States

We study the entanglement of closed strings degrees of freedom in order to investigate the microscopic structure and statistics of objects as D-branes. By considering the macroscopic pure state (MPS) limit, whenever the entanglement entropy goes to zero (in such a way that the macroscopic properties of the state are preserved), we show that boundary states may be recovered in this limit and, furthermore, the description through closed string (perturbative) degrees of freedom collapses. We also show how the thermal properties of branes and closed strings could be described by this model, and it requires that dissipative effects be taken into account. Extensions of the MPS analysis to more general systems at finite temperature are finally emphasized.Comment: 14 pages. Minor improvements. Published in Phys. Rev.