We will spend only a short time reviewing old fashioned perturbation theory and exploring its application in quantum field theory. The essentials in QFT are the same as they were in non-relativitstic quantum mechanics, where we split the Hamiltonian into two parts a “free" solvable Hamiltonian, H0 and an interacting part Hint 42 This separation is often troublesome, as symmetries may relate ope...

A new perturbative scheme is proposed for the evaluation of the free energy density of field theories at finite temperature. The screened loop expansion takes into account exactly the phenomenon of screening in thermal propagators. The approach is tested in the N -component scalar field theory at 2-loop level and also at 3-loop in the large N limit. The perturbative series generated by the scre...

In this paper classical matrix perturbation theory is approached from a probabilistic point of view. The perturbed quantity is approximated by a rst-order perturbation expansion, in which the perturbation is assumed to be random. This permits the computation of statistics estimating the variation in the perturbed quantity. Up to the higher-order terms that are ignored in the expansion, these st...

Apparent large failures of perturbation theory in lattice gauge theory at g* 5 1.0 result from the use of the bare coupling constant of the lattice regulator in the perturbative expansion. They can be cured by the use of a coupling constant defined by some physical process. Thissituation is thesame as that encountered in dimensionally regularized perturbation theory, where the use of a coupling...

This is a review on cosmological perturbation theory. After an introduction, it presents the problem of gauge transformation. Gauge invariant variables are introduced and the Einstein and conservation equations are written in terms of these variables. Some examples, especially perfect fluids and scalar fields are presented in detail. The generation of perturbations during inflation is studied. ...

Our starting point is the set of eigenstates n of the unperturbed Hamiltonian 0 n H n E n = , notice we are not labeling with a zero, no 0 n E , because with a time-dependent Hamiltonian, energy will not be conserved, so it is pointless to look for energy corrections. What happens instead, provided the perturbation is not too large, is that the system makes transitions between the eigenstates n...

Partially quenched theories are theories in which the valenceand seaquark masses are different. In this paper we calculate the nonanalytic one-loop corrections of some physical quantities: the chiral condensate, weak decay constants, Goldstone boson masses, BK and the K + → π+π0 decay amplitude, using partially quenched chiral perturbation theory. Our results for weak decay constants and masses...

We provide an introduction to the power-counting issue in baryon chiral perturbation theory and discuss some recent developments in the manifestly Lorentz-invariant formulation of the one-nucleon sector. As explicit applications we consider the chiral expansion of the nucleon mass, its convergence properties, and calculations of the electromagnetic and axial form factors of the nucleon.

Perturbation theory (PT) appears in many important theoretical and practical roles in lattice field theory. Conceptual issues related to the continuum limit can be studied in a perturbative framework, such as the renormalizability of lattice gauge theories [1]. Perturbative calculations are used in relating lattice quantities to those defined in continuum schemes and in constructing Symanzik-im...