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Michael G. Schmidt

Publications and source records attributed to Michael G. Schmidt.

69 records · Page 4Linked to original sources
arXiv1998

SUSY Variants of the Electroweak Phase Transition

The MSSM with a light right-handed stop and supersymmetric models with a singlet whose vev is comparable to that of the SU(2)_W Higgs allow for a strongly first-order electroweak phase transition even for a mass of the lightest Higgs around 100 GeV. After a short review of the standard model situation we discuss these supersymmetric models. We also compare perturbative calculations based on the dimensionally reduced 3-dimensional action with lattice results and present an analytic procedure based on an analogue of the stochastic vacuum model of QCD to estimate the nonperturbative contributions.

hep-ph↗
arXiv1998

World Line Path Integrals as a Calculational Tool in Quantum Field Theory

We report on the status of the string-inspired world line path integral formalism, a recently developed powerful tool for the reorganisation of standard perturbative amplitudes in quantum field theory. The method is outlined and the present range of its applicability surveyed. The emphasis is on QED and QCD photon/gluon amplitudes, with a short discussion of axial couplings.

hep-th↗
arXiv1998

Exact Combinatorics of Bern-Kosower-type Amplitudes for Two-Loop $Φ^3$ Theory

Counting the contribution rate of a world-line formula to Feynman diagrams in $ϕ^3$ theory, we explain the idea how to determine precise combinatorics of Bern-Kosower-like amplitudes derived from a bosonic string theory for $N$-point two-loop Feynman amplitudes. In this connection we also present a method to derive simple and compact world-line forms for the effective action.

hep-th↗
arXiv1997

Constant External Fields in Gauge Theory and the Spin 0, 1/2, 1 Path Integrals

We investigate the usefulness of the "string-inspired technique" for gauge theory calculations in a constant external field background. Our approach is based on Strassler's worldline path integral approach to the Bern-Kosower formalism, and on the construction of worldline (super--) Green's functions incorporating external fields as well as internal propagators. The worldline path integral representation of the gluon loop is reexamined in detail. We calculate the two-loop effective actions induced for a constant external field by a scalar and spinor loop, and the corresponding one-loop effective action in the gluon loop case.

hep-th↗
arXiv1997

The Electroweak Phase Transition - Standard and ``Beyond''

We first review why a strongly first-order phase transition needed for baryogenesis is excluded in the electroweak standard model. We also comment on some intriguing effects in the strongly interacting hot phase. In the MSSM with a light stop a strongly first-order phase transition can be achieved. It possibly proceeds in two stages.

hep-ph↗
arXiv1996

Relativistic QFTH - Couplings on the Worldline

In the framework of the worldline path integral approach to QFTH we discuss spin and relativistic couplings, in particular Yukawa and axial couplings to spin 1/2, and the case of spin 1 in the loop.

hep-th↗
arXiv1996

Three Applications of the String-Inspired Technique to Quantum Electrodynamics

We discuss the following recent applications of the ``string-inspired'' worldline technique to calculations in quantum electrodynamics: i) Photon splitting in a constant magnetic field, ii) The two-loop Euler-Heisenberg Lagrangian, iii) A progress report on a recalculation of the three-loop QED beta -- function.

hep-th↗
arXiv1995

Multiloop Calculations in the String-Inspired Formalism: The Single Spinor-Loop in QED

We use the worldline path-integral approach to the Bern-Kosower formalism for developing a new algorithm for calculation of the sum of all diagrams with one spinor loop and fixed numbers of external and internal photons. The method is based on worldline supersymmetry, and on the construction of generalized worldline Green functions. The two-loop QED $β$ -- function is calculated as an example.

hep-th↗
arXiv1995

Axial Couplings on the World-Line

We construct a world-line representation for the fermionic one-loop effective action with axial and also vector, scalar, and pseudo-scalar couplings. We use this expression to compute a few selected scattering amplitudes. These allow us to verify that our method yields the same results as standard field theory. In particular, we are able to reproduce the chiral anomaly. Our starting point is the second-order formulation for the Dirac fermion. We translate the second order expressions into a world-line action.

hep-th↗
arXiv1995

Yukawa Couplings for the Spinning Particle and the World Line Formalism

We construct the world-line action for a Dirac particle coupled to a classical scalar or pseudo-scalar background field. This action can be used to compute loop diagrams and the effective action in the Yukawa model using the world-line path-integral formalism for spinning particles.

hep-th↗
arXiv1995

Inverse Mass Expansions from Worldline Path Integrals - Higher Order Coefficients and Ordering Problems

Higher order coefficients of the inverse mass expansion of one--loop effective actions are obtained from a one--dimensional path integral representation. For the evaluation of the path integral with Wick contractions a suitable Green function has to be chosen. We consider the case of a massive scalar loop in the background of both a scalar potential and a (non--abelian) gauge field. For the pure scalar case the method yields the coefficients of the expansion in a minimal set of basis terms whereas complicated ordering problems arise in gauge theory. An appropriate reduction scheme is discussed.

hep-th↗
arXiv1994

Sphaleron Effects Near the Critical Temperature

We discuss one-loop radiative corrections to the sphaleron-induced baryon number-violating transition rate near the electroweak phase transition in the standard model. We emphasize that in the case of a first-order transition a rearrangement of the loop expansion is required close to the transition temperature. The corresponding expansion parameter, the effective 3-dimensional gauge coupling approaches a finite $λ$ dependent value at the critical temperature. The $λ$ (Higgs mass) dependence of the 1-loop radiative corrections is discussed in the framework of the heat kernel method. Radiative corrections are small compared to the leading sphaleron contribution as long as the Higgs mass is small compared to the W mass. To 1-loop accuracy, there is no Higgs mass range compatible with experimental limits where washing-out of a B+L asymmetry could be avoided for the minimal standard model with one Higgs doublet.

hep-ph↗