263 research outputs found
The Charge Quantum Numbers of Gauge Invariant Quasi-free Endomorphisms
The representations of a group of gauge automorphisms of the canonical
commutation or anticommutation relations which appear on the Hilbert spaces of
isometries H_\rho implementing quasi-free endomorphisms \rho on Fock space are
studied. Such a representation, which characterizes the "charge" of \rho in
local quantum field theory, is determined by the Fock space structure of H_\rho
itself: Together with a "basic" representation of the group, all higher
symmetric or antisymmetric tensor powers thereof also appear. Hence \rho is
reducible unless it is an automorphism. It is further shown by the example of
the massless Dirac field in two dimensions that localization and
implementability of quasi-free endomorphisms are compatible with each other.Comment: 15 pages, no figure
Scattering matrix in external field problems
We discuss several aspects of second quantized scattering operators
for fermions in external time dependent fields. We derive our results on a
general, abstract level having in mind as a main application potentials of the
Yang--Mills type and in various dimensions. We present a new and powerful
method for proving existence of which is also applicable to other
situations like external gravitational fields. We also give two complementary
derivations of the change of phase of the scattering matrix under generalized
gauge transformations which can be used whenever our method of proving
existence of applies. The first is based on a causality argument i.e.\
(including phase) is determined from a time evolution, and the second
exploits the geometry of certain infinite-dimensional group extensions
associated with the second quantization of 1-particle operators. As a special
case we obtain a Hamiltonian derivation of the the axial Fermion-Yang-Mills
anomaly and the Schwinger terms related to it via the descent equations, which
is on the same footing and traces them back to a common root.Comment: AmsTex file (uses amstex.tex and amsppt.sty) 22 ouput page
Unitary evolution in Gowdy cosmology
Recent results on the non-unitary character of quantum time evolution in the
family of Gowdy T**3 spacetimes bring the question of whether one should
renounce in cosmology to the most sacred principle of unitary evolution. In
this work we show that the answer is in the negative. We put forward a full
nonperturbative canonical quantization of the polarized Gowdy T**3 model that
implements the dynamics while preserving unitarity. We discuss possible
implications of this result.Comment: 5 pages, no figures. V2 discussion expanded, references added. Final
version to appear in PR
QED in external fields from the spin representation
Systematic use of the infinite-dimensional spin representation simplifies and
rigorizes several questions in Quantum Field Theory. This representation
permutes ``Gaussian'' elements in the fermion Fock space, and is necessarily
projective: we compute its cocycle at the group level, and obtain Schwinger
terms and anomalies from infinitesimal versions of this cocycle. Quantization,
in this framework, depends on the choice of the ``right'' complex structure on
the space of solutions of the Dirac equation. We show how the spin
representation allows one to compute exactly the S-matrix for fermions in an
external field; the cocycle yields a causality condition needed to determine
the phase.Comment: 32 pages, Plain TeX, UCR-FM-01-9
Neutral Particles and Super Schwinger Terms
Z_2-graded Schwinger terms for neutral particles in 1 and 3 space dimensions
are considered.Comment: 13 page
Slowly decaying classical fields, unitarity, and gauge invariance
In classical external gauge fields that fall off less fast than the inverse
of the evolution parameter (time) of the system the implementability of a
unitary perturbative scattering operator (-matrix) is not guaranteed,
although the field goes to zero. The importance of this point is exposed for
the counter-example of low-dimensionally expanding systems. The issues of gauge
invariance and of the interpretation of the evolution at intermediate times are
also intricately linked to that point.Comment: 8 pages, no figure
Kakutani Dichotomy on Free States
Two quasi-free states on a CAR or CCR algebra are shown to generate
quasi-equivalent representations unless they are disjoint.Comment: 12 page
On the ultraviolet behaviour of quantum fields over noncommutative manifolds
By exploiting the relation between Fredholm modules and the
Segal-Shale-Stinespring version of canonical quantization, and taking as
starting point the first-quantized fields described by Connes' axioms for
noncommutative spin geometries, a Hamiltonian framework for fermion quantum
fields over noncommutative manifolds is introduced. We analyze the ultraviolet
behaviour of second-quantized fields over noncommutative 3-tori, and discuss
what behaviour should be expected on other noncommutative spin manifolds.Comment: 10 pages, RevTeX version, a few references adde
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