# The Concept of Particle Weights in Local Quantum Field Theory

May 06, 2000

The concept of particle weights has been introduced by Buchholz and the
author in order to obtain a unified treatment of particles as well as (charged)
infraparticles which do not permit a definition of mass and spin according to
Wigner's theory. Particle weights arise as temporal limits of physical states
in the vacuum sector and describe the asymptotic particle content. Following a
thorough analysis of the underlying notion of localizing operators, we give a
precise definition of this concept and investigate the characteristic
properties. The decomposition of particle weights into pure components which
are linked to irreducible representations of the quasi-local algebra has been a
long-standing desideratum that only recently found its solution. We set out two
approaches to this problem by way of disintegration theory, making use of a
physically motivated assumption concerning the structure of phase space in
quantum field theory. The significance of the pure particle weights ensuing
from this disintegration is founded on the fact that they exhibit features of
improper energy-momentum eigenstates, analogous to Dirac's conception, and
permit a consistent definition of mass and spin even in an infraparticle
situation.

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