### Helicity decoupling in the massless limit of massive tensor fields

#### Resumo

Massive and massless potentials play an essential

role in the perturbative formulation of particle interactions. Many

difficulties arise due to the indefinite metric in gauge theoretic

approaches, or the increase with the spin of the UV dimension of

massive potentials. All these problems can be evaded in one stroke:

modify the potentials by suitable terms that leave unchanged the field

strengths, but are not polynomial in the momenta. This feature

implies a weaker localization property: the potentials are

``string-localized''. In this setting, several old issues can be

solved directly in the

physical Hilbert space of the respective particles: We construct

\set s for massless fields of any helicity (thus

evading the Weinberg-Witten theorem). We can control the separation

of helicities in the massless limit of higher spin fields and

conversely we recover massive potentials with $2s+1$ degrees of

freedom by a smooth deformation of the massless potentials

(``fattening''). We arrive at a simple understanding of the van

Dam-Veltman-Zakharov discontinuity concerning, e.g., the distinction

between a massless or a very light graviton. Finally, the

use of string-localized fields opens new perspectives for

interacting quantum field theories with, e.g., vector bosons

or gravitons.

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