Positive energy ray representations of the Poincaré group are naturally subdivided into three classes according to their mass and spin content: m > 0, m = 0 finite helicity and m = 0 infinite spin. For a long time the localization properties of the massless infinite spin class remained unknown, until it became clear that such matter does not permit compact spacetime localization and its generating covariant fields are localized on semi-infinite spacelike strings. Using a new perturbation theory for higher spin fields we present arguments which support the idea that infinite spin matter cannot interact with normal matter and we formulate conditions under which this also could happen for finite spin s > 1 fields. This raises the question of a possible connection between inert matter and dark matter.
Wigner's infinite spin representation and string-localizationWigner's famous 1939 theory of unitary representations of the Poincaré group P was the first systematic and successful attempt to classify relativistic particles according to the intrinsic principles of relativistic quantum theory [1]. As we know nowadays, his massive and massless spin/helicity class of positive energy ray representations of P does not only cover all known particles, but their "covariantization" [2] leads also to a complete description of all covariant pointlocal (pl) free fields. For each covariant Lorentz transformation property compatible with physical spin or helicity there exists a pl field.It is not necessary that these free fields permit a characterization as Euler-Lagrange fields; in fact the attempt to describe higher spin fields in this way and pass to quantum fields by canonical quantization or use other ways to Dedicated to the memory of Robert Schrader. a e-mail: schroer@zedat.fu-berlin.de start from a classical field creates problems of their own. On the other hand the construction of free fields starting from Wigner's representation theory does not require one to rely on any "quantization parallelism" to classical structures.The idea that a more fundamental theory should not rely on structural properties of a less basic one can also be upheld in the presence of interactions. Defining a first order Lorentz invariant interaction densities in terms of products of free fields and using the Epstein-Glaser [3] formulation of renormalization theory permits a perturbative cutoff-and regularization-free higher order inductive implementation of the causal localization principle in a positivity-obeying Wigner-Fock Hilbert space without a reference to the classical field formalism. Renormalizable theories in this setting are those for which this higher order induction increases the model-defining first order parameters by an at most finite number of "counter term" parameters. When higher spin s ≥ 1 fields participate in the interaction there are no positivity-obeying renormalizable couplings. The traditional way out has been to skip positivity in the calculations and save it by restricting the result. A total abandonment would have...