Towards a Localised S-Matrix Theory

TL;DR

This paper develops a localized S-matrix theory incorporating interaction localization effects, using a solvable model to explore quantum phenomena like diffraction and obliquity, with implications for collider and neutrino experiments.

Contribution

It introduces a localized S-matrix formalism with spatial spreads and analyzes its effects using a solvable model, revealing new angular dependence features and a quantum obliquity factor.

Findings

Confirmation of earlier S-matrix amplitude behaviors in Fresnel and Fraunhofer regimes.

Discovery of a quantum obliquity factor suppressing backward propagation.

Novel angular dependence features in the localized S-matrix.

Abstract

We formulate an S-matrix theory in which localisation effects of the particle interactions involved in a scattering process are consistently taken into account. In the limit of an infinite spread of all interactions, the S-matrix assumes its standard form. To better understand the significance of the emerging quantum phenomena in this formalism, we consider a solvable field-theoretic model with spatial Gaussian spreads at the interaction vertices. This solvable model, which was previously introduced in the literature, enables accurate descriptions of detection regions that are either close to or far from the source. In close analogy with light diffraction in classical optics, we call these two regions near-field and far-field zones, or the Fresnel and Fraunhofer regions. We revisit the question whether mixed mediators produce an oscillating pattern if their detection occurs in the Fresnel region. Besides corroborating certain earlier findings of the S-matrix amplitude in the forward Fresnel and Fraunhofer regimes, we observe several novel features with respect to its angular dependence which have not been accounted before in the literature. In particular, we obtain a ``quantum obliquity factor'' that suppresses particle propagation in the backwards direction, thereby providing an explicit quantum field-theoretic description for its origin in diffractive optics. Present and future colliders, as well as both short and long baseline neutrino experiments, would greatly benefit from the many predictions that can be offered from such a holistic localised S-matrix theory.