Scattering on self-dual Taub-NUT

TL;DR

This paper derives exact solutions and amplitudes for massless fields on self-dual Taub-NUT and dyon backgrounds, revealing new insights into their structure, growth at infinity, and a double copy relation between gauge and gravity theories.

Contribution

It introduces exact solutions for massless fields on self-dual Taub-NUT and dyon backgrounds, and explores their amplitudes and double copy relations, extending previous flat space results.

Findings

Massless scalar and spinning particle amplitudes vanish in certain helicity configurations.

Non-vanishing amplitudes are computed exactly on the backgrounds.

A double copy relation links gluon and graviton amplitudes in these backgrounds.

Abstract

We derive exact solutions of massless free field equations and tree-level two-point amplitudes up to spin 2 on self-dual Taub-NUT space-time, as well as on its single copy, the self-dual dyon. We use Killing spinors to build analogues of momentum eigenstates, finding that, in the spirit of color-kinematics duality, those for the self-dual dyon lift directly to provide states on the self-dual Taub-NUT background if one replaces charge with energy. We discover that they are forced to have faster growth at infinity than in flat space due to the topological non-triviality of these backgrounds. The amplitudes for massless scalars and spinning particles in the $(+\,+)$ and $(+\,-)$ helicity configurations vanish for generic kinematics as a consequence of the integrability of the self-dual sector. The $(-\,-)$ amplitudes are non-vanishing and we compute them exactly in the backgrounds, which are treated non-perturbatively. It is explained how spin is easily introduced via a Newman-Janis imaginary shift along the spin-vector leading directly to the additional well-known exponential factor in the dot product of the spin with the momenta. We also observe a double copy relation between the gluon amplitude on a self-dual dyon and graviton amplitude on a self-dual Taub-NUT space-time.