Goldilocks Modes and the Three Scattering Bases

TL;DR

This paper explores the relationships between different bases in massless scattering, focusing on the subleading soft theorems and celestial symmetries, and clarifies how the celestial basis effectively reveals the underlying symmetry structure.

Contribution

It translates the infrared triangle into the celestial basis, resolves tensions in gauge transformations, and extends the analysis to $w_{1+ty}$ symmetries in celestial CFT.

Findings

Resolved tension between overleading gauge transformations and Goldstone modes.

Generalized to $w_{1+ty}$ symmetries, revealing higher multipole moments.

Clarified order-of-limits issues in celestial formalism.

Abstract

We consider massless scattering from the point of view of the position, momentum, and celestial bases. In these three languages different properties of physical processes become manifest or obscured. Within the soft sector, they highlight distinct aspects of the infrared triangle: quantum field theory soft theorems arise in the limit of vanishing energy $\omega$, memory effects are described via shifts of fields at the boundary along the null time coordinate $u$, and celestial symmetry algebras are realized via currents that appear at special values of the conformal dimension $\Delta$. We focus on the subleading soft theorems at $\Delta=1-s$ for gauge theory $(s=1)$ and gravity $(s=2)$ and explore how to translate the infrared triangle to the celestial basis. We resolve an existing tension between proposed overleading gauge transformations as examined in the position basis and the `Goldstone-like' modes where we expect celestial symmetry generators to appear. In the process we elucidate various order-of-limits issues implicit in the celestial formalism. We then generalize our construction to the tower of $w_{1+\infty}$ generators in celestial CFT, which probe further subleading-in-$\omega$ soft behavior and are related to subleading-in-$r$ vacuum transitions that measure higher multipole moments of scatterers. In the end we see that the celestial basis is `just right' for identifying the symmetry structure.