Do all 5d SCFTs descend from 6d SCFTs?

TL;DR

This work challenges the conjecture that all $5d$ SCFTs arise solely from integrating out BPS particles in $6d$ SCFTs compactified on a circle, by constructing explicit counter-examples. It shows these theories can be reached only when one also integrates out BPS strings, using Calabi-Yau geometry and decompactification of surfaces to realize the necessary RG flows from untwisted $6d$ SCFTs. A general, geometry-based surface-decoupling criterion via Mori cone analysis is developed to determine when such decompactifications are possible, and concrete examples are worked out for algebras $\mathfrak{f}_4$, $\mathfrak{e}_6$, $\mathfrak{e}_7$, and related cases. The results motivate a revised conjecture and imply substantial changes to the classification program of $5d$ SCFTs, highlighting the role of BPS strings in connecting higher- and lower-rank theories.

Abstract

We present examples of 5d SCFTs that serve as counter-examples to a recently actively studied conjecture according to which it should be possible to obtain all 5d SCFTs by integrating out BPS particles from 6d SCFTs compactified on a circle. We further observe that it is possible to obtain these 5d SCFTs from 6d SCFTs if one allows integrating out BPS strings as well. Based on this observation, we propose a revised version of the conjecture according to which it should be possible to obtain all 5d SCFTs by integrating out both BPS particles and BPS strings from 6d SCFTs compactified on a circle. We describe a general procedure to integrate out BPS strings from a 5d theory once a geometric description of the 5d theory is given. We also discuss the consequences of the revised conjecture for the classification program of 5d SCFTs.