Notes on S-folds and $\mathcal{N}=3$ theories

TL;DR

This paper investigates the structure and supersymmetry enhancement of 4d $ =3$ SCFTs arising from D3 branes near S-folds, analyzing BPS spectra, wall-crossing, and the absence of certain vector multiplets.

Contribution

It provides an explicit counting of degrees of freedom, clarifies the conditions for supersymmetry enhancement, and relates string junctions to multiplet structures in S-fold backgrounds.

Findings

No vector multiplet for certain stretched strings in trivial torsion cases.

BPS spectrum matches between $ =3$ and $ =4$ theories for rank 2 cases.

Wall-crossing behavior is consistent with supersymmetry enhancement.

Abstract

We consider D3 branes in presence of an S-fold plane. The latter is a non perturbative object, arising from the combined projection of an S-duality twist and a discrete orbifold of the R-symmetry group. This construction naively gives rise to 4d $\mathcal{N}=3$ SCFTs. Nevertheless it has been observed that in some cases supersymmetry is enhanced to $\mathcal{N}=4$. In this paper we study the explicit counting of degrees of freedom arising from vector multiplets associated to strings suspended between the D3 branes probing the S-fold. We propose that, for trivial discrete torsion, there is no vector multiplet associated to $(1,0)$ strings stretched between a brane and its image. We then focus on the case of rank 2 $\mathcal{N}=3$ theory that enhances to $SU(3)$ $\mathcal{N}=4$ SYM, explicitly spelling out the isomorphism between the BPS-spectrum of the manifestly $\mathcal{N}=3$ theory and that of three D3 branes in flat spacetime. Subsequently, we consider 3-pronged strings in these setups and show how wall-crossing in the S-fold background implies wall crossing in the flat geometry. This can be considered a consistency check of the \emph{conjectured} SUSY enhancement. We also find that the above isomorphism implies that a $(1,0)$ string, suspended between a brane and its image in the S-fold, corresponds to a 3-string junction in the flat geometry. This is in agreement with our claim on the absence of a vector multiplet associated to such $(1,0)$ strings. This is because the 3-string junction in flat geometry gives rise to a $1/4$-th BPS multiplet of the $\mathcal{N}=4$ algebra. Such multiplets always include particles with spin $>1$ as opposed to a vector multiplet which is restricted by the requirement that the spins must be $\leq 1$.