# GSO projections via SPT phases

## TL;DR

This paper links GSO projection phases in string theory to fermionic SPT phases on the worldsheet, explaining the classification of type II and type 0 theories and connecting to D-brane K-theory.

## Contribution

It introduces a novel perspective that relates GSO phases to fermionic SPT phases, clarifying the classification of string theories and their unoriented variants.

## Key findings

- Two types of type II theories explained by SPT phases
- Prediction of n mod 8 classification for unoriented type 0 theories
- Connection established between SPT perspective and D-brane K-theory

## Abstract

We point out that the choice of phases in GSO projections can be accounted for by a choice of fermionic SPT phases on the worldsheet of the string. This point of view not only easily explains why there are essentially two type II theories, but also predicts that there are unoriented type 0 theories labeled by n mod 8, and that there is an essentially unique choice of the type I worldsheet theory. We also discuss the relationship between this point of view and the K-theoretic classification of D-branes.