Towards logarithmic GLSM: the r-spin case

We establish the logarithmic foundation for compactifying the moduli stacks of the gauged linear sigma model using stable log maps. We then illustrate our method via the key example of Witten's r-spin class to construct a proper moduli stack with a reduced perfect obstruction theory whose virtual cycle recovers the r-spin virtual cycle of Chang, Li and Li. Indeed, our construction of the reduced virtual cycle is built upon their work by appropriately extending and modifying the Kiem-Li cosection along certain logarithmic boundary. In a follow-up article, we push the technique to a general situation. One motivation of our construction is to fit the gauged linear sigma model in the broader setting of Gromov-Witten theory so that powerful tools such as virtual localization can be applied. A project along this line is currently in progress, leading to applications including computing loci of holomorphic differentials, and calculating Gromov-Witten invariants of threefolds.