2-dimensional stable pairs on 4-folds

I will discuss a 2-dimensional stable pair theory of nonsingular complex 4-folds parallel to Pandharipande-Thomas' 1-dimensional stable pair theory of 3-folds. The stable pairs of a 4-fold are related to its 2-dimensional subschemes via wall-crossings in the space of polynomial stability conditions. In Calabi-Yau case, Oh-Thomas theory is applied to define invariants counting these stable pairs under some restrains. This is a joint work with Yunfeng Jiang and Jason Lo.