Abstract: We evaluate the light-cone operator product expansion for the unpolarized transverse momentum dependent (TMD) operator in the background-field technique up twist-3 inclusively. The next-to-leading order (NLO) matching coefficient for the Sivers function is derived. The method, as well as many details of the calculation, are presented.

The abstract is short, but the work is not! In fact, it is the first calculation of such kind in physics of TMDs, and in this article, we look at many aspects of TMD distributions from a different perspective. Due to it I specially made the article as simple and pedagogical as I could. You even can find it boring in some places, but you can also give it to your students (well... master students). Here is arXiv-reference. I have also added working notes on it to supplementary materials (entry 8).