Generalized TMDs in the exclusive double Drell-Yan process

Generalized TMDs (GTMDs) of hadrons are the most general two-parton correlation functions. Upon certain projections, several GTMDs reduce to generalized parton distributions (GPDs) and transverse momentum dependent parton distributions (TMDs), respectively. Therefore, GTMDs can be considered as partonic "mother functions". Moreover, two of the GTMDs play an important role in the nucleon spin structure. We show that, by means of the exclusive double Drell-Yan process, GTMDs for quarks can in principle be measured. This is the first known process which is sensitive to these objects. We argue that specific GTMDs can be addressed via suitable polarization observables. We also identify other processes that are directly sensitive to GTMDs.