Gauge invariance, causality and gluonic poles

Understand

We explore the electromagnetic gauge invariance of the hadron tensor of the Drell-Yan process with one transversely polarized hadron.

• The special role is played by the contour gauge for gluon fields.
• The prescription for the gluonic pole in the twist 3 correlator is related to causality property and compared with the prescriptions for exclusive hard processes.
• As a result of we get the extra contributions, which naively do not have an imaginary phase.