Lorentz invariance relations and Wandzura-Wilczek approximation

Understand

A complete list of the so-called Lorentz invariance relations between parton distribution functions is given and some of their consequences are discussed, such as the Burkhardt-Cottingham sum rule.

• The violation of these relations is considered in a model independent way.
• It is shown that several Lorentz invariance relations are not violated in a generalized Wandzura-Wilczek approximation, indicating that numerically their violation may be small.