Twist-three distributions and their appearance in the doubly-polarized Drell-Yan process

The twist-three distributions g_2(x) and h_2(x) are defined as quark-field matrix elements between polarized hadron states. They can be written in terms of quark-mass and gluonic operators, after which the Burkhardt-Cottingham sum rule for g_2 can be derived and a similar sum rule for h_2. Their role in the Drell-Yan double-spin asymmetry A_{LT} is explained