Gauge-invariant TMD factorization for Drell-Yan hadronic tensor at small $x$

The  Drell-Yan hadronic tensor  is calculated in the Sudakov region $s\gg Q^2\gg q_\perp^2$ 

with ${1\over Q^2}$ accuracy, first at the tree level and then with the double-log  accuracy.

It is demonstrated that in the leading order in $N_c$ the higher-twist quark-quark-gluon TMDs reduce to 

leading-twist TMDs due to QCD equation of motion. The resulting tensor for unpolarized hadrons is 

 EM gauge-invariant and depends on two leading-twist TMDs: $f_1$ responsible for total DY cross section,

 and Boer-Mulders function $h_1^\perp$.  The order-of-magnitude estimates of angular distributions for 

 DY process seem to agree with LHC results at corresponding kinematics