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

The  Drell-Yan process is studied in the framework of TMD factorization  in the Sudakov region $s\gg Q^2\gg q_\perp^2$  
corresponding to recent LHC experiments with $Q^2$ of order of mass of Z-boson and transverse momentum 
of DY pair $\sim$ few tens GeV. The  Drell-Yan hadronic tensors are calculated 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 corresponding qualitative and semi-quantitative predictions for angular coefficients of Z-boson
  production seem to agree with LHC data at corresponding kinematics.