Cos(4 phi) azimuthal anisotropy in small-x DIS dijet production beyond   the leading power TMD limit
  We determine the first correction to the quadrupole operator in high-energy QCD beyond the TMD limit of Weizsaecker-Williams and linearly polarized gluon distributions. These functions give rise to isotropic resp. ~ cos 2 phi angular distributions in DIS dijet production. On the other hand, the correction produces a ~ cos 4 phi angular dependence which is suppressed by one additional power of the dijet transverse momentum scale (squared) P^2. 
Adrian Dumitru and Vladimir Skokov
hep-ph