Twist decomposition of Drell-Yan structure functions: phenomenological implications

The forward Drell--Yan process in $pp$ scattering at the LHC at $\sqrt{S}=14$ TeV is considered. We analyze the Drell--Yan structure functions assuming the dominance of a Compton-like emission of a virtual photon from a fast quark scattering off the small $x$ gluons. The color dipole framework is applied to perform quantitatively the twist decomposition of all the Drell--Yan structure functions. Two models of the color dipole scattering are applied: the Golec-Biernat--W\"{u}sthoff model and the dipole cross section obtained from the Balitsky--Fadin--Kuraev--Lipatov evolution equation. The two models have essentially different higher twist content and the gluon transverse momentum distribution and lead to different significant effects beyond the collinear leading twist description. It is found that the gluon transverse momentum effects are significant in the Drell--Yan structure functions for all Drell--Yan pair masses $M$, and the higher twist effects become important for $M \lesssim 10$ GeV. It is found that the structure function $W_{TT}$ related to the $A_2$ angular coefficient and the Lam--Tung observable $A_0 -A_2$ are particularly sensitive to the gluon $k_T$ effects and to the higher twist effects. A procedure is suggested how to disentangle the higher twist effects from the gluon transverse momentum effects.