Dilepton angular distributions in the color-dipole $S$-matrix framework

The dilepton production at forward rapidities in hadronic collisions at the LHC energies is considered and the helicity density matrix elements of gauge bosons that enter the Drell–Yan angular coefficients associated with the gauge boson decay ($G={\gamma }^{\ast },Z^0,W$) are derived using the color-dipole $S$ - matrix framework. We show results for $pp$ collisions at $\sqrt{s}=14$ TeV for the nonvanishing helicity density matrix elements in the rapidity range of $2.0\le y\le 4.0$ of the gauge boson. We consider different models for the proton unintegrated gluon distributions, among others a solution of the Balitsky–Kovchegov equation. We compare the associated predictions with those obtained disregarding the non-linear term in the evolution equation. In addition, the Lam–Tung relation is discussed.