Forward production of Drell-Yan dileptons at high energies and low dilepton invariant masses in a $k_t$-factorization approach: Do we see onset of saturation?

TL;DR

This paper investigates forward Drell-Yan dilepton production at high energies using a hybrid k_t-factorization approach, comparing it with dipole models and experimental data, and finds no clear evidence of saturation effects at small invariant masses.

Contribution

It introduces a high-energy hybrid approach for Drell-Yan production, highlighting limitations of dipole models and including multiple structure functions for more accurate predictions.

Findings

Dipole approach results are too approximate for real experiments.

Dipole formula is valid only at very forward/backward rapidities.

No clear signs of saturation effects at small dilepton invariant masses.

Abstract

We discuss Drell-Yan production of dileptons at high energies in forward rapidity region in a hybrid high-energy approach. This approach uses unintegrated gluon distributions in one proton and collinear quark/antiquark distributions in the second proton. Corresponding momentum-space formula for the differential cross sections in high-energy approximation has been derived. The relation to the commonly used dipole approach is discussed. We conclude and illustrate that some results of the dipole approaches are too approximate, as far as kinemtics is considered, and in fact cannot be used for real experiments. We find that the dipole formula is valid only in very forward/backward rapidity regions ($|y| >$ 5). Some differential cross sections for low-mass dilepton production are shown and compared to the LHCb and ATLAS experimental data. In distinction to dipole approaches, we include four Drell-Yan structure functions (the impact of interference structure functions is rather small for typical experimental cuts). We find that both side contributions ($g q/\bar q$ and $q/\bar q g$) have to be included even for the LHCb rapidity coverage which is in contradiction with what is usually done in the dipole approach. We present results for different unintegrated gluon distributions from the literature (some of them include saturation effects). We see no clear hints of saturation even at small $M_{ll}$.