Drell-Yan phenomenology in the color dipole picture revisited

TL;DR

The paper investigates Drell-Yan production in proton–proton collisions within the color dipole formalism, extending the framework to include $Z^0$ contributions and analyzing observables across a wide kinematic range. It derives inclusive DY cross sections and dilepton–hadron azimuthal correlations in momentum space, employing three dipole cross-section models (GBW, BGBK, IP-SAT) with varying degrees of DGLAP evolution and impact-parameter dependence. The study finds that DGLAP-evolved models (IP-SAT, BGBK) better reproduce high invariant-mass data, while GBW underestimates at large $M$, and it predicts a distinctive double-peak structure in the DY–pion azimuthal correlation near $\Delta\phi=\pi$ at forward rapidities, signaling saturation effects. These results provide a pathway to constrain dipole-model parameters and unintegrated gluon distributions with future RHIC/LHC measurements, enhancing understanding of saturation in high-energy QCD.

Abstract

An extensive phenomenological study of the Drell-Yan (DY) process in $pp$ collisions at various energies is performed in the color dipole framework. Besides previously studied $γ^*$ production we have also included the $Z^0$ contribution relevant at large dilepton invariant masses. We investigate the DY cross section differential in invariant mass, rapidity and transverse momentum of the dilepton pair in $pp$ collisions at RHIC and LHC. We consider three different phenomenological models for the dipole cross section and found a reasonable agreement with the available data. As a further test of the color dipole formalism, we also study the correlation function in azimuthal angle between the dilepton pair and a forward pion $Δφ$ for different energies, dilepton rapidites and invariant masses. The characteristic double-peak structure of the correlation function around $Δφ\simeq π$ found for very forward pions and low-mass dilepton pairs is sensitive to the saturation effects and can be tested by future DY measurements in $pp$ collisions.

Figures (12)

• Figure 1: Diagrams (a) and (b) represent the process of a gauge boson radiation by a quark (antiquark) of flavour $f$ either after or before the interaction with the target color field (denoted by a shaded circle), respectively. For the considered $\gamma,\,Z^0$ radiation $q_k = q_f$. Diagram (c) represents the gauge boson-pion production in the color dipole picture.
• Figure 2: The DY pair invariant mass distribution of the process $pp\rightarrow \gamma^*/Z^0 \rightarrow l \bar{l}$ at $\sqrt{s}=7$ TeV in low (left panel) and high (right panel) invariant mass ranges compared to the data from the ATLAS Collaboration ATLASlowmassATLAShighmass for three different parametrisations of the dipole cross section.
• Figure 3: The DY pair invariant mass distribution of the process $pp\rightarrow \gamma^*/Z^0 \rightarrow l \bar{l}$ at $\sqrt{s} =$ 7 TeV compared to the data from the CMS collaboration cmsDYmass for three different parametrisations of the dipole cross section in the left panel. The corresponding predictions are shown for $\sqrt{s} =$ 14 TeV in the right panel.
• Figure 4: The DY pair invariant mass distribution of the process $pp\rightarrow \gamma^*/Z^0 \rightarrow l \bar{l}$ at RHIC Run I ($\sqrt{s} = 200$ GeV) and II ($\sqrt{s} = 500$ GeV) energies for three different parametrisations of the dipole cross section.
• Figure 5: The $Z^0$ boson rapidity distribution for different center of mass energies: $\sqrt{s} =$ 1.96 TeV (top left panel), 7 TeV (top right panel) and 14 TeV (bottom panel) versus data from the D0 d0-07 and CMS Zcms Collaborations.