The color dipole picture of the Drell-Yan process

TL;DR

This work extends the color-dipole framework, previously successful for DIS, to proton-proton Drell-Yan production by treating dilepton emission as bremsstrahlung in the target rest frame and expressing cross sections through a universal dipole cross section. Using the GBW saturation parameterization, the authors reproduce low-x2 DY data without a K-factor and predict a strong energy rise for RHIC energies. They show that the DY transverse momentum distribution is finite at zero momentum due to saturation, with larger energy growth at high transverse momentum, and analyze angular distributions, finding λ near 0.95 and a Lam–Tung relation violation. Overall, the paper demonstrates the predictive power of the color-dipole approach for DY observables and outlines RHIC tests to probe small-x dynamics and color transparency.

Abstract

At high energies, Drell-Yan (DY) dilepton production viewed in the target rest frame should be interpreted as bremsstrahlung and can be expressed in terms of the same color dipole cross section as DIS. We compute DY cross sections on a nucleon target with the realistic parameterization for the dipole cross section saturated at large separations. The results are compared to experimental data and predictions for RHIC are presented. The transverse momentum distribution of the DY process is calculated and energy growth is expected to be steeper at large than at small transverse momenta. We also calculate the DY angular distribution and investigate deviations from the 1+cos^2(θ) shape.

Figures (4)

• Figure 1: In the target rest frame, DY dilepton production looks like bremsstrahlung. A quark or an antiquark inside the projectile hadron scatters off the target color field and radiates a massive photon, which subsequently decays into the lepton pair. The photon can also be radiated before the quark hits the target.
• Figure 2: The points represent the measured DY cross section in $p\,^2H$ scattering from dydata. Only statistical errors are shown. Note that for these points $0.03\le x_2\le 0.09$. These values of $x_2$ are already quite large for the dipole approach. The curves are calculated with the dipole cross section (\ref{['wuestsigma']}) without any further fitting procedure. The solid curves are calculated at the same kinematics as the data point (center of mass energy $\sqrt{s}=38.8$ GeV). The dashed curves are calculated for RHIC energies, $\sqrt{s}=500$ GeV. For each energy, the lower curve is for quark mass $m_f=200$ MeV, the upper curve for $m_f=0$.
• Figure 3: The transverse momentum distribution for DY pairs calculated from (\ref{['dylcdiff']}) at $x_F=0.625$ and $M=6.5$ GeV. The curves which flatten at small $q_\perp$ are calculated with the realistic dipole cross section (\ref{['wuestsigma']}), while the other two curves are calculated with the small $\rho$ approximation (\ref{['sr']}).
• Figure 4: The left figure shows the dependence of the parameter $\lambda$, which describes the angular distribution of DY pairs (\ref{['angular']}) on the dilepton mass. The calculation is performed for $x_F=0.625$. The figure on the right displays the $q_\perp$ dependence of $\lambda$ at $x_F=0.625$ and $M=6.5$ GeV.