Nuclear modification of vector boson production in proton-lead collisions at the LHC

TL;DR

The paper addresses how proton–lead collisions at the LHC can illuminate cold nuclear matter effects on parton distributions and parton scatterings. It applies Collins-Soper-Sterman resummation to predict the $Z^0$ $p_T$ distribution and RpA under EPS09 and nDS nuclear PDFs, and analyzes transverse momentum broadening for vector bosons, highlighting distinct patterns between heavy quarkonia and $W/Z^0$ that reflect color dynamics during quarkonium formation. The findings show that gluon anti-shadowing in EPS09 can produce high-$p_T$ enhancements in $R_{pA}$, while nDS yields smaller effects, and that heavy quarkonia experience larger broadening due to final-state interactions, offering a direct probe of color neutralization mechanisms. Collectively, these predictions provide clean experimental tests to constrain nuclear PDFs and to illuminate the color dynamics of quarkonium production in cold nuclear matter.

Abstract

In anticipating the upcoming proton-lead run at the LHC in the near future, we present predictions for the nuclear modification factor of transverse momentum spectrum of $Z^0$ production and transverse momentum broadening of vector boson ($J/ψ$, $Υ$, $W/Z^0$) production in proton-lead collisions at $\sqrt{s}=5$ TeV, respectively. We find that the measurement of nuclear modification factor of $Z^0$ production provides a clean and unambiguous test of the nuclear anti-shadowing proposed in the recent EPS09. In addition, the dramatic difference in transverse momentum broadening between the heavy quarkonium and $W/Z^0$ production could be a signature prediction of QCD parton multiple scattering, and it provides valuable informations on color neutralization of a produced heavy quark pair when it transmutes into a physical quarkonium.

Figures (5)

• Figure 1: Theoretical calculations as in Eq. \ref{['css-gen']} for $Z^0$ production in p+p (or p+$\bar{\rm p}$) collisions are compared with experimental data from CDF Affolder:1999jh and CMS Chatrchyan:2011wt at $\sqrt{s}=1.8$ TeV and $\sqrt{s}=7$ TeV, respectively.
• Figure 2: $Z^0$ boson production in both p+p and p+Pb collision at center of mass energy $\sqrt{s}=5$ TeV and rapidity $y=0$. The left panel is for EPS09 Eskola:2009uj, while the right panel is for nDS deFlorian:2003qf. For both panels, the upper plots are the $Z^0$ cross sections per nucleon as a function of transverse momentum $p_T$. The black dashed curve is the p+p baseline, and the red solid curve is for the minimum bias p+Pb collision. The blue dotted curve is exactly the same as the red solid curves except that $g_2$ is not enhanced by $A^{1/3}$. That is, the nuclear size enhanced dynamical power corrections from multiple scattering is absent in blue dotted curve. Lower panel is for the nuclear modification factor, $R_{pA}$.
• Figure 3: The fractional contributions to $Z^0$ production in p+p collisions at $\sqrt{s}=5$ TeV and rapidity $y=0$ from partonic subprocesses with at least one initial-state gluon (the red solid curve) and those not initiated by gluons (the blue dashed curve).
• Figure 4: Ratio of nuclear PDFs over the proton PDFs at scale $Q=M_Z$: $R_{i}^A = f_{i/A}(x, Q^2)/f_{i/p}(x, Q^2)$. The red solid curve is for gluon, and the blue dashed curve is for the valence $u$-quark. Left panel is for EPS09 Eskola:2009uj and right panel is for nDS deFlorian:2003qf.
• Figure 5: Transverse momentum broadening of vector boson production in p+Pb collision is plotted as a function of number of collisions $N_{\rm coll}$, we choose LHC kinematics $\sqrt{s}=5$ TeV and mid-rapidity $y=0$. Red solid is quarkonium $\Upsilon$, red dashed is $J/\psi$, black solid is $W$, and black dashed is $Z^0$ boson.