Charge Asymmetry of Weak Boson Production at the LHC and the Charm Content of the Proton

TL;DR

This work analyzes W± and Z production at the LHC to extract parton distribution information. By using NNLO QCD predictions and experimentally reconstructed W rapidities, it demonstrates how W charge asymmetry constrains the $u/d$ quark PDFs, and how the cross-section ratio $B(Y)$ (and the charm parameter $\epsilon$ through $B(\epsilon)$) can directly probe the charm content of the proton. The results show substantial charm contributions to W production (up to ~34–36%) and a relatively QCD-order–insensitive observable $B(Y)$, enabling a robust determination of the charm sea at 7 and 14 TeV. The findings underscore the LHC’s potential to map both light-quark and charm PDFs with high precision via weak-boson production.

Abstract

We investigate the production of the weak bosons $W^+$, $W^-$ and $Z$ at the LHC as a function of their rapidity, reconstructed {\it experimentally} from their leptonic decay. We show that the measurements provide a powerful tool for constraining the parton distribution functions, as was already the case for the lower energy $p\bar{p}$ collider data. We study the charge asymmetry determining the $u$ and $d$ distribution functions using the reconstructed $W^+$, $W^-$ rapidities. We also show how the ratio of the $W$ and $Z$ boson rapidity distributions directly probes the charm quark distribution function of the proton.

Figures (9)

• Figure 1: The total cross sections for the production of weak bosons, $W^{+(-)}$ and $Z$ in proton-proton collision up to the full NNLO in QCD. The separate contributions from the valence-sea (dashed) and the sea-sea (dot-dashed) interactions are also shown as function of energy.
• Figure 2: The rapidity distribution of the differential cross sections for $W^{\pm}$ and Z boson production. The left and right figures are for energies of $\sqrt{s}$ = 7 TeV and $\sqrt{s}$ = 14 TeV, respectively. The results are presented for MSTW2008 (solid) MSTW08 and CTEQ6.6 (dashed) CTEQ66 PDFs.
• Figure 3: The weights $\omega_1$ (dark black) and $\omega_2$ (bright red) as a function of rapidity for $\sqrt{s}$ = 7 and 14 TeV.
• Figure 4: The distributions of the reconstructed rapidities $Y_\pm$ and the charged lepton rapidity $Y_l$ as function of $Y$ of $W$ boson.
• Figure 5: The charge asymmetry $A(Y)$ for the different PDFs for energies of $\sqrt{s}$ = 7 and 14 TeV as a function of the reconstructed rapidity $Y_{-}$. The results for the PDFs are differentiated as follows: black (solid) for MSTW2008 MSTW08, and red (dashed) for CTEQ6.6 CTEQ66.