Parton densities from LHC vector boson production at small and large transverse momenta

TL;DR

The paper addresses the challenge of poorly constrained gluon densities at large x by proposing the use of high-p_T electroweak vector-boson production at the LHC, where quark-gluon scattering dominates and reliable predictions can be achieved with NLO plus soft-gluon resummation. It analyzes the partonic channel composition, validates the approach against 7 TeV data using a Collins-Soper-Sterman framework with a BLNY non-perturbative function, and demonstrates that high-p_T measurements at 7–14 TeV can meaningfully constrain the gluon PDF at large x. The study finds that the qg channel dominates well into the multi-TeV p_T range, making these observables sensitive probes of the gluon density, while the low-p_T regime remains largely governed by non-perturbative effects but is already well described by Tevatron-fitted parameters. The authors advocate incorporating these measurements into global PDF fits to reduce large-x uncertainties, with NNPDF already planning to include such data.

Abstract

The parton densities of the proton are of fundamental importance not only for our description of hadronic and nuclear structure, but also for reliable predictions for new heavy particle searches at colliders. At the large partonic momentum fractions required for the production of these particles, the parton distribution functions, in particular the one of the gluon, are unfortunately still badly constrained. In this paper, we investigate the possibility to improve on their determination with new data coming from electroweak vector boson production at large transverse momenta at the LHC with center-of-mass energies of 7, 8 or 14 TeV. We demonstrate that this process is dominated by quark-gluon scattering, that theoretical predictions can be reliably made on the basis of next-to-leading order perturbation theory and its resummation, and that these data should thus be used in global fits. We also point out that the non-perturbative parameters determined from Tevatron run-1 Z-boson data at low p_T describe very well the new LHC data at \sqrt{s}=7 TeV.

Figures (9)

• Figure 1: Parton distribution functions (PDFs) on a logarithmic $x$-scale, emphasizing the low-$x$ region, at the factorization scale $Q = M_Z$ (top) and 1 TeV (bottom) from different collaborations. PDF uncertainties are only shown for the CT10 group. PDFs are printed for gluons, up and strange quarks.
• Figure 2: Same as in Fig. \ref{['fig:1']}, but on a linear $x$-scale, emphasizing the high-$x$ region.
• Figure 3: Transverse-momentum spectra of $Z$ (top) and $W$ (bottom) bosons at the LHC with $\sqrt{s}=7$ TeV, normalized to the total cross section. CMS (top) and ATLAS (bottom) data are compared with our theoretical calculation at NLL+NLO in the rapidity range $|y|<2.1$ using CT10 PDFs. The data points are positioned at the theoretical center of gravity of the bins (red) and at the center of the bins (black).
• Figure 4: Relative contributions at NLO of the quark-antiquark (dashed) and QCD "Compton" (full) subprocesses to the production of $Z$ (top) and $W$ bosons (bottom) at the LHC with $\sqrt{s}=7$ TeV. Subdominant partonic subprocesses which enter only at NLO or higher order are not shown.
• Figure 5: Same as Fig. \ref{['fig:4']} for $\sqrt{s}=14$ TeV.