Determination of the parton distribution functions of the proton from ATLAS measurements of differential $W^\pm$ and $Z$ boson production in association with jets

TL;DR

The paper delivers ATLASepWZVjet20, a NNLO QCD proton PDF set incorporating W±/Z production with jets at 8 TeV (plus 7 TeV inclusive data) and HERA DIS data to tighten high-x sea-quark constraints and refine the strange-quark density. Using a sophisticated xFitter-based fit with GM-VFNS heavy-quark treatment and cross-dataset correlations, the analysis shows that including vector-boson plus jet data substantially improves the W+jets description and constrains x(d̄) at high x, while Rs remains close to unity at low x but decreases at higher x. The results reduce some tensions with global PDFs (notably CT18A) yet persistent differences remain in the mid-to-high x region, highlighting ongoing challenges in global PDF fits. Overall, ATLASepWZVjet20 enhances predictions for SM processes at the LHC by providing tighter high-x sea-quark and strange-quark information, informing both collider phenomenology and future PDF determinations.

Abstract

This article presents a new set of proton parton distribution functions, ATLASepWZVjet20, produced in an analysis at next-to-next-to-leading order in QCD. The new data sets considered are the measurements of $W^+$ and $W^-$ boson and $Z$ boson production in association with jets in $pp$ collisions at $\sqrt{s} = 8~\mathrm{TeV}$ performed by the ATLAS experiment at the LHC with integrated luminosities of $20.2~\mathrm{fb}^{-1}$ and $19.9~\mathrm{fb}^{-1}$, respectively. The analysis also considers the ATLAS measurements of differential $W^{\pm}$ and $Z$ boson production at $\sqrt{s} = 7~\mathrm{TeV}$ with an integrated luminosity of $4.6~\mathrm{fb}^{-1}$ and deep-inelastic-scattering data from $e^{\pm}p$ collisions at the HERA accelerator. An improved determination of the sea-quark densities at high Bjorken $x$ is shown, while confirming a strange-quark density similar in size to the up- and down-sea-quark densities in the range $x \lesssim 0.02$ found by previous ATLAS analyses.

Figures (11)

• Figure 1: The differential cross-section measurements of (a) $W^+$ + jets and (b) $W^-$ + jets in Ref. STDM-2016-14 (black points) as a function of the transverse momentum of the $W$ boson, $p_{\text{T}}^W$. The bin-to-bin uncorrelated part of the data uncertainties is shown as black error bars, while the total uncertainties are shown as a yellow band. The cross sections are compared with the predictions computed with the predetermined PDFs resulting from the fits ATLASepWZ20 (red lines) and ATLASepWZVjet20 (blue lines). The solid lines show the predictions without shifts of the systematic uncertainties, while for the dashed lines the $b_j$ parameters associated with the experimental systematic uncertainties as shown in \ref{['eqn:chi2-1']} are allowed to vary to minimise the $\chi^2$. The ratios of each set of predictions to the data are shown in the bottom panel in each case.
• Figure 2: The differential cross-section measurements of $Z$ + jets as a function of the absolute rapidity of inclusive jets, $\lvert y^{\text{jet}}\rvert$, in bins of (a) $25 < p_{T}^{\mathrm{jet}} < 50~\text{Ge V}$, (b) $50 < p_{T}^{\mathrm{jet}} < 100~\text{Ge V}$, (c) $100 < p_{T}^{\mathrm{jet}} < 200~\text{Ge V}$ and (d) $200 < p_{T}^{\mathrm{jet}} < 300~\text{Ge V}$, where the transverse momentum of inclusive jets is labelled $p_{T}^{\mathrm{jet}}$. The bin-to-bin uncorrelated part of the data uncertainties is shown as black error bars, while the total uncertainties are shown as a yellow band. The cross sections are compared to the predictions computed with the PDFs resulting from the fits ATLASepWZ20 (red lines) and ATLASepWZVjet20 (blue lines). The solid lines show the predictions without shifts of the systematic uncertainties, while for the dashed lines the shifts with fitted $b_j$ parameters as shown in \ref{['eqn:chi2-1']} are applied. The ratios of each set of predictions to the data are shown in the bottom panel in each case.
• Figure 3: The differential cross-section measurements of $Z$ + jets as a function of the absolute rapidity of inclusive jets, $\lvert y^{\text{jet}}\rvert$, in bins of (a) $300 < p_{T}^{\mathrm{jet}} < 400~\text{Ge V}$ and (b) $400 < p_{T}^{\mathrm{jet}} < 1050~\text{Ge V}$, where the transverse momentum of inclusive jets is labelled $p_{T}^{\mathrm{jet}}$. The bin-to-bin uncorrelated part of the data uncertainties is shown as black error bars, while the total uncertainties are shown as a yellow band. The cross sections are compared to the predictions computed with the PDFs resulting from the fits ATLASepWZ20 (red lines) and ATLASepWZVjet20 (blue lines). The solid lines show the predictions without shifts of the systematic uncertainties, while for the dashed lines the shifts with fitted $b_j$ parameters as shown in \ref{['eqn:chi2-1']} are applied. The ratios of each set of predictions to the data are shown in the bottom panel in each case.
• Figure 4: PDFs multiplied by Bjorken $x$ at the scale $Q^2 = 1.9~\text{Ge V}^2$ as a function of Bjorken $x$ obtained for the (a)-(b) valence quarks and (c)-(d) up and down sea quarks when fitting $W$ + jets, $Z$ + jets, inclusive $W$ and $Z$, and HERA data (ATLASepWZVjet20, blue bands), compared with a similar fit without $W$ + jets or $Z$ + jets data (ATLASepWZ20, green bands). Inner error bands indicate the experimental uncertainty, while outer error bands indicate the total uncertainty, including parameterisation and model uncertainties. The relative uncertainties around the nominal value of each PDF centred on 1 is displayed in the bottom panel in each case.
• Figure 5: PDFs multiplied by Bjorken $x$ at the scale $Q^2 = 1.9~\text{Ge V}^2$ as a function of Bjorken $x$ obtained for the (a) strange sea quark, (b) gluon, (c) the total of the down-type quarks and (d) the total of the anti-down-type quarks when fitting $W$ + jets, $Z$ + jets, inclusive $W$ and $Z$, and HERA data (ATLASepWZVjet20, blue bands), compared with a similar fit without $W$ + jets or $Z$ + jets data (ATLASepWZ20, green bands). Inner error bands indicate the experimental uncertainty, while outer error bands indicate the total uncertainty, including parameterisation and model uncertainties. The relative uncertainties around the nominal value of each PDF centred on 1 is displayed in the bottom panel in each case.