Impact of new measurements of light quarks at hadron colliders

TL;DR

The paper investigates deviations between new hadron-collider Drell--Yan measurements and current PDFs in the region $x\sim0.1$, focusing on the light-quark flavor balance via the valence ratio $d_v/u_v$. It deploys correlation analyses using the cosine of the Pearson angle $C_H(E,f)$ and an ePump-based weighted PDF updating to assess compatibility and impact on the PDFs, particularly the valence sectors. The results show a coherent trend across three collider datasets toward a larger $d_v/u_v$ around $x\sim0.1$, with $u_v$ suppressed and $d_v$ enhanced, while fixed-target data like NMC and NuSea remain in tension with these adjustments. The work argues for incorporating these new measurements into a full PDF global analysis to refine the proton’s light-quark structure and improve global fits for collider phenomenology.

Abstract

Recently a series of new measurements with both the neutral and charge current Drell--Yan processes have been performed at hadron colliders, showing deviations from the predictions of the current parton distribution functions (PDFs). In this article, the impact of these new measurements is studied by using their results to update the PDFs. Although these new measurements correspond to different boson propagators and colliding energies, they are found to have a similar impact to the light quark parton distributions with the momentum fraction $x$ around 0.1. It manifests that the deviations are consistent with each other and favor a larger valence $d_v/u_v$ ratio than the modern PDF predictions. Further study indicates that such tension arises dominantly from the deep inelastic scattering measurements of NMC and the fixed target experiments of NuSea, both of which play pivotal roles in detecting the relative $u$ and $d$ type quark contributions for modern PDFs. According to the conclusions of the impact study, it would be essential to include these new measurements into the complete PDF global analysis in the future.

Figures (4)

• Figure 1: Comparison between experimental measurements and theoretical predictions obtained using the modern PDF sets CT18NNLO CT18NNLO, MSHT20 MSHT20, and NNPDF4.0 NNPDF4, for observables sensitive to the flavor composition of the proton in hadron collider environments. The left panel shows the D0 measurement of the inverse $1/R$ in the Drell--Yan process at $\sqrt{s} = 1.96$ TeV, while the middle panel presents the CMS measurement of $R$ at $\sqrt{s} = 8$ TeV in a similar process. The right panel shows the ATLAS measurement of the $W$ boson charge asymmetry $A^W_\text{ch}$ at $\sqrt{s}$ = 13 TeV. According to Equation \ref{['equ:apr2']}, $R$ is defined as a function of $x_1$, with $x_1 \sim 0.1$ being a valid approximation only in the rapidity ranges $1<|Y|<1.5$ for the D0 data and $1.25<|Y|<2.4$ for the CMS data. The complete set of rapidity bins in the left (middle) panel covers a wider range of $0.05 < x_1 < 0.4$ ($0.005 < x_1 < 0.1$) values.
• Figure 2: $C_H(E, f_\text{parton})$ (parton = $u_v$, $d_v$, $d_v/u_v$, $u$, $d$, $d/u$, $\bar{u}$, $\bar{d}$ and $\bar{d}/\bar{u}$, respectively) as a function of $x$, showing the correlation between each dataset and the parton PDF $f_\text{parton}$ from CT18NNLO.
• Figure 3: Impact on the global fit by increasing the weight (from 0 to 15) of the three new input datasets ($R_\text{D0~}$, $R_\text{CMS}$, $A^W_\text{ch ATLAS}$). The effect is quantified by the changes in $\mathcal{X}^2$ for the five selected datasets. Each curve shows $\Delta\mathcal{X}^2 = \mathcal{X}^2(E)_\text{original PDF} - \mathcal{X}^2(E)_\text{updated PDF}$ as a function of the updating weight applied to the corresponding dataset. An increasing slope reflects aligned PDF preferences, whereas a decreasing slope indicates opposite tendencies and potential tension.
• Figure 4: Comparison between updated PDFs, which are updated by the three new datasets with weight 15, and the original CT18NNLO central predictions. Each plot shows the ratio of the updated PDF to the original one, with associated uncertainty bands.