A reassessment of the role of high $x$ data on the MSHT global PDF fit

TL;DR

The paper reassesses high-$x$ physics in the MSHT global PDF fit by implementing explicit target-mass corrections and freely parameterized higher-twist terms at approximate N$^3$LO, updating fixed-target data, and incorporating SeaQuest Drell–Yan and high-$x$ ZEUS DIS data. The analysis finds that HT corrections are more subtle and that aN$^3$LO reduces their impact, while TMCs become crucial for looser $W^2$ cuts and have limited effect for the baseline setup; the SeaQuest data constrain the light-quark sea at high $x$ but introduce tensions with NuSea, and ZEUS high-$x$ data yield only mild PDF changes. The combined updates lead to modest shifts in PDFs and a small downward tendency in the preferred $\alpha_S(M_Z^2)$, all within the MSHT dynamic tolerance, implying increased stability of high-$x$ PDFs as perturbative order rises. The work informs data selection and theoretical treatment for precision LHC phenomenology and sets the stage for future EIC data to further sharpen high-$x$ determinations.

Abstract

We present updates within the MSHT global PDF fit that focus on the high $x$ region, and on improving our understanding of the interplay of various theoretical contributions and experimental constraints here. We revisit the question of target mass and higher twist corrections, considering their impact for the first time at approximate N$^3$LO order in a global PDF analysis. Their inclusion is found to be moderate but not negligible on both the PDFs and preferred value of the strong coupling. Increased stability in these at aN$^3$LO is observed in comparison to lower orders. We also study the impact of an updated treatment of various fixed-target DIS data, the inclusion of Seaquest fixed-target Drell Yan data, and new ZEUS data that extends coverage into the high $x$ region. The Seaquest data have the largest effect of these, in particular on the light quark separation at high $x$, while the impact of the other updates is rather mild.

Figures (21)

• Figure 1: A selection of PDFs at $Q^2=10^4$${\rm GeV}^2$ that result from aN${}^3$LO MSHT fits with the old and new treatment of the fixed target datasets.
• Figure 2: Impact of ZEUS high data ZEUS:2020ddd on MSHT20 baseline PDFs, via Hessian reweighting procedure. Results with the full dataset described in the reference, and with only the $x>0.35$ data included, in order to remove double counting with the HERA combined data, are shown. The gluon and $u_V$ are shown as they have the largest impact on the PDF central values.
• Figure 3: As in Fig. \ref{['fig:ZeusPDF']} but for ratio of the PDF uncertainty to the MSHT20 baseline.
• Figure 4: As in Fig. \ref{['fig:ZeusPDF']} but with the tolerance factors incorrectly set to 1 in the profiling.
• Figure 5: As in Fig. \ref{['fig:ZEUSunc']} but with the tolerance factors incorrectly set to 1 in the profiling.