Update of MRST Parton Distributions

TL;DR

MRST updates address uncertainties in proton parton distributions arising from input assumptions and theoretical corrections across NLO and NNLO fits, including isospin violation, strangeness asymmetry, QED effects, NNLO coefficient functions, and low-$Q^2$/small-$x$ dynamics. The study combines data-driven fits with exploratory theoretical adjustments (e.g., kinematic cuts to define conservative partons and small-$x$ resummations) to assess stability and identify regions of sensitivity. Key findings include a best-fit isospin-violation parameter $\kappa \approx -0.2$ that reduces the NuTeV anomaly toward $1.5\sigma$, a small but non-negligible QED-induced isospin effect with $\Delta\alpha_S(M_Z^2) \approx +0.0003$, and that full NNLO corrections modify PDFs only modestly but influence predictions for gluon-dominated observables; the work also emphasizes the persistent uncertainties at small $x$ and low $Q^2$, and the need for targeted data (e.g., low-energy HERA) and refined theory. The results highlight the interplay between experimental errors and modeling choices in high-precision hadronic predictions and motivate further higher-order and resummed treatments.

Abstract

I present a brief summary of the most important updates in the MRST determinations of the NLO and NNLO parton distributions. I concentrate on uncertainties due to input assumptions and possible theoretical corrections, particularly isospin violation, QED corrections, NNLO and corrections at low Q^2 and small x.

Figures (3)

• Figure 1: Variation of global $\chi^2$ with the value of $\kappa$
• Figure 2: Diagrams for QED corrections to DIS
• Figure 3: Predictions for $F_L(x,Q^2)$ at LO, NLO, NNLO and with a resummed calculation