QED Effects in PDFs -- A Les Houches Comparison Study

TL;DR

This study compares the effects of including QED in proton PDF fits, highlighting how it alters the shape and size of PDFs, especially in the NNPDF4.0 set, which is crucial for future high-precision physics analyses.

Contribution

The paper provides a detailed comparison of QCD+QED and QCD-only PDFs, emphasizing subtle effects and their potential importance for future precision measurements.

Findings

QED inclusion significantly affects the shape of PDFs.

NNPDF4.0 shows the largest impact when QED is included.

Subtle effects can influence future high-precision studies.

Abstract

In the last decade, and even more so in the last few years, our knowledge of the internal structure of the proton has become more accurate and precise thanks to the large amount of data available and developments in theory and methodology. The reduction of the uncertainties associated to these developments has brought previously neglected effects into focus as their typical magnitude are competitive with the size of the uncertainties. One such effect is the inclusion of QED into PDF fits. Typically this is a percent effect, and thus while theoretically important, it has had a relatively limited impact on phenomenological studies up to this point. In this proceeding we study some of the effects which, while peripheral to the inclusion of QED in the proton, can considerably change the relative size and shape of the QCD+QED fit with respect to the QCD only determination. These may become important in the future as precision continues to increase. After a comparison of the QCD+QED PDFs with the QCD only PDFs of various global PDF fitting groups, we focus largely upon NNPDF4.0, which shows the biggest effect when including QED. Focusing largely on a single set of PDFs also enables more subtle effects to be analysed, making it an ideal candidate for this study.