The persistent nonperturbative charm enigma

TL;DR

The paper discusses the ongoing challenge of identifying and quantifying nonperturbative charm in the proton, highlighting current theoretical and experimental limitations, and presenting an updated analysis with future directions.

Contribution

It provides an updated NNLO analysis of fitted charm and discusses the theoretical and experimental challenges in resolving the nonperturbative charm enigma.

Findings

Current data and theory are insufficient to definitively resolve the nonperturbative charm contribution.

The updated CT18 FC analysis offers new insights into fitted charm.

Future data and theory developments are necessary for progress.

Abstract

The question of the existence and possible magnitude of nonperturbative (often called "intrinsic") charm in the proton has long confounded attempts to cleanly isolate such a contribution in global analyses of high-energy experiments. In this letter, we show that the available (non)perturbative QCD theory and hadronic data have still not developed to a sufficient level to clearly resolve this problem. We highlight a number of challenging aspects that must be confronted in extracting nonperturbative charm in PDF fits, and in so doing, present an updated next-to-next-to-leading order CT analysis of fitted charm, CT18 FC, which we also compare to recent studies. We outline the theory developments and future data needed to make progress on this subject.