Charm total cross sections with nonuniversal fragmentation treatment

TL;DR

This paper introduces a data-driven method to accurately extrapolate total charm-pair cross sections from differential data, incorporating nonuniversal fragmentation effects, and aligns well with NNLO QCD predictions.

Contribution

It develops the ddFONLL approach, a novel empirical parametrization fitting data directly to account for nonuniversal charm fragmentation effects.

Findings

Total charm cross sections agree with NNLO QCD predictions.

The method reduces bias in extrapolation of differential to total cross sections.

First sensitivity studies to charm-quark mass and gluon PDFs at low x.

Abstract

Total charm-pair cross sections in $pp$ collisions are interesting because they can be calculated to NNLO in QCD without any reference to fragmentation effects. On the other hand, the fiducial differential charm cross sections from which the total cross sections must be extrapolated are currently known to NLO+NLL at most (e.g. FONLL), and must be treated for known effects of nonuniversal charm fragmentation. A new procedure using the FONLL framework as input for an empirical parametrization of the data in both shape and normalization, with all its parameters actually fitted to data, is used to derive so-called data-driven FONLL (ddFONLL) parametrizations which can be used to extrapolate the differential cross sections to total cross sections with minimal bias. This includes an empirical treatment of all known non-universal charm fragmentation effects, in particular for the baryon-to-meson ratio as a function of transverse momentum. The total charm-pair cross sections obtained in this way, which supersede all previous ones obtained using the assumption of charm-fragmentation universality, are consistent with NNLO predictions, and allow first studies of their sensitivity e.g. to the charm-quark mass and/or the NNLO gluon PDF at very low transverse momentum fractions $x$.