Abstract
We scrutinize the recent LHCb data for D0-meson production in p+Pb collisions within a next-to-leading order QCD framework. Our calculations are performed in the SACOT-mT variant of the general-mass variable-flavour-number scheme (GM-VFNS), which has previously been shown to provide a realistic description of the LHC p+p data. Using the EPPS16 and nCTEQ15 nuclear parton distribution functions (PDFs) we show that a very good agreement is obtained also in the p+Pb case both for cross sections and nuclear modification ratios in the wide rapidity range covered by the LHCb data. Encouraged by the good correspondence, we quantify the impact of these data on the nuclear PDFs by the Hessian reweighting technique. We find compelling direct evidence of gluon shadowing at small momentum fractions x, with no signs of parton dynamics beyond the collinear factorization. We also compare our theoretical framework to a fixed-order calculation supplemented with a parton shower. While the two frameworks differ in the absolute cross sections, these differences largely cancel in the nuclear modification ratios. Thus, the constraints for nuclear PDFs appear solid.
Highlights
Large masses of the W± and Z0 bosons, the interaction scale is high and a significant sensitivity to gluons via evolution effects will eventually set constraints on gluons, as has been shown in ref. [27]
Using the EPPS16 and nCTEQ15 nuclear parton distribution functions (PDFs) we show that a very good agreement is obtained in the p+Pb case both for cross sections and nuclear modification ratios in the wide rapidity range covered by the LHCb data
As the nPDF sets we consider in this work, EPPS16 [22] and nCTEQ15 [21], are of a variable-flavour type, where the charm and bottom quarks are “active” partons above their mass thresholds, our default setup for the heavy-meson cross section calculations is based on the general-mass variable-flavour-number scheme (GM-VFNS) approach
Summary
The general idea of D-meson hadroproduction in the GM-VFNS approach [59, 65] is to reproduce the results of (3-flavour) fixed flavour-number scheme (FFNS) at the small pT limit and match to the massless calculation at high values of pT. As Powheg generates only events where the heavy-quark pair is produced in the Born-level process or in the first (hardest) splitting, it ignores the component where the QQ would be created only later on in the shower e.g. starting from a hard gg → gg process Such contributions are, effectively included in any GM-VFNS framework via the scale-dependent PDFs and FFs. Since charm quarks are abundantly produced in parton showers at the LHC energies [81], truncating the resummation of the splittings to the first one may miss a significant source of heavy quarks, as was pointed out in ref. Within its large scale uncertainties the Powheg+Pythia method agrees with the D-meson data measured by LHCb even at PT mcharm, though the central predictions are generally below the data [44]
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