A QCD analysis of LHCb D-meson data in p+Pb collisions

TL;DR

The work tests QCD collinear factorization for D0 meson production in p+Pb collisions using LHCb data within a NLO GM-VFNS framework (SACOT-mT) and compares to Powheg+Pythia. It employs Hessian reweighting to quantify the data’s impact on nuclear PDFs (EPPS16 and nCTEQ15), finding compelling evidence for gluon shadowing at small x and a substantial reduction of gluon-nPDF uncertainties. The analysis shows excellent agreement with RpPb and differential cross sections for PT ≳ 3 GeV, supporting the validity of the approach and the universality of nPDFs in this kinematic region. The results remain robust across different theoretical implementations, highlighting D-meson data as a valuable input for future global nPDF fits and the exploration of small-x nuclear dynamics.

Abstract

We scrutinize the recent LHCb data for D$^0$-meson production in p+Pb collisions within a next-to-leading order QCD framework. Our calculations are performed in the SACOT-$m_{\rm T}$ 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.

Figures (14)

• Figure 1: Double-differential cross section for $\mathrm{D^{0}}$ production in p+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.0~\text{TeV}$ in five different $Y$ bins at backward rapidities. Data from LHCb Aaij:2017gcy are compared to the GM-VFNS calculations with EPPS16 (solid black) and nCTEQ15 (dashed purple) nPDFs, and to a Powheg+Pythia setup with EPPS16 nPDFs (dot-dashed green). The theoretical uncertainties related to the PDFs are shown with dark grey and the combination of the scale variations and PDF uncertainties with light blue.
• Figure 2: Same as figure \ref{['fig:dsigma_pPb_backward']} but at forward rapidities.
• Figure 3: Nuclear modification ratio for $\mathrm{D^{0}}$ production in p+Pb collisions in different backward-rapidity bins from the LHCb measurement Aaij:2017gcy (black points with error bars) and the SACOT-$m_{\mathrm{T}}$ calculation with the EPPS16 (left) and nCTEQ15 (right) nPDFs. In addition to the central result (solid) and the nPDF-originating uncertainties (coloured bands), the scale variations (dotted band) and the result with massless definition of the fragmentation variable (dashed) are shown, as well as the Powheg+Pythia predictions (dot-dashed).
• Figure 4: Same as figure \ref{['fig:R_pPb_backward']} but at forward rapidities.
• Figure 5: The histograms of the $R_{\rm pPb}^{\rm D^0}$-data residuals obtained before (left) and after (right) the reweighting with EPPS16 (top) and nCTEQ15 (bottom). Ideal Gaussian distributions (dashed) are compared to fitted ones (solid) in the reweighted case.