Identify the origin of the forward-rapidity LHCb D0 discrepancy

Identify the physical or theoretical source of the discrepancy between nCTEQ26 predictions and the forward-rapidity LHCb Run-II $D^0$-meson production data in proton-lead collisions at 8.16 TeV.

Background

The nCTEQ26 fit describes most heavy-quark production data well, but the forward-rapidity LHCb Run-II D0D^0 dataset is poorly described, whereas the corresponding backward-rapidity data and earlier Run-I measurements are described satisfactorily.

The discrepancy occurs at low momentum fractions, where saturation and other nuclear final-state effects could potentially become relevant. The authors do not determine which effect, if any, accounts for the mismatch.

References

On the other hand, the Run-II data in the forward region (cf. Fig.~\ref{fig:data-vs-theory:HQ3133}) are poorly described, with no clear indication of what might account for this discrepancy.

— nCTEQ26 nuclear parton distribution functions  (2610.13633 - Collaboration et al., 9 Oct 2026) in Section 5.2, subsection “Description of selected datasets”