D bar D asymmetry at low and high energies and possible consequences for prompt atmospheric neutrinos

We discuss the role of unfavoured light quark/antiquark to $D$ meson fragmentation. The unknown parameters of fragmentation process are adjusted to describe the asymmetry for $D^+$ and $D^-$ production measured by the LHCb. Predictions for similar asymmetry for neutral $D$ mesons are presented. The predicted asymmetry at large rapidity (or $x_F$) are very large which is related to the valence-quark contribution. As a result, prompt atmospheric neutrino flux at high neutrino energies can be much larger than for the conventional $c \to D$ fragmentation. We predict large rapidity-dependent $D^+/D^-$ and $D^0/{\bar D}^0$ asymmetries for low ($\sqrt{s} =$ 20 - 100 GeV) energies. The $q/\bar q \to D$ fragmentation leads to enhanced production of $D$ mesons at low energies. Predictions for fixed target $p+^{4}\!\textrm{He}$ collisions relevant for a fixed target LHCb experiment are discussed.