Abstract
We consider unfavoured light quark/antiquark to $D$ meson fragmentation. We discuss nonperturbative effects for small transverse momenta. The asymmetry for $D^+$ and $D^-$ production measured by the LHCb collaboration provides natural constraints on the parton (quark/antiquark) fragmentation functions. We find that already a fraction of $q/{\bar q} \to D$ fragmentation probability is sufficient to account for the measured asymmetry. We make predictions for similar asymmetry for neutral $D$ mesons. Large $D$-meson production asymmetries are found for large $x_F$ which is related to dominance of light quark/antiquark $q/\bar q \to D$ fragmentation over the standard $c \to D$ fragmentation. As a consequence, prompt atmospheric neutrino flux at high neutrino energies can be much larger than for the conventional $c \to D$ fragmentation. The latter can constitute a sizeable background for the cosmic neutrinos claimed to be observed recently by the IceCube Observatory. Large rapidity-dependent $D^+/D^-$ and $D^0/{\bar D}^0$ asymmetries are predicted 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. At $\sqrt{s}$ = 20 GeV the enhancement factor with respect to the conventional contribution is larger than a factor of five. In the considered picture the large-$x_F$ $D$ mesons are produced dominantly via fragmentation of light quarks/antiquarks. Predictions for fixed target $p+^{4}\!\textrm{He}$ collisions relevant for a fixed target LHCb experiment are presented.
Highlights
It is believed that the D mesons are produced dominantly via c → D fragmentation
Asymmetries for Dþ and D− production were obtained at lower energies for π−nucleus collisions [1] and Σ−-nucleus collisions [2] and recently at the LHC for proton-proton collisions [3]
We consider a simple alternative phenomenological explanation using so-called unfavored fragmentation functions responsible for light quark/antiquark fragmentation to D mesons. Such unfavored fragmentation functions are known to be important, e.g., for Kþ=K− production and corresponding asymmetries obtained at SPS [18] and RHIC [19]
Summary
It is believed that the D mesons are produced dominantly via c → D fragmentation. asymmetries for Dþ and D− production were obtained at lower energies for π−nucleus collisions [1] and Σ−-nucleus collisions [2] and recently at the LHC for proton-proton collisions [3]. Rather small asymmetries of the order of 1% were found by the LHCb collaboration [3]. The predicted effect was, rather small (< 1%), at least for the LHCb (pseudo)rapidity coverage 2 < η < 4. The simplest model includes only nonperturbative gluon splitting and gives equal amount of charm and anticharm in the initial states and cannot explain observed asymmetries in the production of D and Dmesons to be discussed in the present paper. The intrinsic charm was discussed recently in the context of atmospheric neutrino production The string model approach to the problem of heavy meson production and asymmetries in the production of heavy mesons was discussed in extent in Ref. The LHCb asymmetry was discussed in the framework of heavy-quark recombination approach [16]
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