Abstract
We discuss the impact of the new LHCb fixed-target $p\!+\!^{4}\!He$ data for $D^0/{\bar D}^0$ production on the intrinsic $c \bar c$ component in the nucleon wave function. Within the scenario presented here neither the traditional gluon-gluon fusion or quark-antiquark annihilation mechanisms calculated in the $k_T$-factorization approach nor their counterparts from the collinear next-to-leading order collinear framework are sufficient to describe the transverse momentum and rapidity distributions of $D^0/{\bar D}^0$ mesons. First the $c{\bar c}$-pair production within the standard frameworks is considered. Here a crucial role of the $c \to D$ hadronization effects at low energies and low transverse momenta is found and discussed, which was not analyzed in previous studies. A contribution related to intrinsic $c \bar c$ component in the nucleon wave function is included in addition. Two models of the symmetric ($c(x) = \bar c(x)$) intrinsic charm (IC) component are considered. The intrinsic charm $g^* c \to g c$ (or $g^* {\bar c} \to g {\bar c}$) contribution needs to be regularized in order to obtain a suppression of the minijet $p_{T}$ spectrum present in the phenomenological minijet model, commonly used in Monte Carlo generators. We show that in our model the regularization parameter can be obtained from the fit to the LHCb fixed-target data under consideration here. We discuss uncertainties of our calculations (scale, charm quark mass, fragmentation function) as well as set limits on the IC probability. According to our model the intrinsic charm probability $P_{IC} = 1.65\%$ allows to significantly improve description of the LHCb data but the number is rather uncertain.
Highlights
The proton is known to be a complicated object built of QCD degrees of freedom: quarks, antiquarks and gluons or even their conglomerates
Let us start presentation of our numerical results with the differential cross pp-scattering at pseffisffic1⁄4tio8n6s.6foGr ecVha.rmIn quark Fig. 4 production in we show the transverse momentum and rapidity distributions of charm quark for the standard gg →
Numerical results for the mechanism driven by the intrinsic charm are obtained within the hybrid approach with off-shell gluon and with collinear intrinsic charm distribution taken from the CT14nnloIC PDFs
Summary
The proton is known to be a complicated object built of QCD degrees of freedom: quarks, antiquarks and gluons or even their conglomerates. An interesting d − uasymmetry confirmed by the Gottfried sum rule violation [1] and fixed target DrellYan experiment [2] is one example This is usually explained in terms of the pion (meson) cloud in the nucleon The pion cloud probability is of the order of 30% It contributes to nonperturbative light quark/ antiquark distributions in the nucleon. [7] the cðxÞ and cðxÞ instrinsic charm/anticharm distributions are calculated postulating heavy charmed meson–heavy charmed baryon component in the nucleon wave function. In this approach typically cðxÞ is somewhat different than cðxÞ (asymmetric IC) but the most probable range of x is similar as for the BHPS model. Combining conclusions from both of them may be very useful
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