Charm-quark fragmentation fractions and production cross section at midrapidity in pp collisions at the LHC

Recent $p_{\rm T}$-integrated cross section measurements of the ground-state charm mesons and baryons, D$^{\rm 0}$, D$^+$, D$_{\rm s}^{+}$, $\Lambda_{\rm c}^{+}$, and $\Xi_{\rm c}^0$, are used to evaluate the charm fragmentation fractions and production cross section per unit of rapidity at midrapidity ($|y|<~0.5$), in pp collisions at $\sqrt{s} = 5.02$ TeV at the LHC. The latter is ${\rm d} \sigma^{\rm c \overline{c}}/{\rm d} y|_{|y| <~ 0.5}$ =1165 $\pm 44(\rm{stat})^{+134}_{-101}(\rm{syst})$ $\mu b$. These measurements were obtained for the first time in hadronic collisions at the LHC including the charm baryon states, recently measured by ALICE at midrapidity. The charm fragmentation fractions differ significantly from the values measured in e$^+$e$^-$ and ep collisions, providing evidence of the dependence of the parton-to-hadron fragmentation fractions on the collision system, indicating that the assumption of their universality is not supported by the measured cross sections. An increase of a factor of about 3.3 for the fragmentation fraction for the $\Lambda_{\rm c}^{+}$ with a significance of $5\,\sigma$ between the values obtained in pp collisions and those obtained in e$^+$e$^-$ (ep) collisions is reported. The fragmentation fraction for the $\Xi_{\rm c}^0$ was obtained for the first time in any collision system. The measured fragmentation fractions were used to update the $\rm c \overline{c}$ cross sections per unit of rapidity at $|y|<~0.5$ at $\sqrt{s} = 2.76$ and 7 TeV, which are about 40% higher than the previously published results. The data were compared with perturbative-QCD calculations and lie at the upper edge of the theoretical bands.

 

Submitted to: PRL
e-Print: arXiv:2105.06335 | PDF | inSPIRE
CERN-EP-2021-088

Figure 1

Transverse-momentum integrated production cross sections of the various charm meson~ and baryon~ species per unit of rapidity at midrapidity normalised to that of the \Dzero{} meson measured in pp collisions at $\s = 5.02~\TeV$. The measurements are compared with PYTHIA~8 calculations~ (left panel) and with results from a SHM~ (right panel) (see text for details). For \Jpsi the inclusive cross section was used. The $\Jpsi/\Dzero$ ratio, as well as the model calculations for the $\Omegac/\Dzero$ ratio, are multiplied by a factor 30 for visibility.

Figure 2

Left: Charm-quark fragmentation fractions into charm hadrons measured in pp collisions at $\s = 5.02~\TeV$ in comparison with experimental measurements performed in \ee{} collisions at LEP and at B factories, and in ep collisions at HERA~. The \Dstar{} meson is depicted separately since its contribution is also included in the ground-state charm mesons. Right: Charm production cross section at midrapidity per unit of rapidity as a function of the collision energy. STAR~ and PHENIX~ results, slightly displaced in horizontal direction for better visibility, are reported. Comparisons with FONLL~ (red band) and NNLO~ (violet band) pQCD calculations are also shown.