Measurements of the production cross sections of prompt ${ \rm D^0}$, ${ \rm D^+}$, ${ \rm D^{ \ast +}}$, ${ \rm D_s^+}$, ${ \rm \Lambda_{ c}^{ +}}$, and ${ \rm \Xi_{ c}^{ +}}$ charm hadrons at midrapidity in proton$-$proton collisions at $\sqrt{ s}=13$ TeV with the ALICE detector are presented. The D-meson cross sections as a function of transverse momentum ($p_{ \rm T}$) are provided with improved precision and granularity. The ratios of $p_{ \rm T}$-differential meson production cross sections based on this publication and on measurements at different rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-$x$ ($10^{ -5}-10^{ -4}$). The measurements of ${ \rm \Lambda_{ c}^{ +}}$ (${ \rm \Xi_{ c}^{ +}}$) baryon production extend the measured $p_{ \rm T}$ intervals down to $p_{ \rm T}=0(3)$~GeV$/c$. These measurements are used to determine the charm-quark fragmentation fractions and the ${ \rm c\overline{ c}}$ production cross section at midrapidity ($|y|<0.5$) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons ${ \rm D^0}$, ${ \rm D^+}$, ${ \rm D_s^+}$, ${ \rm \Lambda_{ c}^{ +}}$, ${ \rm \Xi_{ c}^{ 0}}$ and, for the first time, ${ \rm \Xi_{ c}^{ +}}$, and of the strongly-decaying J/$psi$ mesons. The first measurements of ${ \rm \Xi_{ c}^{ +}}$ and ${ \rm \Sigma_{ c}^{ 0,++}}$ fragmentation fractions at midrapidity are also reported. A significantly larger fraction of charm quarks hadronising to baryons is found compared to e$^+$e$^-$ and ep collisions. The ${ \rm c\overline{ c}}$ production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations.
Categories: hep-ex nucl-ex