CERN Accelerating science

The ALICE experiment at the LHC at CERN is devoted to the study of collisions of heavy nuclei accelerated at ultra-relativistic energies. These collisions are employed to reproduce in laboratory the quark-gluon plasma (QGP), a deconfined state of the strongly-interacting matter. The existence of this state is predicted by the QCD theory under extreme conditions of energy-density and temperature. The QGP is supposed to constitute the early universe in the first $\sim 10$ $\mu$s after the Big Bang. Charm and beauty quarks are unique probes for investigating the QGP. Given a mass of the GeV order, they are produced in the hard-scattering processes in the early stages of the nucleus-nucleus collision, experiencing the full evolution of the system. Charm and beauty quarks lose energy by interacting with the plasma constituents. These phenomena can be studied by measuring the heavy-flavour hadron production and exploited to infer properties of the system. In this thesis, the production of electrons from charm and beauty hadron decays in central (0-10%), semicentral (30-50%) and peripheral (60-80%) Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV measured with the ALICE experiment is presented. These results are compared with those in pp collisions by means of the nuclear modification factor ($R_{\rm AA}$) and a significant suppression with respect to what expected in absence of a deconfined medium is observed in central and semicentral events. This behaviour indicates that charm and beauty quarks are subject to in-medium energy loss. The observed effect decreases in more peripheral events. The measurement is pushed down to $p_{\rm T}^{\rm e^-}=500$ MeV/$c$, where the heavy quark production is sensitive to the shadowing effects. The $R_{\rm AA}$ does not overcome unity, signalling that the production of heavy-flavour hadrons is suppressed. The QGP formation in heavy-ion collisions is expected to induce a modification of the heavy quark hadronisation mechanisms. In order to disentangle the effects of the medium produced in Pb-Pb collisions, a deep comprehension of the mechanisms that govern the hadronisation in pp collisions is required. Recent results on baryon-to-meson production ratios in pp collisions at the LHC showed an enhancement with respect to $\rm e^+e^-$ and $\rm e^-p$ collisions. In this thesis, the measurement of the $\Lambda_{\rm c}^{+}$ and $\Sigma_{\rm c}^{0.++}$ baryon production cross section in pp collisions at $\sqrt{s}=13$ TeV with the ALICE experiment is described. The ratio to the $\rm D^{0}$ meson production of both baryons is significantly higher than what measured in $\rm e^+e^-$ and $\rm e^-p$ collisions. The measurements are described by several model calculations assuming different mechanisms for the charm quark hadronisation. Moreover, the first measurement of the prompt $\Lambda_{\rm c}^{+}$ feed-down from $\Sigma_{\rm c}^{0,+,++}$ decays in pp collisions is presented and observed to be $\sim 2$ times larger than $\rm e^+e^-$ collisions in the interval $2 < p_{\rm T} < 12$ GeV/$c$. The investigation of the charm quark hadronisation mechanisms in hadronic collisions can significantly benefit from production measurements of more charm baryons. In the last Chapter, a few studies on the capabilities to perform such measurements in the future with the ALICE experiment are discussed. New frontiers in the heavy-flavour hadron production measurements will be allowed by the significantly higher statistics that the experiment will collect in the upcoming years, as well as the improved pointing resolution provided by the upgraded ITS detector.