CERN Accelerating science

Nearly $10^{-6}$ seconds after the Big Bang, our Universe was in an extremely hot and dense state. It started expanding and cooling down, eventually evolving to its current state. During its evolution, when the temperature drops below the chemical freeze-out temperature, nuclear matter undergoes a phase transition from a deconfined state to a confined hadronic phase, where quarks and gluons are bound together into hadrons. In the Standard Model of particle physics, the gauge theory governing the strong interaction between quarks and gluons is known as quantum chromodynamics (QCD). In conditions of extreme energy density and temperature similar to those in the early Universe, QCD predicts the formation of the quark-gluon plasma (QGP) -- a deconfined state of strongly interacting matter, in which quarks and gluons are not confined inside hadrons. These conditions can be reached in the laboratory via relativistic heavy-ion collisions. At the Large Hadron Collider (LHC), A Large Ion Heavy Collider Experiment (ALICE) was the experiment designed to study the QGP. Heavy flavours, i.e. charm and beauty quarks ( $m_{\rm c} \sim 1.28 \ {\rm GeV}/c^{2}$ and $m_{\rm b} \sim 4.5 \ {\rm GeV}/c^{2}$ ), represent ideal probes of the QGP, since they are produced predominantly via hard-scattering processes in the early stages of the nucleus-nucleus collisions and experience the full system evolution. Measurements of the production of charm hadrons in high-energy hadronic collisions provide important tests for calculations based on perturbative quantum chromodynamics (pQCD). Moreover, investigations of the production ratios of different hadron species as a function of the transverse momentum can shed light on the charm-quark hadronisation mechanism. Generally, the QGP is not expected to exist in small hadronic collision systems, however, enhancement of baryon production, as well as other observed features similar to those in Pb--Pb collisions, such as strangeness enhancement, has also been observed in proton-proton (pp) collisions at the LHC. These phenomena challenge our current understanding of QGP formed only in nucleus-nucleus collisions and of charm hadronisation, therefore, more specific measurements are needed to further explore the underlying physics. In particular, the production of heavy-flavour hadrons containing strange quarks is an intriguing topic in heavy-ion collisions. As the production of strange quarks is enhanced in the QGP with respect to smaller hadronic collision systems, the possible coalescence of heavy quarks with other medium constituents may lead to an enhanced abundance of heavy-flavour hadrons with strange content compared to those without strangeness. The focus of studies presented in this thesis is on the precise measurements of charmed strange baryons, specifically $\Xi^{0}_{\rm c} \rightarrow \Xi^{-}\rm e^{+}\nu_{e}$ and $\Omega^{0}_{\rm c} \rightarrow \Omega^{-}\rm e^{+}\nu_{e}$, where the former contains one strange quark and the latter contains two. $\Xi^{0}_{\rm c}$-baryon production is measured with the ALICE detector in pp collisions at a centre-of-mass energy of $\sqrt{s}=$ 5.02 TeV. The $p_{\rm T}$-differential cross section is compared with previously published measurements at $\sqrt{s}=$ 7 and 13 TeV, respectively. The results suggest a hardening of the $p_{\rm T}$-differential spectrum with increasing collision energy. The observed $p_{\rm T}$ dependence of the $\Xi^{0}_{\rm c}/ \rm D^{0}$ yield ratio across the three different collision energies is similar to what was measured for the $\Lambda^{+}_{\rm c}/ \rm D^{0}$ and $\Sigma^{0,++}_{\rm c}/ \rm D^{0}$ yield ratios. Model calculations capture the data well for $\Lambda^{+}_{\rm c}$ and $\Sigma^{0,++}$ baryons, however, they fail to describe the heavier strange-charm baryon $\Xi^{0}_{\rm c}$, indicating that the charm-baryon hadronisation is not fully understood. Therefore, charm-strange baryon measurements have a large constraining power on model predictions. Currently, the interpretation of the production measurements of even heavier strange-charm $\Omega^{0}_{\rm c}$ baryons is limited by the absence of measured absolute branching fraction values. The $p_{\rm T}$-differential cross section of $\Omega^{0}_{\rm c}$ baryons production multiplied by the branching ratio into $\Omega^{-}{\rm e}^{+}\nu_{\rm e}$ is measured in pp collisions at $\sqrt{s}=$ 13 TeV in this thesis and divided by existing measurements in the hadronic decay channel $\Omega^{-}\pi^{+}$. The resulting branching-fraction ratio ${\rm BR}(\Omega^0_{\rm c} \rightarrow \Omega^{-}{\rm e}^{+}\nu_{\rm e})/ {\rm BR}(\Omega^0_{\rm c} \rightarrow \Omega^{-}{\pi}^{+})$ is calculated to be 1.12 $\pm$ 0.22 (stat.) $\pm$ 0.27 (syst.). The present result is consistent with theory calculations and within one standard deviation with a corresponding measurement from the CLEO Collaboration. However, it is $2.3\sigma$ lower than the measurement reported by the BELLE Collaboration. It is also compatible within the uncertainties with the ratio ${\rm BR}(\Xi^0_{\rm c} \rightarrow \Xi^{-}{\rm e}^{+}\nu_{\rm e})/ {\rm BR}(\Xi^0_{\rm c} \rightarrow \Xi^{-}{\pi}^{+})$ measured previously by the ALICE Collaboration. More precise measurements are expected to be carried out in Runs 3 and 4 of the LHC, allowing us to obtain a more complete understanding of charm quark hadronisation mechanisms and dynamics in hadronic collisions.