CERN Accelerating science

The quark-gluon plasma (QGP) is an extremely hot and dense form of matter in which quarks and gluons exist in a deconfined state. It was present at the very beginning of the universe just after the Big Bang, before the universe cooled down enough for hadrons to exist. Ultra-relativistic heavy-ion collisions such as the lead-lead collisions at the Large Hadron Collider (LHC) at CERN (Geneva, Switzerland) can be used to recreate this form of matter in the laboratory, creating a very short lived small droplet of QGP just after the collision. These collisions are therefore a way to study not only the properties of the QGP, but also the hadronisation mechanisms at play when the QGP droplet cools down and undergoes a phase transition from free quarks and gluons to hadronic matter. Heavy quarks, such as the charm or bottom quark, are created at the very beginning of the collision, and interact with the QGP as it expands and cools down, eventually undergoing hadronisation. This means they experience the entire evolution of the system, making heavy-flavour hadrons such as the $\Lambda$ $^{+}_{c}$ baryon (udc) an excellent probe into the QGP. The relative production of different hadrons, in particular the relative production of baryons and mesons, is an especially useful probe of hadronisation mechanisms. As heavy-flavour hadrons have a very short lifetime, they decay before reaching the detector, and their production has to be reconstructed via their decay products. In this thesis, the production of pK$^{0}_{s}$ measured as a function of transverse momentum $_{pT}$ in Pb–Pb collisions at a centre-of-mass energy per nucleon-nucleon collision of $\sqrt{sNN}$ = 5.02 TeV via the $\Lambda$ $^{+}_{c}$ → pK−π+ decay channel is presented. The $\Lambda$ $^{+}_{c}$ production yield is measured in two collisional centrality classes, central and semi-central, in two and three pT bins respectively. A previous measurement of charmed D meson production by the ALICE collaboration is used to calculate the $\Lambda$ $^{+}_{c}$ /D$^{0}$ production ratio, and a measurement of $\Lambda$ $^{+}_{c}$ production in proton-proton (pp) collisions is used to determine the nuclear modification factor R$_{AA}$. The experimental results of this thesis work are merged with other ALICE measurements performed using a previous analysis done via the $\Lambda$ $^{+}_{c}$ → pK$^{0}_{s}$ decay channel, and compared to theoretical models which implement charm quark transport and hadronisation in different ways. It is found that the $\Lambda$ $^{+}_{c}$ /D$^{0}$ ratio is enhanced in central Pb–Pb collisions with respect to pp collisions at intermediate pT, while it approaches that of pp collisions in the high-$_{pT}$ region. This indicates that hadronisation differs in Pb–Pb collisions compared to pp collisions. This measurement significantly improves precision at high pT when added to previous results from the $\Lambda$ $^{+}_{c}$ → pK$^{0}_{s}$ channel, and this work gives the most precise measurement of $\Lambda$ $^{+}_{c}$ production in Pb–Pb collisions to date.