CERN Accelerating science

Quantum chromodynamics (QCD) predicts the existence of an extreme state of nuclear matter in which quarks and gluons are deconfined and thermalised: this is the so-called Quark Gluon Plasma (QGP). The QGP has been studied experimentally at colliders such as the LHC at CERN in Geneva, during the LHC Run-1 (2009-2013) and Run-2 (2015-2018) data taking periods. The 5$^{\rm th}$ of July 2022, the LHC has restarted for a third data taking campaign (LHC Run-3), as well as the experiment in which this thesis is carried out, ALICE. This thesis proposes to analyse - possibly, one last time - the data recorded during the LHC Run-2 before moving on to the ones from the LHC Run-3, in order to fully exploit them and push them to their precision limits. To that end, two analyses have been performed. The main analysis consists in a test of the CPT (Charge-Parity-Time) symmetry via the mass difference measurement of multi-strange baryons ($\Xi^{-}$ [$dss$] and $\overline{\Xi}^{+}$ [$\bar{d}\bar{s}\bar{s}$], and $\Omega^{-}$ [$sss$] and $\overline{\Omega}^{+}$ [$\bar{s}\bar{s}\bar{s}$]) in proton-proton collisions at $\sqrt{s}$ = 13 TeV. The current mass and mass difference values given by the Particle Data Group (PDG) for these two baryons relying on measurements with relatively low statistics, it becomes now possible to improve them in order to test the CPT symmetry to an unprecedented level of precision, thanks to the abundant production and detection of these baryons by ALICE at the LHC. The total uncertainty on the mass values has been reduced by a factor 1.19 for the $\Xi^{-}$ and $\overline{\Xi}^{+}$, and 9.26 for the $\Omega^{-}$ et $\overline{\Omega}^{+}$. Concerning the mass differences, their precision has been improved by 20% for the $\Xi$, and by more than a factor two for the $\Omega$. The second analysis aims to provide a better understanding of the production mechanisms of strange quarks in proton-proton collisions at $\sqrt{s}$ = 13 TeV. This is achieved by studying the correlations between identified particles. In practice, this analysis focuses specifically on correlations between a multi-strange baryon - $\Xi^{\pm}$ or $\Omega^{\pm}$ - and a $\phi(1020)$ [$s\bar{s}$] resonance. The first results show no correlation with the rapidity separation while the production of $\phi(1020)$ increases in the vicinity (in azimuth) of a $\Xi^{\pm}$ in both minimum-bias and high-multiplicity proton-proton collisions. A similar trend can be observed for $\Omega^{\pm}-\phi(1020)$ correlation in high-multiplicity events. The comparison to QCD-inspired Monte Carlo predictions shows that PYTHIA 8 overestimates $\Xi^{\pm}-\phi(1020)$ correlation with the azimuth in minimum-bias proton-proton collisions, while EPOS 4 underestimates it. This suggests that the correlated production of strange hadrons is likely an interplay between soft and hard hadronisation mechanisms.