Study of Λ – Hadron Correlations in Pb–Pb Collisions at √sNN = 5.36 TeV Using the ALICE Detector

Ultrarelativistic heavy-ion collisions, which take place at particle accelerators like the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC), are known to produce a strongly-interacting, fluid-like state of matter referred to as the Quark-Gluon Plasma (QGP). Despite the medium’s short lifetime of approximately 10^-23 s, its presence is known to influence particle production in a collision event. This is expected to include the production of particles carrying strangeness and baryon number, two quantum numbers that are conserved in interactions. If the QGP does affect the production of conserved quantities, this will be reflected in the distribution of the final-state particles that carry the corresponding conserved quantum numbers. Furthermore, it will be worthwhile to study the dependence of these final-state distributions on the collision centrality, as this concept is closely related to the size of the QGP medium.

The analysis presented in this thesis aims to gain an understanding of the QGP’s influence on the production mechanisms of strangeness and baryon number. To this end, data from the ALICE detector of Pb–Pb collisions at √s_NN = 5.36 TeV is used to construct Λ – h angular correlation functions, where h is either of π, K or p. Separating the correlation functions into the four possible (electric) charge configurations (Λ – h⁺, Λ – h¯, Λ^† – h⁺, Λ^† – h¯) allows one to produce opposite- and same-sign correlations, where the sign refers to the relative sign between the quantum numbers of the Λ baryon and the chosen hadron. By subtracting the same-sign correlations from the opposite-sign correlations, a distribution known as the balance function is obtained. This distribution is expected to be free of any correlations apart from those arising due to opposite-sign quantum numbers, allowing one to study the evolution of quantum number balancing.

The results show that the opposite-sign Λ – K and Λ – p correlations are greater on the near-side (−π/2 < Δφ ≤ π/2 ) and almost identical on the away-side ( π/2 < Δφ < 3π/2 ) compared to their same-sign counterparts. This leads to a prominent near-side peak in the Λ – K and Λ – p balance functions, indicating that strangeness and baryon number are conserved locally between particles in close proximity to each other. This is an unexpected outcome, as the presence of a thermal, deconfined QGP in Pb–Pb collisions should “wash out” correlations between balancing quantum numbers. Furthermore, comparing the balance functions to predictions from the Lund string model reveals that the widths (σ) of the balance functions are considerably greater in Pb–Pb collisions than in pp collisions. This seems to suggest that the mechanisms governing strangeness and baryon number conservation in Pb–Pb occur over a wider range compared to pp, being neither entirely local nor completely global.

For Λ– π, the opposite- and same-sign correlations show no discernible differences, leading to a flat balance function. Since the Λ – π correlations are free from strangeness and baryon number balancing, this is to be expected.