Study of Ξ-Hadron Correlations in pp Collisions at √s = 13 TeV Using the ALICE Detector
Abstract: By colliding heavy nuclei at high energies, which is done at RHIC and the LHC, a strongly interacting Quark Gluon Plasma (QGP) is created. This manifests itself through several different signatures, which until recently was thought to uniquely probe the QGP. Recently, however, similar signatures have been observed also in small systems, such as pp collisions with high charged-particle multiplicity, which is quite puzzling since a QGP is not expected to be formed in such dilute systems with short lifetimes. One such observable is the enhanced relative yields of multistrange baryons, such as the Ξ baryon, which has been observed in e.g. Pb-Pb collisions. More recently, this yield enhancement has been observed to scale smoothly with multiplicity also in pp collisions.The main analysis presented in this thesis aims at understanding the production mechanism of strange quarks in pp collisions at √s = 13 TeV, and in this way reach an explanation of the origin of the strangeness enhancement observed there. This is done by studying angular Ξ-h correlations, where h is either of π, K, p, Λ, or Ξ hadrons, by using data from the ALICE detector. The results are compared with four phenomenological models; three flavours of the QCD inspired PYTHIA8 which is based on colour strings, and the core-corona model EPOS LHC. The PYTHIA tunes are the Monash tune, the Junction Mode 0 tune, which has an additional mechanism for baryon formation, and a yet unofficial tune including rope hadronisation, which is a proposed mechanism for the observed strangeness enhancement. In EPOS, the enhanced strangeness is modelled by an increasing fraction of a core that behaves like a medium.The results show that the Ξ-π correlation function is dominated by a narrow near-side peak. This is not present in any of the other correlations, which on the other hand have a wide extension in rapidity. This means that pions decouple later in the evolution from the Ξ baryon compared to the other species, likely within the jet, which was concluded to be due to charge balance, whereas the other correlations are attributed to strangeness and baryon decoupling. In all PYTHIA flavours, strong correlations within the jet are present for all combinations except Ξ-p correlations, meaning that strangeness and baryon number are produced earlier in the evolution in data than in PYTHIA. The junction model however gave a description of the Ξ-baryon correlation that was closer to data than the Monash tune, indicating that the additional baryon mechanism included there is more likely to be correct. For EPOS, on the other hand, the correlation function is very dilute for most species, which was concluded to be due to local conservation of quantum numbers not being properly accounted for. Therefore, it is not yet possible to use this measurement to test the underlying mechanism provided by this model. Based on this, the observations in data indicate that the strangeness production mechanism is likely either due to a core-corona like state, or some hybrid mechanism where string interactions are also important.Correlations were also measured as a function of multiplicity, yielding very similar results across multiplicity classes. Therefore it was concluded that the strangeness and baryon production mechanisms in pp collisions are likely the same regardless of multiplicity.
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