Thesis defense of Lars Kolk

A measurement of multiplicity-dependent strangeness production in fixed-target collisions is conducted using data from the LHCb experiment at CERN. The measurement is motivated by observations of strangeness enhancement, the increase of strangeness production as a function of multiplicity, in various collision systems reported by the ALICE collaboration. Additionally, in the context of astroparticle physics, the presence of strangeness enhancement is of interest as it could explain the muon excess observed in extensive air showers, known as the Muon Puzzle. The analysis is performed on data collected with the fixed-target configuration of the LHCb experiment called SMOG, employing helium and neon as targets at a nucleon-nucleon centre-of-mass energy of 110 GeV. Differential cross-section ratios of the single strange particles K-short, Lambda, and anti-Lambda to charged pions are measured in intervals of rapidity in the centre-of-mass frame and transverse momentum; for the study of strangeness enhancement, these ratios are additionally provided in intervals of multiplicity. The resulting cross-section ratios are compared with predictions of state-of-the-art hadronic event generators. None of the generators provide a consistent description of the data within experimental uncertainties. No significant hints of multiplicity dependent strangeness production are observed.