Production of strange hadrons in charged jets in Pb–Pb collisions measured with ALICE at the LHC

Abstract

First measurements of the baryon-to-meson ratio in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) showed an relatively enhanced baryon production at intermediate transverse momentum (pt around 3 GeV/c) relative to pp collisions. Measurements at much higher energy at the Large Hadron Collider (LHC) at CERN corroborated this observation. Although the mechanisms of this enhanced baryon production is not yet known, there are several scenarios proposed to explain this. These are e.g. collective effects and string fragmentation in a hydro-dynamically expanding environment. These collective phenomena, like the particle flow, are a characteristic feature of the Underlying Event (UE) in Pb-Pb collisions. Partons that are produced by hard parton scatterings in an early stage of the heavy-ion collision and which can be measured as a collimated spray of particles, a particle jet, are assumed to hadronise via fragmentation. The production of the particles from the Underlying Event (UE) is in principle indepen-dent of the fragmentation process but could have a possible impact on the jet fragments. In this way it is possible that interactions between partons that stem from the fragmentation and those inside the hot and dense medium created in heavy-ion collisions, could change the jet pattern. So far several measurements observed these medium-modifications in terms of jet quenching. Furthermore a modification of the fragmentation functions in Pb-Pb collisions was lately seen by the CMS and the ATLAS experiment. There are models that consider alternative hadronisation mechanisms to explain the baryon anomaly. They expect hadrons at low pt to be produced via recombination, which is a soft process that is expected to favour baryon over meson production. Particles with a momentum larger than pt = 4-6 GeV/c would, on the other hand, be produced in hard processes via fragmentation, that does not lead to an enhanced production of baryons (compared to production in vacuum). The study of identified particle yields and ratios like lambda(antilambda) and K0s in jets in Pb-Pb collisions will help to disentangle energy loss and hadronisation scenarios and their pt dependence. For the analysis presented in this thesis, the pt spectra of strange particles associated with jets are studied for the 10% most central events in Pb-Pb collisions at sqrt(snn) = 2.76 TeV. Furthermore they are compared to the inclusive particle measurement and the (lambda+antilambda)/2K0s ratio in jets in p-Pb collisions at sqrt(snn) = 5.02 TeV. For the particles within jets, that are produced via hard parton fragmentation, the (lambda+antilambda)/2K0s ratio is found to be much less enhanced than the corresponding ratio for inclusive particles. Within uncertainties, it is furthermore con-sistent with a reference measurement in jets in p-Pb collisions. The measured (lambda+antilambda)/2 and K0s pt spectra however give strong hints about possible modifications. The comparison to the scaled V0 pt spectra in jets in Pb-Pb collisions indicates medium-modifications of the spectra in Pb-Pb collisions. The K0s yields seem to be enhanced in Pb-Pb for all hadron pt intervals w.r.t. the reference. The lambda yields at low hadron pt indicate an enhancement while for higher hadron pt they hint on a suppression of the yields. This could point to a modification of the fragmentation pattern similar as observed by CMS and ATLAS. Furthermore the observation raises the question whether there could be a particle-species dependent strength of the medium-modification of the fragmentation functions and probably also as a function of the jet pt.