Open Access

Simone Schuchmann

• Measurements of the transverse momentum (pt) spectra of K0 s and Λ(Λ̄) in Pb–Pb and pp collisions at √sNN = 2.76TeV with the ALICE detector at the LHC at CERN up to pt = 20GeV/c and pt = 16GeV/c, respectively, are presented in this thesis. In addition, the particle rapidity densities at mid-rapidity and nuclear modification factors of K0 s and Λ(Λ̄) are shown and discussed. The analysis was performed using the Pb–Pb data set from 2010 and the pp data set from 2011. For the identification of K0 s and Λ(Λ̄), the on-the-fly V0 finder was employed on tracking information from the TPC and ITS detectors. The Λ and Λ̄ spectra were feed-down corrected using the measured published Ξ− spectra as input. Regarding the rapidity density at mid-rapidity, a suppression of the strange particle production in pp as compared to Pb–Pb collisions is observed at all centralities, whereas the production per pion rapidity density stays constant as a function of dNch/dη including both systems. Furthermore, the relative increase of the individual particle species in pp and AA collisions is compatible for non- and single-strange particles when going from RHIC (√sNN = 0.2TeV) to LHC energies. On the other hand, in case of multi-strange baryons, a stronger increase in the particle production in pp is seen. The Λ̄ and Λ production in Pb–Pb and pp collisions was found to be equal. Concerning the nuclear modification factors, at lower pt (pt <5GeV/c), an enhancement of the RAA of Λ with respect to that of K0 s and charged hadrons is observed. This baryon-to-meson enhancement appearing in central Pb–Pb collisions at RHIC and LHC is currently explained by the interplay of the radial flow and recombination as the dominant particle production mechanism in this pt sector. The effect of radial flow is thus also seen in the low and intermediate pt region of RAA, where a mass hierarchy is discovered among the baryons and mesons, respectively, with the heaviest particle being least suppressed. When comparing the results from RHIC and LHC, the RCP is found to be similar at low-to-intermediate pt, while a significantly smaller RAA of K0 s and Λ in central and peripheral events at the LHC is observed in this pt region as compared to the RHIC results. This can be attributed to the larger radial flow in AA collisions and to the harder spectra at the LHC. At high pt (pt > 8GeV/c), a strong suppression in central Pb–Pb collisions with respect to pp collisions is found for K0 s and Λ(Λ̄). A significant high-pt suppression of these hadrons is also observed in the ratio of central-to-peripheral collisions. The nuclear modification of K0 s and Λ(Λ̄) is compatible with the modification of charged hadrons at high pt. The calculations with the transport model BAMPS agree with these results suggesting a similar energy loss for all light quarks, i.e. u, d and s. Moreover, a compatible suppression for c-quarks appears in the ALICE measurements via the D meson RAA as well as in the BAMPS calculations, which hints to a flavour-independent suppression if light- and c-quarks are regarded. Within this consideration, no indication for a medium-modified fragmentation is found yet. To summarize, for the particle production in Pb–Pb collisions at the LHC relative to pp neither at lower pt (rapidity density) nor at higher pt (nuclear modification factor) a significant difference of K0 s and Λ(Λ̄) carrying strangeness to hadrons made of u- and d-quarks was found.
• 10−6 Sekunden nach dem Urknall. Elementare Materie unter hohem Druck und hoher Temperatur. Ihr Zustand, ein Plasma aus stark wechselwirkenden Elementarteilchen - ein Quark-Gluon-Plasma (QGP), das am Large Hadron Collider (LHC) des Kern- und Teilchenforschungszentrum CERN1 in Genf erforscht wird. Allerdings werden die Erkenntnisse dort nicht durch Himmelsbeobachtungen gewonnen. Vielmehr wird dieser spezielle Materiezustand künstlich zu erzeugen versucht: Der LHC beschleunigt Blei-Kerne (Pb-Kerne) auf (fast) Lichtgeschwindigkeit und lässt sie kollidieren, wodurch sehr viel Energie auf sehr kleinem Raum konzentriert wird. Somit werden ähnliche energetische Bedingungen wie wenige Mikrosekunden nach dem Urknall (Big-Bang) hergestellt, in sogenannten "Little-Bangs". Da das Kollisionssystem innerhalb von 10−23 s in viele Teilchen zerfällt, ist jedoch eine direkte Verfolgung der verschiedenen Systemstadien, die Ausbildung des Plasmas und seine Expansion mit anschließendem Erkalten und Bildung von neuen Teilchen (in Analogie zur Expansion des Universums nach dem Urknall), experimentell nicht möglich. Die Detektoren, die das Kollisionszentrum umgeben und beobachten, nehmen lediglich die Signale auf, die von den Produkten der Kollision generiert werden. Die Untersuchung der Little-Bangs ist daher ein komplexer Prozess bestehend aus der Auswertung der Detektor-Signale, ausgelöst durch die erzeugten Teilchen, der darauf aufbauenden Rekonstruktion der Teilchenspuren und der abschließenden Analyse der Teilcheneigenschaften. Die vorliegende Arbeit dokumentiert die Auswertung von Messungen der Teilchen K0 s und Λ(Λ̄) in Pb–Pb und Proton-Proton Kollisionen bei einer Schwerpunktenergie von √sNN = 2.76TeV, die von der ALICE Kollaboration am CERN aufgezeichnet wurden.