Photoproduction of ρ0 and Two–photon Production of Lepton Pairs in Ultra–peripheral Pb–Pb Collisions at the CERN LHC

This thesis is based on the analysis of ultra–peripheral collisions collected by the ALICE Collaboration at the CERN LHC. ALICE is a general purpose detector designed to study heavy-ion collisions at ultra–relativistic energies with the purpose of investigating the properties of strongly interacting matter, similar to the matter that existed shortly after the Big Bang. In this analysis the charged particle tracking system of the central barrel in ALICE is used. The tracking system consists of an Inner Tracking System, with six layers of silicon detectors, and a large Time Projection Chamber. Trigger information is provided by the following detectors: The Silicon-Pixel Detector (SPD), a part of the Inner Tracking System; the Time–Of–Flight (TOF), located outside the TPC; the V0 detectors, plastic scintillators located outside of the central barrel, covering roughly two units of pseudorapidity on either side of mid-rapidity. Ultra–peripheral collisions are collisions between hadrons, they can be protons or nuclei, where they geometrically miss each other. This implies that the impact parameter is larger than two times the radii of the colliding hadrons. Because of the short range of the strong force, the interactions will be mediated by the electromagnetic field. The electromagnetic field of a moving charged particle can be treated as a flux of virtual photons. Ultra-peripheral collisions can be divided into two categories: two-photon and photonuclear interactions. In a photonuclear interaction a photon from the field of one of the nuclei interacts with the other (target) nucleus. In two–photon interactions one photon from each nucleus interacts and create for example a lepton pair. In this thesis the focus is the analysis of coherent photoproduction of the vector meson ρ0 and two–photon production of e+e− pairs. First an introduction to heavy–ion physics and the physics of ultra–peripheral collisions is given and then ALICE detector is described. In the following the analysis procedure is discussed: How the events are reconstructed; the event selection, track cuts and particle identification; how the selected events are corrected for acceptance and efficiency; and how the luminosity of the data set is estimated. In the end the physics results of this thesis are presented. The method for signal extraction, error estimation, and cross section calculation for photoproduction of ρ0 is described, and the resulting differential cross section is presented. For the process γγ →e+e− the event characteristics, event selection and cross section calculation is discussed. The differential cross section for the mass interval 0.45 ≤ Minv ≤ 2.5 and with pseudorapidity |η| < 1.5, and the differential cross section as a function of invariant mass is presented.