Electron-Positron Production in Ultra-Peripheral Heavy-Ion Collisions with the STAR Experiment

Abstract

This thesis presents a measurement of the cross-section of the purely electromagnetic production of $e^+e^-$ pairs accompanied by mutual nuclear Coulomb excitation $AuAu\to Au^*Au^*+e^+e^-$, in ultra-peripheral gold-gold collisions at RHIC at the center-of-mass collision energy of $\sqrt{s_{NN}} = 200$ GeV per nucleon. These reactions were selected by detecting neutron emission by the excited gold ions in the Zero Degree Calorimeters. The charged tracks in the $e^+e^-$ events were reconstructed with the STAR Time Projection Chamber. The detector acceptance limits the kinematical range of the observed $e^+e^-$ pairs; therefore the measured cross-section is extrapolated to $4\pi$ with the use of Monte Carlo simulations. We have developed a Monte Carlo simulation for ultra-peripheral $e^+e^-$ production at RHIC based on the Equivalent Photon Approximation, the lowest-order QED $e^+e^-$ production cross-section by two real photons and the assumption that the mutual nuclear excitations and the \ee production are independent (EPA model). %We present cross-section predictions from this model in various kinematic regions. We compare our experimental results to two models: the EPA model and a model based on full QED calculation of the \ee production, taking the photon virtuality into account. The measured differential cross-section $d\sigma /dM_{inv}$ ($M_{inv}$ -- $e^+e^-$ invariant mass) agrees well with both theoretical models. The measured differential cross-section $d\sigma /dp_{\perp}^{tot}$ ($p_{\perp}^{tot}$ -- $e^+e^-$ total transverse momentum) favors the full QED calculation over the EPA model.