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 → Au*Au* + e+e-, in ultra-peripheral gold-gold collisions at RHIC at the center-of-mass collision energy of √SNN = 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 4π 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 e+e- production are independent (EPA model). We compare our experimental results to two models: the EPA model and a model based on full QED calculation of the e+e- production, taking the photon virtuality into account. The measured differential cross-section dσ/dMinv (Minv - e+e- invariant mass) agrees well with both theoretical models. The measured differential cross-section dσ/dp $tot\atop{\perp}$ (p$tot\atop{\perp}$- e+e- total transverse momentum) favors the full QED calculation over the EPA model.