Atomic collisions at ultrarelativistic energies: 6.4 TeV S and 33.2 TeV Pb

We have used beams of 6.4 TeV S ions and 33.2 TeV Pb ions obtained from the CERN-SPS facility to study collision processes of heavy ions at ultrarelativistic energies. Measurements have been made of electron-positron pair production, and capture of an electron produced from the negative continuum onto bound states of the moving ion. Here we report on some aspects of these results. In a new formulation for the theory of electronic stopping power of ions at ultrarelativistic energies/Lindhard and Sørensen (LS) find that nuclear size effects should act to reduce the momentum transfer to electrons and hence the stopping power. Using the 33.2-Pb ions, we measured dE dx in C, Si, Cu, Sn and Pb targets. The LS theory was confirmed.