Coherent lepton pair production from relativistic heavy-ion collisions

摘要

During the collision of heavy-ions at relativistic velocities, law transient electromagnetic fields are formed in a space-time region near the collision. These fields are sufficiently intense to produce large numbers of electron, muon and tauon pairs, as well as other more massive particles. It is also possible to produce the Higgs via its coupling to two-photons through charged fermion and boson loops. The central issue in searching for the yet-undiscovered Higgs boson is separating the decay products of the signal from background events. Electromagnetic production may have a competitive advantage insofar as the transverse response of the fields near the colliding nuclei is sufficiently large that triggering on peripheral events provides a simple way of removing the backgrounds from hadronic decays. The coherent production of electron pairs is important at RHIC energies in several aspects. In regions where the beams cross, up to 10(sup 7) electron pairs are produced each second from the long-range fields. These pairs can either be captured by the heavy-ions, leading to a depletion of the beam, or they can propagate into the detection region where thay form backgrounds for the hadronic signals. The relevant cross sections are poorly understood. Several theoretical calculations yield results which differ by as much as an order of magnitude, and there are no experiments appropriate to this energy domain. However, experiments are being planned for the AGS and an experiment has just been completed at CERN, althought the data analysis is uncompleted. The uncertainties in the theory are due in part to higher order QED effects and to the treatment of the electromagnetic form factors of the nuclei. The coherent production of muon pairs is extremely active to the nuclear form factor and will be discussed in more detail in this paper.