On Detection of Narrow Angle ${\rm e}!+!{\rm e}-$ Pairs From Dark Photon Decays

A class of models of a “dark sector” of particles consider new very weak interaction between the ordinary and dark matter transmitted by U´(1) gauge bosons A´ (dark photons) mixing with ordinary photons. If such A´s exist, they could be searched for in a light-shining-through-a-wall experiment in a high energy electron beam from the CERN SPS. A proposed search project suggests detection of the e+e- pairs produced in the A ^´ -> e+e - decays with a very small opening angle. Coordinate detectors based on the thin-wall drift tubes with a minimal material budget and a two-hit resolution for e+ and e- tracks separated by more than 0.5 mm are considered as an option for detecting such events.