The 8Be excess and search for the X → e+e− decay of a new light boson with NA64 detector

The X → e+e− decay of a new short-lived neutral boson X with a mass mX = 16.7 MeV and coupling to electrons in the range 10−4 < εe < 10−3 could explain the excess of e+e− pairs recently observed in the excited 8Be nucleus decays. If such X’s exist, they could be searched for in a light-shining-through-a-wall experiment with a high energy electron beam. The electron energy absorption in a calorimeter (WCAL) is accompanied by the emission of bremsstrahlung X’s in the reaction eZ → eZX of electrons scattering on nuclei due to the e−X coupling. A part of the primary beam energy is deposited in the WCAL, while the rest of the energy is transmitted by the X through the ”WCAL wall” and deposited in another downstream calorimeter ECAL by the e+e− pair from the X → e+e− decay in flight. Thus, the X’s could be observed by looking for an excess of events with the two-shower signature generated by a single high energy electron in the WCAL and ECAL. In October 2016 NA64 took a short run to study feasibility of the search for the X → e+e− decay at H4 beamline with this method. A proposal based on the results of this run to perform an experiment aiming to probe the region of the coupling strength 10−4 <εe < 10−3 and mass mX = 16.7 MeV by using 100-150 GeV electron beams from the H4 line is presented. The experiment can test for the first time the X parameter space with the sensitivity allowing either to exclude or observe the X boson in the run 2017 with the NA64 detector.