Hunting down the X17 boson at the CERN SPS.

Recently, the ATOMKI experiment has reported new evidence for the excess of events with a mass 17 MeV in the nuclear transitions of He, that they previously observed in measurements with Be. These observations could be explained by the existence of a new vector boson. So far, the search for the decay with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the decay with the proposed method. To reach this goal an optimization of the production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible.