Hunting down the X17 boson at the CERN SPS

Recently, the ATOMKI experiment has reported new evidence for the excess of [Formula: see text] events with a mass [Formula: see text] 17 MeV in the nuclear transitions of [Formula: see text] He, that they previously observed in measurements with [Formula: see text] Be. These observations could be explained by the existence of a new vector [Formula: see text] boson. So far, the search for the decay [Formula: see text] 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 [Formula: see text] parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the [Formula: see text] decay with the proposed method. To reach this goal an optimization of the [Formula: see text] 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.