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.