AbstractRecently, the ATOMKI experiment has reported new evidence for the excess of $$e^+e^-$$
e
+
e
-
events with a mass $$\sim $$
∼
17 MeV in the nuclear transitions of $$^4$$
4
He, that they previously observed in measurements with $$^8$$
8
Be. These observations could be explained by the existence of a new vector $$X17$$
X
17
boson. So far, the search for the decay $$X17 \rightarrow e^+ e^-$$
X
17
→
e
+
e
-
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 $$X17$$
X
17
parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the $$X17$$
X
17
decay with the proposed method. To reach this goal an optimization of the $$X17$$
X
17
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.