Hunting down the X17 boson at the CERN SPS

TL;DR

This paper proposes a new detection technique for the hypothetical X17 boson at CERN SPS, aiming to improve sensitivity and confirm previous negative results through invariant mass reconstruction of decay events.

Contribution

It introduces a novel method for X17 boson detection that enhances sensitivity and allows unambiguous identification, building on and validating previous experimental approaches.

Findings

The proposed method can improve detection sensitivity for X17 boson.

Monte Carlo simulations show the feasibility of the new detection setup.

Analysis confirms the tracking procedure's reliability in identifying decay events.

Abstract

Recently, the ATOMKI experiment has reported new evidence for the excess of $e^+ e^-$ events with a mass $\sim$17 MeV in the nuclear transitions of $^4$He, that they previously observed in measurements with $^8$Be. These observations could be explained by the existence of a new vector $X17$ boson. So far, the search for the decay $X17 \rightarrow e^+ e^-$ 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 $X17$ parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the $X17$ decay with the proposed method. To reach this goal an optimization of the $X17$ 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 [Phys. Rev. D101, 071101 (2020)], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate shows that the goal of the proposed search is feasible.