Abstract
We propose a direct search for the X17 particle, which was conjectured to explain the ATOMKI ^{8}Be and ^{4}He anomalies, through the dilepton photoproduction process on a nucleon in the photon energy range below or around the pion production threshold. For the scenarios of either pseudoscalar, vector, or axial-vector quantum numbers of the conjectured X17, we use existing constraints to estimate the X17 signal process. For dilepton invariant mass resolutions which have been achieved in previous experiments, a signal-to-background ratio of up to an order of magnitude is found for a neutron target, and, in particular, for the pseudoscalar and vector X17 scenarios.
Highlights
A few years ago, the ATOMKI group measured electronpositron angular correlations for two magnetic dipole transitions to the ground state taking place in 8Be [1]
In view of a vigorous program worldwide to search for dark sector particles with a very weak coupling to standard model particles, from sub-eV mass scales to multi-TeV mass scales [6–8], the ATOMKI observations have sparked the prospect that the conjectured X17 might fall in this category
Direct searches by the NA64 Collaboration at CERN have not found any X17 evidence so far [28,29], putting constraints on the allowed parameter ranges for new physics explanations
Summary
A few years ago, the ATOMKI group measured electronpositron angular correlations for two magnetic dipole transitions to the ground state taking place in 8Be [1]. In this Letter, we propose a direct search for the conjectured X17 particle through the dilepton photoproduction on a nucleon, the γN → eþe−N process, in the 100–150 MeV photon energy range, below or around the production threshold for pions, at high-luminosity fixed target electron scattering facilities.
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