Else Lytken

The so-called X17 particle has been proposed in order to explain a very significant resonant behaviour (in both the angular separation and invariant mass) of e⁺e⁻ pairs produced during a nuclear transition of excited ⁸Be, ⁴He and ¹²C nuclei. Fits to the corresponding data point, as most probable explanation, to a spin-1 object, which is protophobic and has a mass of approximately 16.7 MeV, which then makes the X17 potentially observable in Coherent Elastic neutrino (ν) Nucleus Scattering (CEνNS) at the European Spallation Source (ESS). By adopting as theoretical framework a minimal extension of the Standard Model (SM) with a generic U(1)′ gauge group mixing with the hypercharge one of the latter, which can naturally accommodate the X17 state compliant with all available measurements from a variety of experiments, we predict that CEνNS at the ESS will constitute an effective means to probe this hypothesis, even after allowing for the inevitable systematics associated to the performance of the planned detectors therein.