Hunting the elusive $X17$ in CE$\nu$NS at the ESS

Abstract

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 $^8$Be, $^4$He and $^{12}$C 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 ($\nu$) Nucleus Scattering (CE$\nu$NS) 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 CE$\nu$NS 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.