Yasar Hicyilmaz (Balikesir University, Turkey & Uppsala University, Sweden): Hunting the Elusive X17 in CEvNS: From Nuclear Reactor Neutrino Data to the European Spallation Source
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Europe/Stockholm
Beurling Å10238
Beurling Å10238
Description
The persistent anomaly observed by the ATOMKI experiment— a significant enhancement in the angular separation and invariant mass of electron-positron pairs during nuclear transitions of Beryllium-8, Helium-4, and Carbon-12 — has sparked immense interest in a new physics candidate: the X17 particle. Best interpreted as a light, spin-1 boson with a mass of approximately 16.7 MeV, the X17 could be the carrier of a postulated fifth force. While standard high-energy searches have heavily constrained pure-vector extensions, this talk proposes a powerful, independent, and low-energy avenue to test the X17 hypothesis: Coherent Elastic Neutrino-Nucleus Scattering (CEvNS).
By synthesizing recent CEvNS measurements across different energy regimes, we reveal compelling hints of the X17 particle's potential existence. First, we demonstrate that an existing tension between the Dresden-II and CONUS+ nuclear reactor experiments, when analyzed strictly under the Standard Model, can be remarkably resolved by the introduction of a ~17 MeV gauge boson corresponding to the X17. Furthermore, conducting a combined statistical fit using reactor data, spallation source data from the COHERENT experiment, and Non-Standard Interaction constraints from IceCube, we show that the data singles out a unique, highly favored region for the effective neutrino-nucleus couplings. This preferred parameter space features a negative effective coupling for electron anti-neutrinos and a positive effective coupling for muon neutrinos.
Looking to the near future, we project the discovery reach of high-statistics spallation sources, with a specific focus on the European Spallation Source (ESS). By embedding the X17 within a minimal family-dependent U(1)' extension of the Standard Model, we show that the presence of the X17 mediator will distinctly modify both the total CEvNS interaction rate and the shape of the nuclear recoil energy spectrum, especially at low recoil energies due to propagator enhancement. Ultimately, this talk highlights how the synergy between current reactor anomalies, existing spallation data, and the upcoming ESS facility provides a robust and highly sensitive frontier for confirming the X17 particle and unlocking physics beyond the Standard Model.
