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Glimpses of the X17 from coherent elastic neutrino nucleus scattering

Johan Rathsman, Joakim Cederkäll, Yasar Hicyilmaz, Else Lytken, Stefano Moretti

Abstract

We show that the process of Coherent Elastic neutrino (v) Nucleus Scattering (CEvNS) at nuclear reactor experiments has significant sensitivity to the so-called X17 particle, which has been invoked to explain the ATOMKI anomaly, wherein electron-positron pairs emerging from a nuclear transition of excited Be-8, He-4 and C-12 nuclei are studied. Such a new state has potentially been identified as a spin-1 object, with axial-vector couplings and a mass around 16.7 MeV, hence, in the kinematic range accessible by the aforementioned experimental settings. Specifically, we fit CONUS+ and Dresden-II data and show that a robust statistical analysis renders these more compatible with the X17 hypothesis, in turn interfering with the Standard Model, than with that of the latter alone. The same stays true when also adding COHERENT data from pi+ decays at rest, singling out two regions of preferred couplings of the X17 to electron and muon neutrinos as well as nuclei.

Glimpses of the X17 from coherent elastic neutrino nucleus scattering

Abstract

We show that the process of Coherent Elastic neutrino (v) Nucleus Scattering (CEvNS) at nuclear reactor experiments has significant sensitivity to the so-called X17 particle, which has been invoked to explain the ATOMKI anomaly, wherein electron-positron pairs emerging from a nuclear transition of excited Be-8, He-4 and C-12 nuclei are studied. Such a new state has potentially been identified as a spin-1 object, with axial-vector couplings and a mass around 16.7 MeV, hence, in the kinematic range accessible by the aforementioned experimental settings. Specifically, we fit CONUS+ and Dresden-II data and show that a robust statistical analysis renders these more compatible with the X17 hypothesis, in turn interfering with the Standard Model, than with that of the latter alone. The same stays true when also adding COHERENT data from pi+ decays at rest, singling out two regions of preferred couplings of the X17 to electron and muon neutrinos as well as nuclei.
Paper Structure (10 sections, 50 equations, 12 figures, 2 tables)

This paper contains 10 sections, 50 equations, 12 figures, 2 tables.

Table of Contents

  1. Introduction
  2. The theoretical framework embedding the $X_{17}$
  3. The experimental framework sensitive to the $X_{17}$
  4. Statistical and systematic errors
  5. Combining reactor and spallation source datasets
  6. Conclusions and outlook
  7. Detector performance for COHERENT data
  8. Argon
  9. Caesium Iodine
  10. Germanium

Figures (12)

  • Figure 1: Neutrino spectra from fission reactors as calculated by Kopeikin (in red dots) together with data from Daya Bay (in blue dots) and fit used (solid line). The spectra are normalised per fission.
  • Figure 2: Nuclear recoil spectrum in the SM from reactor neutrinos and assuming a Germanium detector before smearing and quenching. Arbitrary normalisation.
  • Figure 3: QFs used: ${\rm QF}_1$ is the Lindhard model with $k=0.157$ and ${\rm QF}_2$ is the Lindhard model with $k=0.162$ and a modification added to describe the data points.
  • Figure 4: Shape of nuclear recoil spectra for Dresden-II (left) and CONUS+ (right) after smearing and quenching with different QFs as well as the inherent resolution of the detector given by $R\left(y_{\rm rec},0\right)$ normalised such that $R\left(0,0\right)=1$.
  • Figure 5: QF uncertainty for reactor (left) and $\pi^+$ decay at rest (right) neutrinos with the same detector resolution as Dresden-II (black) and CONUS+ (blue).
  • ...and 7 more figures