New anomaly observed in ${}^4\mathrm{He}$ supports the existence of the hypothetical X17 particle

Angular correlation spectra of $e^{+}{e}^{-}$ pairs produced in the ${}^3\mathrm{H}(p,e^{+}{e}^{-}){}^4\mathrm{He}$ nuclear reaction have been studied at $E_p=510$, 610, and 900 keV proton energies. The main features of the spectra can be understood by taking into account the internal and external pair creations following the proton capture by ${}^3\mathrm{H}$. However, these processes cannot account for an observed peak around ${115}^{\circ }$ in the angular correlation spectra. This anomalous excess of $e^{+}{e}^{-}$ pairs can be described by the creation and subsequent decay of a light particle during the direct capture process. The derived mass of the particle is $m_{\mathrm{X}}{c}^2=16.94\pm 0.12\left(\mathrm{stat}\right)\pm 0.21\left(\mathrm{syst}\right)$ MeV. According to the mass this is likely the same X17 particle, which we recently suggested [Phys. Rev. Lett. 116, 042501 (2016)] for describing the anomaly observed in the decay of ${}^8\mathrm{Be}$.

Figure 1

Schematical drawing of the target cooling system.

Figure 2

CAD drawing of the $e^{+}{e}^{-}$ spectrometer. The target (black/blue spot in the center of the figure) is evaporated onto 10 $\mu \mathrm{m}$ Al strip foil spanned between 3 mm thick Perspex rods to minimize the scattering and external pair creation in the vicinity of the target. The beam pipe is shown in black around which the DSSD detectors are arranged. The scintillators are shown in yellow while their light guides are in green. The PMT tubes are not shown.

Figure 3

Detector response for the setup as a function of correlation angle ($\theta$) for isotropic emission of $e^{+}{e}^{-}$ pairs (solid line crosses) compared with the results of the Monte Carlo simulations (dashed line histogram) as explained in the text.

Figure 4

$e^{+}{e}^{-}$ angular correlations obtained for the 17.6 MeV transition of ${}^8\mathrm{Be}$ by using thin target backing.

Figure 5

$e^{+}{e}^{-}$ angular correlations obtained for the 17.6 MeV transition of ${}^8\mathrm{Be}$ by using thick target backing.

Figure 6

(a) Experimental energy-sum spectra of the $e^{+}{e}^{-}$ pairs derived, respectively, for “All” different detector combinations (dashed-line histogram with counts multiplied by 0.08) and for detectors at ${120}^{\circ }$ relative angles (solid-line histogram). The cosmic-ray background contributions are subtracted from both spectra. The CRB spectrum corresponding to the “All” spectrum is plotted with dotted line. (b) The markers with error bars show the difference of the solid-line and dashed-line distributions from (a). The solid line represents the same difference, calculated using the simulated response of the spectrometer for $\gamma$ rays (through external $e^{+}{e}^{-}$ pair creation) created in the direct proton capture on ${}^3\mathrm{H}$, and for the $e^{+}{e}^{-}$ pairs expected from the decay of the hypothetical X17 particle.

Figure 7

Angular correlations of the $e^{+}{e}^{-}$ pairs for the “Signal” region (see Fig. 6). Symbols with error bars indicate experimental data measured in the ${}^3\mathrm{H}(p,\gamma ){}^4\mathrm{He}$ reaction at different proton beam energies, while solid-line histograms correspond to the respective data obtained in the simulations described in the text.

Figure 8

Angular correlations of the $e^{+}{e}^{-}$ pairs for the “Background” region (see Fig. 6). See the caption of Fig. 7 for more details.

Figure 9

Comparison of the experimental and the simulated angular correlations of the $e^{+}{e}^{-}$ pairs. The best fitted sum (dashed line) as sum of the the simulated background (dotted line) and the simulated contribution of the hypothetical X17 boson is compared with the experimental signal values (dots with error bars).