Observation of Anomalous Internal Pair Creation in $^8$Be: A Possible Signature of a Light, Neutral Boson

TL;DR

The study revisits anomalous internal pair creation (IPC) in $^8$Be, focusing on the 17.6 MeV isovector and 18.15 MeV isoscalar M1 transitions to the ground state. It uses the $^{7}$Li(p,$\gamma$)$^{8}$Be reaction at selected resonances and a high-efficiency $e^+e^-$ detector setup, with GEANT-based simulations to model IPC, external pair creation, and backgrounds. A significant deviation from standard IPC is observed for the 18 MeV transition at large angles, not explainable by E1 admixture alone. Interpreting the anomaly as production and decay of a neutral boson with mass $m_0c^2 = 16.70 \pm 0.35$ (stat) $\pm 0.50$ (sys) MeV/$c^2$ and a branching ratio $BR(e^+e^-)/BR(\gamma) \approx 5.8\times10^{-6}$ yields a good fit, suggesting a potential dark-sector mediator (dark Z or U(1)$_d$) with a lifetime on the order of $10^{-14}$ s and a very short flight distance.

Abstract

Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV state ($J^π=1^+$, $T=1$) $\rightarrow$ ground state ($J^π=0^+$, $T=0$) and the isoscalar magnetic dipole 18.15 MeV ($J^π=1^+$, $T=0$) state $\rightarrow$ ground state transitions in $^{8}$Be. Significant deviation from the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of $> 5σ$. This observation might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70$\pm0.35 $ (stat)$\pm 0.5 $ (sys) MeV$/c^2$ and $J^π= 1^+$ was created.

Figures (5)

• Figure 1: Measured total energy spectrum (a) and angular correlation (b) of the $e^{+}e^{-}$ pairs originated from the decay of the 17.6 MeV resonance compared with the simulated angular correlations gu15 assuming M1 (full curve) and M1+2%E1 mixed transitions (dashed line).
• Figure 2: Measured total energy spectrum (a) and angular correlations (b) of the $e^{+}e^{-}$ pairs created in the different transitions labelled in the figure, compared with the simulated angular correlations assuming E0 (from the $^{16}$O peak) and M1+E1 mixed transitions from the other peaks.
• Figure 3: Measured angular correlations of the $e^{+}e^{-}$ pairs originated from the ground state decay of the $^{7}$Li(p,$\gamma$)$^{8}$Be reaction (dots with error bars) compared with the simulated ones (full curves) assuming M1+E1 mixed transitions with the same mixing ratio for all curves at different beam energies.
• Figure 4: Experimental angular $e^{+}e^{-}$ pair correlations measured in the $^7$Li(p,$e^{+}e^{-}$) reaction at E$_p$=1.10 MeV with -0.5 $\leq$ y $\leq$ 0.5 (closed circles) and $\vert y \vert\geq$ 0.5 (open circles). The results of simulations of boson decay pairs added to those of IPC pairs are shown for different boson masses as described in the text.
• Figure 5: Determination of the mass of the new particle by the $\chi^{2}/f$ method, by comparing the experimental data with the results of the simulations obtained for different particle masses.