Observation of Anomalous Internal Pair Creation inBe8: A Possible Indication of a Light, Neutral Boson

Abstract

Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV (J^{π}=1^{+}, T=1) state→ground state (J^{π}=0^{+}, T=0) and the isoscalar magnetic dipole 18.15 MeV (J^{π}=1^{+}, T=0) state→ground state transitions in ^{8}Be. Significant enhancement relative to 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 could possibly be due to nuclear reaction interference effects or might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70±0.35(stat)±0.5(syst) MeV/c^{2} and J^{π}=1^{+} was created.

Review of dark photon searches

“…In the following we will discuss electron scattering experiments as well as results from hadron and e + e − accelerators. Very recently, an anomaly was observed in a nuclear decay of Berryllium [4]. This could be a first hint for a 17 MeV Dark Photon signal, however the confirmation of the anomaly by an independent group is pending.…”

New determination of proton spectroscopic factors and reduced widths forBe8states in the 16.5–18.0 MeV excitation energy region via the study of theLi7(Belhout1, Ouichaoui2, Beaumevieille3 et al. 2017Phys. Rev. C4000View full textAdd to dashboardCite

“…anomaly at E x ∼ 17 MeV in 8 Be [25] decaying via internal electron-positron pair creation, further perspectives for a particle physics solution to the cosmological Lithium problem have been proposed (see [26] and references therein).…”

Explanation of the 17 MeV Atomki anomaly in a U(1)′ -extended two Higgs doublet model

“…1 the parameter space explaining the Atomki anomaly together with the most constraining experimental results. The orange region, where the Z ′ gauge couplings comply with the best-fit of the 8 Be * decay rate in the mass range M Z ′ = 16.7 MeV − 17.6 MeV [1,3], encompasses the uncertainties on the computation of the nuclear matrix elements [17]. The region above it is excluded by the non-observation of the same transition in the isovector excitation 8 Be * ′ [1].…”

“…Upon analysis of the electron-positron properties, the spectra of both their opening angle θ and invariant mass M presented the characteristics of an excess consistent with an intermediate boson X being produced on-shell in the decay of the 8 Be * state, with the X object subsequently decaying into e + e − pairs. The best fit to the mass M X of X was given as [1] M X = 16.7 ± 0.35 (stat) ± 0.5 (sys) MeV, in correspondence of a ratio of Branching Ratios (BRs) obtained as BR( 8 Be * → X + 8 Be) BR( 8 Be * → γ + 8 Be) × BR(X → e + e − ) = 5.8 × 10 −6 .…”

“…The Atomki pair spectrometer experiment [1] was set up for searching e + e − internal pair creation in the decay of excited 8 Be nuclei (henceforth, 8 Be * ), the latter being produced with the help of a beam of protons directed on a Lithium ( 7 Li) target. The proton beam was tuned in such a way that the different 8 Be excitations could be separated with high accuracy.…”