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A few years ago we observed anomalous electron-positron angular correlations for the 18.15 MeV M1 transition of8Be. This was interpreted as the creation and decay of an intermediate bosonic particle with a mass ofm0c2=16.70±0.35(stat )±0.5(sys) MeV, which is now called X17. The possible relation of the X17 boson to the dark matter problem triggered an enormous interest in the wider physics community. We also observed a similar anomaly in4He, which could be described also by the creation and subsequent decay of the same X17 particle. Very recently, the11B proton capture reaction was used for exciting the 17.2 MeV broad (Γ=1.15 MeV) resonance in12C and studying their internal pair creation decay. Anomalies were observed in the angular correlation of the electron-positron pairs, which together with the8Be and4He data provides kinematic evidence for the X17 particle and supports their vector boson and fifth force explanation.

Angular correlation spectra of ${e}^{+}{e}^{\ensuremath{-}}$ pairs produced in the $^{3}\mathrm{H}(p,{e}^{+}{e}^{\ensuremath{-}})^{4}\mathrm{He}$ 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 ${115}^{\ensuremath{\circ}}$ in the angular correlation spectra. This anomalous excess of ${e}^{+}{e}^{\ensuremath{-}}$ 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 ${m}_{\mathrm{X}}{c}^{2}=16.94\ifmmode\pm\else\textpm\fi{}0.12(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.21(\mathrm{syst})$ 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 $^{8}\mathrm{Be}$.

In a 2016 paper, an anomaly in the internal pair creation on the M1 transition depopulating the 18.15 MeV isoscalar 1+ state on 8Be was observed. This could be explained by the creation and subsequent decay of a new boson, with mass mXc2 = 16.70 MeV. Further experiments of the same transition with an improved and independent setup were performed, which constrained the mass of the X17 boson (mXc2) and its branching ratio relative to the γ-decay of the 8Be excited state (BX), to mXc2 = 17.01(16) MeV and BX = 6(1) 10−6, respectively. Using the latter setup, the e+e− pairs depopulating the 21 MeV Jπ = 0− 0+ transition in 4He were investigated and a resonance in the angular correlation of the pairs was observed, which could be explained by the same X17 particle, with mass mXc2 = 16.98 ± 0.16(stat) ± 0.20(syst) MeV.

Electron-positron angular correlations were remeasured for the 17.6 MeV ( J π = 1 + → 0 + ) ground state transition in 8 Be using an improved setup compared to the one we used previously. Significant deviations from the internal pair creation was observed at large angles in the angular correlations, which supports that, in an intermediate step, a neutral isoscalar particle with a mass of 17.0 ±0.2(stat) ±0.5(sys) MeV/ c 2 and J π = 1 + was created.

The emerging experimental program for photofission research at ELI-NP is briefly summarized. GEANT4 Monte Carlo simulations, providing estimates of the collimated beam profile of the ELI-NP gamma beam: beam energy bandwidth and beam spot dimensions, the required target dimensions, and fission fragment emission rates, their mass and energy distributions, have been reported, which ensure the feasibility of high-resolution studies of the multiple-humped fission barrier. A spectrometer consisting of four ionization chambers coupled to Si strip detectors, called ELI-BIC, is being developed for these measurements, and is briefly discussed.

Electron-positron angular correlations were remeasured for the 17.6 MeV $(J^π = 1^+ \to 0^+)$ ground state transition in $^8Be$ using an improved setup compared to one we used previously. Significant deviations from the internal pair creation was observed at large angles in the angular correlations, which supports that, in an intermediate step, a neutral isoscalar particle with a mass of 17.0±0.5 (stat)±0.5 (sys) $MeV/c^2$ and $J^π = 1^+$ was created.

Recently, we observed anomalous internal pair creation for the M1 transition depopulating the 18.15 MeV isoscalar 1$^+$ state in $^8$Be. We observed a significant (σ = 7.37) peak-like deviation from the predicted angular correlation of the e+ e pairs at θ = 140°. To the best of our knowledge no nuclear physics related description of such deviation can be made. However, the deviation can be described by assuming the creation and subsequent decay of a boson with mass of ≈17 MeV. In order to clarify the interpretation, we re-investigated the $^8$Be anomaly with an improved, and independent setup. We have confirmed the signal of the assumed X(17) particle and constrained its mass (m$_0$c $^2$ = 17.01(16) MeV) and branching ratio compared to the γ-decay (B$_x$ = 6(1) × 10$^{−6}$).

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.

Y. Fujita1,2 H. Fujita1 T. Adachi1 C.L. Bai3 A. Algora4,5 G.P.A. Berg6 P. von Brentano7 G. Colo8 M. Csatlos5 J.M. Deaven9 E. Estevez-Aguado4 C. Fransen7 D. De Frenne10 K. Fujita1 E. Ganioglu11 C.J. Guess9 J. Gulyas5 K. Hatanaka1 K. Hirota1 M. Honma12 D. Ishikawa1 E. Jacobs10 A. Krasznahorkay5 H. Matsubara1 K. Matsuyanagi13 R. Meharchand9 F. Molina4 K. Muto14 K. Nakanishi1 A. Negret15 H. Okamura1 H.J. Ong1 T. Otsuka16 N. Pietralla7 G. Perdikakis9 L. Popescu17 B. Rubio4 H. Sagawa13 P. Sarriguren18 C. Scholl7 Y. Shimbara19 Y. Shimizu1 G. Susoy11 T. Suzuki1 Y. Tameshige1 A. Tamii1 J.H. Thies20 M. Uchida1 T. Wakasa1 M. Yosoi1 R.G.T. Zegers1 K.O. Zell7 J. Zenihiro1

Y. Fujita et al.; 15 pags.; 6 figs.; 7 tabs.; PACS number(s): 21.10.Hw, 25.55.Kr, 27.40.+z, 25.40.Ep

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 of5σ. 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.

At the Extreme Light Infrastructure - Nuclear Physics facility (ELI-NP), high-power laser systems together with high-brilliance gamma beams are the main research tools. The status of the construction of the facility is reported. The emerging photofission experimental program with brilliant gamma beams at ELI-NP is presented with emphasis on the prepared day-one experiments. The physics cases of the flagship experiments at ELI-NP are discussed, as well as the related instruments which are under construction for their realization.

The leading theory for dark matter used to be WIMPs (weakly interacting massive particles) that only interacted via gravity and the weak force, making them very hard to detect. Following recent research results, though, a new theory of dark matter actually postulates bosons in the 10 MeV to 10 GeV range [1]. A number of attempts were made to find such particles by using data from running facilities or reanalysing data of preceding experiments, but no evidence has been found yet [1, 2]. In the near future, many ongoing experiments are expected to extend those regions in mass and coupling strength which are so far unexplored [3–5].

A neutron spectrometer, the European Low-Energy Neutron Spectrometer (ELENS), has been constructed to study exotic nuclei in inverse kinematics experiments. The spectrometer consisting of scintillator bars can be used in the neutron energy range of 100 keV to 10 MeV. To increase the light collection efficiency a special wrapping method was developed for each bars of ELENS. By using the specially heat treated reflector foil 15-20% better light collection is available. The development of wrapping process, the results of the test experiments are also presented.