Anomalous internal pair conversion signaling elusive light neutral particles

In this paper we report on a systematic search for a neutral boson in the mass range between 5 and 15 MeV/c$^{2}$ in the decay of highly excited nuclei. Its signature is found a deviation in the angular correlation of the $e^+e^-$ pairs from conventional internal pair conversion (IPC) resulting from of its two-body decay kinematics. With an $e^{+}e^{-}$ pair-spectrometer, a number of transitions has been investigated in the ${\alpha}$--nuclei $^{8}$Be, $^{12}$C and $^{16}$O, following light ion induced reactions at low bombarding energies, first at IKF in Frankfurt and during the last years at ATOMKI in Debrecen. Startlingly, in all isoscalar transitions excess $e^{+}e^{-}$ pairs are found at large angles with branching ratios with respect to the total yield ranging from 10$^{-2}$ to 10$^{-6}$. If these deviations are all related to the two-body decay of an $X$-boson, this observation implies plural $X$-bosons. An analysis of all angular spectra with a boson search program, yields a pandemonium of more than ten candidate bosons.