The formation of the superheavy hydrogen isotope $^6$H in the absorption of stopped $\pi^-$-mesons by nuclei

An experimental search for the superheavy hydrogen isotope $^6$H was conducted through studying the absorption of stopped $pi^-$-mesons by $^9$Be and $^{11}$B nuclei. A structure in the missing mass spectrum caused by the resonance states of $^6$H was observed in three reaction channels, namely, $^9$Be($\pi^-, pd$)X, $^{11}$B($\pi^-,d^3$He)X, and $^{11}$B($\pi^-, p^4$He)X. The parameters of the lowest state $E_r=6.6\pm 0.7\,$MeV and $\Gamma=5.5\pm 2.0\,$MeV ($E_r$ is the resonance energy with respect to the disintegration into the triton and three neutrons) are evidence that $^6$H is a more weakly bound system than $^4$H and $^5$H. Three excited states of $^6$H were observed. Their resonance levels ($E_{1r}=10.7\pm 0.7\,$MeV, $\Gamma_{1r}=4\pm 2\,$MeV, $E_{2r}=15.3\pm 0.7\,$MeV, $\Gamma_{2r}=3\pm 2\,$MeV, and $E_{3r}=21.3\pm 0.4\,$MeV, $\Gamma_{3r}=3.5\pm 1.0\,$MeV ) are energetically capable of disintegrating into six free nucleons.