Resonant states in $^{7}$H. I. Experimental studies of the $^2$H($^8$He,$^3$He) reaction

The extremely neutron-rich system $^{7}$H was studied in the direct $^2$H($^8$He,$^3$He)$^{7}$H transfer reaction with a 26 AMeV secondary $^{8}$He beam [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502]. Missing mass and center-of-mass (c.m.) angular distributions of $^{7}$H were reconstructed, as well as the momentum distribution of the $^{3}$H fragment in the $^{7}$H frame. In addition to the investigation reported in Ref. [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502], we carried out another experiment with the same beam but a modified setup. In the second experiment the cross-check of the experimental calibration was performed by the studies of the $^2$H($^{10}$Be,$^3$He)$^{9}$Li reaction. There is solid experimental evidence for the population of two resonant states in $^{7}$H at $2.2(5)$ and $5.5(3)$ relative to $^3$H+4$n$ threshold. There is some evidence for resonance states at $7.5(3)$ and $11.0(3)$ MeV. Based on the energy and angular distributions, it is argued that the weakly populated $2.2(5)$ MeV peak is the $^7$H ground state. It is highly plausible that the firmly ascertained 5.5(3) MeV state is the $5/2^+$ member of the $^7$H excitation doublet, build on $2^+$ state of valence neutrons. The $7.5$ MeV state can be a member of this $5/2^+$-$3/2^+$ doublet of excited states, could not be resolved in Ref. [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502], and which appears in [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502] as a $6.5$ MeV state of $^{7}$H.