Search for the $^{15}$Be ground state

The ground state of the unbound nuclide $^{15}$Be remains an open question. The MoNA collaboration has performed two experiments to study the structure of $^{15}$Be at the NSCL. In a first attempt to populate $^{15}$Be, a two-proton removal reaction from a $^{17}$C beam was used and decays were searched for in the $^{14}$Be+n channel. This led to a non-observation due to a lack of $^{14}$Be fragments detected. A follow-up experiment made the first observation of a $^{15}$Be state through the use of a neutron-pickup reaction with a $^{14}$Be beam impinging on a deuterated plastic target. Because of the observed state’s relatively high decay energy, the existence of a $^{15}$Be state lower in energy decaying sequentially through the first excited state in $^{14}$Be resulting in $^{12}$Be+3n is possible. A first attempt to search for this state in the two-proton removal data set yielded low statistics and the data did not indicate the presence of a lower state. The neutron-pickup data are now being reanalyzed to search for the ground state in $^{15}$Be by simultaneously fitting 2-, 3-, 4-body decay energies. Preliminary results indicate evidence for a state in $^{15}$Be decaying by three neutrons that is lower in energy than the previously measured state.