Search for 21C and constraints on 22C

A search for the neutron-unbound nucleus 21C was performed via the single-proton removal reaction from a beam of 22N at 68 MeV/u. Neutrons were detected with the Modular Neutron Array (MoNA) in coincidence with 20C fragments. No evidence for a low-lying state was found, and the reconstructed C20+n decay-energy spectrum could be described with an s  -wave line shape with a scattering length limit of |as|<2.8 fm|as|<2.8 fm, consistent with shell-model predictions. A comparison with a renormalized zero-range three-body model suggests that 22C is bound by less than 70 keV.