On the existence of a bound tetraneutron
  Following recent work in which events which may correspond to a bound tetraneutron ($^4$n) were observed, it is pointed out that from the theoretical perspective the two-body nucleon-nucleon force cannot by itself bind four neutrons, even if it can bind a dineutron.   A very strong phenomenological four-nucleon (4N) force is needed in order to bind the tetraneutron. Such a 4N force, if it existed, would bind $^4$He by about 100 MeV. Alternative experiments such as ($^8$He,$^4$n) are proposed to search for the tetraneutron. 
N.K. Timofeyuk