π− d → γnn and the Effective Range Theory Parameters ann and rnn

Among all reactions used to determine ann, π− d → γnn and μ− d → υnn are unique to study the two neutrons final state interactions since no other strong interacting particle is present. This fact makes the theoretical calculations necessary to extract ann more reliable. The “complete” experiment of R.M. Salter, R.P. Haddock et al.1) yielded a value of −16.7 ± 1.3 fm. To this uncertainty mainly coming from the detection of the two neutrons must be added the theoretical uncertainty of the Bander’s theory2) (1 fm) used to extract ann. Two recent calculations3,4) give lower uncertainties of about 0.3 fm if the neutron-neutron relative momentum is restricted to about a few tens of MeV/c.