Neutron-deuteron breakup experiment at En=13 MeV:: Determination of the 1S0 neutron-neutron scattering length ann

We report on results of a kinematically complete neutron-deuteron breakup experiment performed at Triangle Universities Nuclear Laboratory using an E-n = 13MeV incident neutron beam. The S-1(0) neutron-neutron scattering length ann has been determined for four production angles of the neutron-neutron final-state interaction configuration. The absolute cross-section data were analyzed with rigorous three-nucleon calculations. Our average value of a(nn) = -18.7 +/- 0.7 fm is in excellent agreement with a(nn) = -18.6 +/- 0.4 fm obtained from capture experiments of negative pions on deuterons. We also performed a shape analysis of the final-state interaction cross-section enhancements by allowing the normalization of the data to float. From these relative data, we obtained an average value of a(nn) = -18.8 +/- 0.5 fm, in agreement with the result obtained from the absolute cross-section measurements. Our result deviates from the world average of a(nn) = -16.7 +/- 0.5 fm determined from previous kinematically complete neutron-deuteron breakup experiments, including the most recent one carried out at Bonn. However, this low value for ann is at variance with theoretical expectation and other experimental information about the sign of charge-symmetry breaking of the nucleon-nucleon interaction. In agreement with theoretical predictions, no evidence was found of significant three-nucleon force effects on the neutron-neutron final-state interaction cross sections.