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Abstract

A measurement of the ¹S₀ neutron-neutron scattering length a[sub nn] has been carried out at TRIUMF by studying the shape of the photon energy spectrum from the reaction π⁻d —» γnn in the region near the endpoint. A 40.5 M eV pion beam was degraded and stopped in a liquid deuterium target and all three final state particles from the reaction were detected in triple coincidence. The photon was detected in a large Nal(Tl) crystal, while the neutrons were detected in a 2 m x 2 m position-sensitive array of plastic scintillation counters, located at a distance of 3 m from the target. The experimental photon energy spectrum was reconstructed to a resolution of 40 keV F W H M from the measured momenta of the two neutrons, and contains 123,000 counts in the top 450 keV region near the endpoint after background subtraction. The value of a[sub nn] is determined from a comparison of this experimental spectrum to simulated spectra that are being developed simultaneously at the University of Kentucky. These spectra are derived from a new model of this reaction that is based on a half off-shell NN T matrix and the elementary γΠ operator due to Lee and Nozawa. The experimental geometry and resolution are taken into account using Monte Carlo techniques. A comparison of our final experimental spectrum to a preliminary set of the simulated spectra has yielded the provisional result of a[sub nn] = -21.8 ± 0.3 fm (theoretical errors excluded) before correction for electromagnetic effects. This preliminary result is in disagreement with the currently accepted experimental value of a[sub nn] = -18.5 ± 0.3 fm, however we stress that the theoretical model is still under development. We anticipate that our final result will make a significant contribution to the discussion of charge symmetry breaking in the strong interaction, particularly with regard to the current uncertainty that surrounds the contribution of the (⍴-ω) mixing term in standard meson-theoretic potentials.