We have determined a value for the S-1(0) neutron-neutron scattering length (a(nn)) from high-precision measurements of time-of-flight spectra of neutrons from the H-2(pi(-), n gamma) n capture reaction. The measurements were done at the Los Alamos Meson Physics Facility by the E1286 Collaboration. The high spatial resolution of our gamma-ray detector enabled us to make a detailed assessment of the systematic uncertainties in our techniques. The value obtained in the present work is a(nn) = -18.63 +/- 0.10 (statistical) +/- 0.44 (systematic) +/- 0.30 (theoretical) fm. This result is consistent with previous determinations of a(nn) from the pi(-)d capture reaction. We found that the analysis of the data with calculations that use a relativistic phase-space factor gives a more negative value for a(nn) by 0.33 fm over the analysis done using a nonrelativistic phase-space factor. Combining the present result with the previous ones from pi(-)d capture gives a(nn) = -18.63 +/- 0.27(expt) +/- 0.30 fm (theory). For the first time the combined statistical and systematic experimental uncertainty in a is smaller than the theoretical uncertainty and comparable to the uncertainty in the proton-proton S-1(0) scattering length (a(pp)). This average value of a(nn) when corrected for the magnetic-moment interaction of the two neutrons becomes - 18.9 +/- 0.4 fm, which is 1.6 +/- 0.5 fm different from the recommended value of a(pp), thereby confirming charge symmetry breaking at the 1% confidence level.

We have determined a value for the {sup 1}S{sub 0} neutron-neutron scattering length (a{sub nn}) from high-precision measurements of time-of-flight spectra of neutrons from the {sup 2}H({pi}{sup -},n {gamma}) n capture reaction. The measurements were done at the Los Alamos Meson Physics Facility by the E1286 Collaboration. The high spatial resolution of our {gamma}-ray detector enabled us to make a detailed assessment of the systematic uncertainties in our techniques. The value obtained in the present work is a{sub nn}=-18.63{+-}0.10 (statistical) {+-} 0.44 (systematic) {+-} 0.30 (theoretical) fm. This result is consistent with previous determinations of a{sub nn} from the {pi}{sup -}d capture reaction. We found that the analysis of the data with calculations that use a relativistic phase-space factor gives a more negative value for a{sub nn} by 0.33 fm over the analysis done using a nonrelativistic phase-space factor. Combining the present result with the previous ones from {pi}{sup -}d capture gives a{sub nn}=-18.63{+-}0.27(expt){+-}0.30 fm (theory). For the first time the combined statistical and systematic experimental uncertainty in a{sub nn} is smaller than the theoretical uncertainty and comparable to the uncertainty in the proton-proton {sup 1}S{sub 0} scattering length (a{sub pp}). This average value of a{sub nn} when corrected formore » the magnetic-moment interaction of the two neutrons becomes -18.9 {+-} 0.4 fm, which is 1.6 {+-} 0.5 fm different from the recommended value of a{sub pp}, thereby confirming charge symmetry breaking at the 1% confidence level.« less

This paper reports two recent high-accuracy determinations of the 1 S 0 neutron-neutron scattering length, a nn . One was done at the Los Alamos National Laboratory using the π− d capture reaction to produce two neutrons with low relative momentum. The neutron-deuteron (nd) breakup reaction was used in other measurement, which was conducted at the Triangle Universities Nuclear Laboratory. The results from the two determinations were consistent with each other and with previous values obtained using the π − d capture reaction. The value obtained from the nd breakup measurements is a nn = −18.7 ± 0.1 (statistical) ± 0.6(systematic) fm, and the value from the π− d capture experiment is a nn = −18.50 ± 0.05 ± 0.53 fm. The recommended value is a nn = − 18.5 ± 0.3 fm.

Asymmetries for the (pi(-), pi(0)) reaction on polarized He-3 were measured using a 200-MeV pion beam at LAMPF. The pi(0)'s were detected with the neutral meson spectrometer in coincidence with the recoiling tritons. A recoil triton detector, consisting of scintillation-counter telescopes and a wire chamber, was used to measure the time-of-flight, energy loss, and direction of the tritons. The polarized gaseous He-3 target, developed at TRIUMF, was modified to run with two diode lasers; the polarization reached 65%. The asymmetries between theta(lab)=60 degrees and 105 degrees were found to be strongly angle dependent. The results are compared with theoretical calculations. [S0556-2813(99)02208-6].

We report a high-precision determination of the 1S0 neutron–neutron scattering length (ann) using the 2H(π−,nγ)n reaction. The value obtained in the present work is −18.50± 0.05 (statistical) ± 0.44 (systematic) ± 0.30 (theoretical) fm, which is consistent with the values from previous measurements. Combining our result with previous measurements reduces the total uncertainty in the world average of ann to ±0.4 fm, matching the accuracy to which the charge-symmetric parameter app is determined.