Abstract:
We have determined a value for the 1S0 neutron-neutron scattering length (ann) from high-precision measurements of time-of-flight spectra of neutrons from the H2(π-,nγ)n capture reaction. The measurements were done at the Los Alamos Meson Physics Facility by the E1286 Collaboration. The high spatial resolution of our γ-ray detector enabled us to make a detailed assessment of the systematic uncertainties in our techniques. The value obtained in the present work is ann=-18.63±0.10 (statistical) ± 0.44 (systematic) ± 0.30 (theoretical) fm. This result is consistent with previous determinations of ann from the π-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 ann by 0.33 fm over the analysis done using a nonrelativistic phase-space factor. Combining the present result with the previous ones from π-d capture gives ann=-18.63±0.27(expt) ±0.30 fm (theory). For the first time the combined statistical and systematic experimental uncertainty in ann is smaller than the theoretical uncertainty and comparable to the uncertainty in the proton-proton 1S0 scattering length (app). This average value of ann 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 app, thereby confirming charge symmetry breaking at the 1% confidence level. © 2008 The American Physical Society.
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