Measurement of the neutron-neutron scattering length using the {pi}{sup -}d capture reaction

• Published in: Physical review. C, Nuclear physics Vol. 77; no. 5
• Main Authors: Chen, Q., Howell, C. R., Kiser, M. R., Roper, C. D., Salinas, F., Setze, H. R., Tornow, W., Walter, R. L., Carman, T. S., Gibbs, W. R., Gibson, B. F., Morris, C., Obst, A., Sterbenz, S., Whitton, M., Hussein, A., Mertens, G., Moore, C. F., Whiteley, C. R., Pasyuk, E.
• Format: Journal Article
• Language: English
• Published: United States 15-05-2008
• Subjects: CAPTURE; DEUTERIUM TARGET; GAMMA RADIATION; LAMPF LINAC; MAGNETIC MOMENTS; NEUTRON-NEUTRON INTERACTIONS; NEUTRONS; NUCLEAR PHYSICS AND RADIATION PHYSICS; NUCLEAR REACTIONS; PHASE SPACE; PIONS MINUS; PROTONS; RELATIVISTIC RANGE; SCATTERING LENGTHS; SPATIAL RESOLUTION; SPECTRA; SYMMETRY BREAKING; TIME-OF-FLIGHT METHOD
• ISSN: 0556-2813; 1089-490X
• DOI: 10.1103/PHYSREVC.77.054002

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 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{sub pp}, thereby confirming charge symmetry breaking at the 1% confidence level.