n−n Scattering Length from the Photon Spectra of the Reactions π−d→γnn and π−p→γn

Abstract

A high-statistics photon spectrum from the reaction ${\pi }^{-}d\to \gamma \mathrm{nn}$, measured with a 0.72-meV-resolution pair spectrometer is compared with different theoretical spectra folded with the measured photon line from the reaction ${\pi }^{-}p\to \gamma n$. A value of $a_{\mathrm{nn}}=-18.5\mathrm{}\pm 0.5$ fm (theoretical error included) is deduced assuming a fixed value of ${\mathcal{r}}_{\mathrm{nn}}=2.8\mathrm{}\pm 0.1$ fm. The problem of extracting ${\mathcal{r}}_{\mathrm{nn}}$ using the present theories is indicated.