Measurement of theN*−−N*++Mass Difference

A measurement of the $N^{*-}-N^{*++}$ mass and width differences is described, where $N^{*}$ is the nucleon resonance with $I\left(J^P\right)=\frac{3}{2}\left({\frac{3}{2}}^{+}\right)$ and mass approximately 1240 MeV. The resonances were produced in the inelastic reactions $\mathrm{nn}\to \mathrm{pn}{\pi }^{-}$ and $\mathrm{pp}\to \mathrm{np}{\pi }^{+}$, which are known to proceed almost entirely via $N^{*-}$ and $N^{*++}$ production, respectively, in the observed energy region. A comparison of the ($n{\pi }^{-}$) and ($p{\pi }^{+}$) effective-mass distributions gives a mass difference of 7.9±6.8 MeV and a width difference of 25±23 MeV for $N^{*}-N^{*++}$. This result agrees with predictions based on the $\mathrm{SU}\left(3\right)\mathrm{}$ and $\mathrm{SU}\left(6\right)\mathrm{}$ symmetry schemes.