Study of a Narrow pi+ pi- Peak at about 755 MeV/c**2 in pbar n --> 2 pi+ 3 pi- Annihilation at Rest

A narrow peak in the pi+ pi- mass distribution was seen by the Rome-Syracuse Collaboration in pbar n --> 2 pi+ 3 pi- annihilation at rest in 1970. It was ignored for 40 years. The reanalysis of this peak finds that it has the mass 757.4 ± 2.8_stat ± 1.1_sys MeV/c**2 and a width consistent with the experimental resolution. The evidence of the peak is 5.2 standard deviations. The peak is generated in (1.03 ± 0.21_stat ± 0.21_sys)% of the pbar n annihilations at rest. No spin analysis is possible with the statistics of the experiment but there are arguments suggesting that it has JP = 0+.