Study of a narrow pi+ pi- peak at about 755 MeV/c² in bar-p 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 bar-p n --> 2 pi+ 3 pi- annihilation at rest 39 years ago. The reanalysis of this peak finds a mass of 757.4 +- 2.6 MeV/c^2 and a width slightly narrower than the experimental resolution. The evidence of the peak is 5.2 standard deviations. This state is generated in (12.4 +- 2.4)% of the bar-p n --> 2 pi+ 3 pi- annihilations at rest. No spin analysis is possible with the statistics of the experiment.