Analysis of pπ+π− enhancements produced in the reaction π+p → π+pπ+π− at 5 GeV/c

Abstract The p π + π − effective mass spectrum is studied in the mass range from 1.2 to 2.3 GeV in the reaction π + p→ π + p π + π − at 5 GeV/ c . A resonant-type-enhancement is observed at 1.71 ± 0.01 GeV whose position is consistent with that of the S 11 pion-nucleon resonance rather than the D 15 or F 15 states, but whose width 66 ± 26 MeV is considerably narrower than that of the S 11 . No evidence for an intermediate two-body decay is found. From the small value of the four-momentum transfer distribution slope 3.2 ± 0.4 GeV −2 a diffractive mechanism can be excluded. The p π + π − spectrum also shows a rapid increase near threshold, which, after elimination of peripheral Δϱ events appears as a broad diffractive-like enhancement (average slope 11.0 ± 1.0 GeV −2 ; the shape of this threshold effect has been obtained from a study of the variation of the four-momentum transfer distribution along the mass spectrum.