Observation of a New $J^{PC} = 1^{- +}$ Exotic State in the Reaction $π^{-} p \to π^{+} π^{-} π^{-} p$ at $18 GeV / c$

A partial-wave analysis of the reaction

π^{-} p \to π^{+} π^{-} π^{-} p

18 GeV / c

has been performed on a data sample of 250 000 events obtained by Brookhaven experiment E852. The expected

J^{PC} {= 1}^{+ +} a_{1} (1260)

2^{+ +} a_{2} (1320)

, and

2^{- +} π_{2} (1670)

resonant states are clearly observed. The exotic

J^{PC} {= 1}^{- +}

wave produced in the natural parity exchange processes shows distinct resonancelike phase motion at about

1.6 GeV {/ c}^{2}

in the

ρ π

channel. A mass-dependent fit results in a resonance mass of

1593 \pm 8_{- 47}^{+ 29} MeV {/ c}^{2}

and a width of

168 \pm 20_{- 12}^{+ 150} MeV {/ c}^{2}

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Observation of a NewJPC=1−+Exotic State in the Reactionπ−p→π+π−π−pat18GeV/c