Observation of Exotic Meson Production in the Reaction $π^{-} p \to η^{'} π^{-} p$ at $18 GeV / c$

An amplitude analysis of an exclusive sample of 5765 events from the reaction

π^{-} p \to η^{'} π^{-} p

18 GeV / c

is described. The

η^{'} π^{-}

production is dominated by natural parity exchange and by three partial waves: those with

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

2^{+ +}

, and

4^{+ +}

. A mass-dependent analysis of the partial-wave amplitudes indicates the production of the

a_{2} (1320)

meson as well as the

a_{4} (2040)

meson, observed for the first time decaying to

η^{'} π^{-}

. The dominant, exotic (non-

q \bar{q}

)

1^{- +}

partial wave is shown to be resonant with a mass of

1.597 \pm {0.010}_{- 0.010}^{+ 0.045} GeV / c^{2}

and a width of

0.340 \pm 0.040 \pm 0.050 GeV / c^{2}

. This exotic state, the

π_{1} (1600)

, is produced with a

t

dependence which is different from that of the

a_{2} (1320)

meson, indicating differences between the production mechanisms for the two states.

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Observation of Exotic Meson Production in the Reactionπ−p→η′π−pat18GeV/c