Observation of exotic meson production in the reaction {pi}{sup -} p {yields} {eta}{prime} {pi}{sup -} p at 18 GeV/c

An amplitude analysis of an exclusive sample of 5765 events from the reaction {pi}{sup -} p {yields} {eta}{prime}{pi}{sup -} p at 18 GeV/c is described. The {eta}{prime}{pi}{sup -} production is dominated by natural parity exchange and by three partial waves: those with J{sup PC} = 1{sup -+}, 2{sup ++}, and 4{sup ++}. A mass-dependent analysis of the partial-wave amplitudes indicates the production of the a{sub 2}(1320) meson as well as the a{sub 4}(2040) meson, observed for the first time decaying to {eta}{prime}{pi}{sup -}. The dominant, exotic (non-q{bar q}) 1{sup -+} partial wave is shown to be resonant with a mass of 1.597 {+-} 0.010{sub -0.010}{sup +0.045} GeV/c{sup 2} and a width of 0.340 {+-} 0.040 {+-} 0.050 GeV/c{sup 2}. This exotic state, the {pi}{sub 1}(1600), is produced with a t dependence which is different from that of the a{sub 2}(1320) meson, indicating differences between the production mechanisms for the two states.