Observation of a J{sup PC}=1{sup -+} Exotic Resonance in Diffractive Dissociation of 190 GeV/c {pi}{sup -} into {pi}{sup -{pi}-{pi}+}

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the {pi}{sup -{pi}-{pi}+} final state using a 190 GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420 000 events taken at values of the squared 4-momentum transfer t{sup '} between 0.1 and 1 GeV{sup 2}/c{sup 2}. The well-known resonances a{sub 1}(1260), a{sub 2}(1320), and {pi}{sub 2}(1670) are clearly observed. In addition, the data show a significant natural-parity exchange production of a resonance with spin-exotic quantum numbers J{sup PC}=1{sup -+} at 1.66 GeV/c{sup 2} decaying to {rho}{pi}. The resonant nature of this wave is evident from the mass-dependent phase differences to the J{sup PC}=2{sup -+} and 1{sup ++} waves. From a mass-dependent fit a resonance mass of (1660{+-}10{sub -64}{sup +0}) MeV/c{sup 2} and a width of (269{+-}21{sub -64}{sup +42}) MeV/c{sup 2} are deduced, with an intensity of (1.7{+-}0.2)% of the total intensity.