Observation of a J**PC = 1-+ exotic resonance in diffractive dissociation of 190-GeV/c pi- into pi- pi- pi+

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the π - π - π + 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' between 0.1 and 1 GeV 2 /c 2 . The well-known resonances a 1 (1260), a 2 (1320), and π 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 PC = 1 -+ at 1.66 GeV/c 2 decaying to ρπ. The resonant nature of this wave is evident from the mass-dependent phase differences to the J PC = 2 -+ and 1 ++ waves. From a mass-dependent fit a resonance mass of (1660 ± 10 +0 -64 ) MeV/c 2 and a width of (269 ± 21 +42 -64 MeV/c 2 are deduced, with an intensity of (1.7 ± 0.2)% of the total intensity.