Observation of a J^PC = 1-_s16 exotic resonance in diffractive dissociation of 190 GeV/c pi- into pi- pi- pi_s16

Abstract

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the pi- pi- pi_s16 final state using a 190 GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420000 events taken at values of the squared 4-momentum transfer t_s14 between 0.1 and 1 GeV^2/c^2. The well-known resonances a1(1260), a2(1320), and pi2(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-_s16 at 1.66 GeV/c^2 decaying to rho pi. The resonant nature of this wave is evident from the mass-dependent phase differences to the J^PC = 2-_s16 and 1_s16_s16 waves. From a mass-dependent fit a resonance mass of 1660 _s16- 10_s160-64 MeV/c^2 and a width of 269_s16-21_s1642-64 MeV/c^2 is deduced.