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

TL;DR

The COMPASS experiment observed a spin-exotic J^PC = 1-+ resonance at 1.66 GeV/c^2 in diffractive dissociation of 190 GeV/c pi- into three pions, providing evidence for an exotic meson state.

Contribution

First observation of a J^PC = 1-+ exotic resonance in diffractive pion dissociation with detailed partial wave analysis and resonance parameter extraction.

Findings

Detected a resonance at 1.66 GeV/c^2 with a width of 269 MeV/c^2.

Confirmed the resonance's exotic quantum numbers J^PC = 1-+.

Provided mass-dependent phase difference analysis supporting the resonance's resonant nature.

Abstract

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the pi- 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 420000 events taken at values of the squared 4-momentum transfer t' 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-+ 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-+ 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 is deduced.