Observation of a J PC = 1 − + Exotic Resonance in Diffractive Dissociation of 190 GeV / c π − into π − π − π +

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The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ final state using a $190\text{ }\text{ }\mathrm{GeV}/c$ 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 ${t}^{\ensuremath{'}}$ between 0.1 and $1\text{ }\text{ }{\mathrm{GeV}}^{2}/{c}^{2}$. The well-known resonances ${a}_{1}(1260)$, ${a}_{2}(1320)$, and ${\ensuremath{\pi}}_{2}(1670)$ are clearly observed. In addition, the data show a significant natural-parity exchange production of a resonance with spin-exotic quantum numbers ${J}^{\mathrm{PC}}={1}^{\ensuremath{-}+}$ at $1.66\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ decaying to $\ensuremath{\rho}\ensuremath{\pi}$. The resonant nature of this wave is evident from the mass-dependent phase differences to the ${J}^{\mathrm{PC}}={2}^{\ensuremath{-}+}$ and ${1}^{++}$ waves. From a mass-dependent fit a resonance mass of $(1660\ifmmode\pm\else\textpm\fi{}{10}_{\ensuremath{-}64}^{+0})\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and a width of $(269\ifmmode\pm\else\textpm\fi{}{21}_{\ensuremath{-}64}^{+42})\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ are deduced, with an intensity of $(1.7\ifmmode\pm\else\textpm\fi{}0.2)%$ of the total intensity.

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