Evidence for a scalar stateI=0 0++(750)from measurements ofπN↑→π+π−Non a polarized target at 5.98, 11.85, and 17.2 GeV/c

Abstract

Measurements of the reactions ${\ensuremath{\pi}}^{+}{n}_{\ensuremath{\uparrow}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}p$ at 5.98 and 11.85 GeV/c and ${\ensuremath{\pi}}^{\ensuremath{-}}{p}_{\ensuremath{\uparrow}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}n$ at 17.2 GeV/c enable model-independent amplitude analyses which yield two solutions for moduli and cosines of certain relative phases of the two $S$-wave and six $P$-wave production amplitudes describing the pion production process below the dipion mass of 1000 MeV. We obtain four solutions for the $S$- and $P$-wave intensities in the physical region of the $\ensuremath{\pi}N\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}N$ reaction. The results for the $S$-wave intensity provide solution-independent evidence for a new resonant state $I=0 {0}^{++}(750)$. Its ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ decay width depends on the solution and is estimated to be in the range of 100-250 MeV. The $I=0 {0}^{++}(750)$ meson is best understood as the lowest-mass scalar gluonium ${0}^{++}(\mathrm{gg})$. Our results emphasize the need for a systematic study of pion production on the level of amplitudes in a new generation of dedicated experiments with spin at the recently proposed high-intensity hadron facilities.

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