Resonance production and ππ S-wave in π−+p→π−π−π++precoil at 190 GeV/c

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Abstract

The COMPASS collaboration has collected the currently largest data set on diffractively produced ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ final states using a negative pion beam of $190\text{ }\text{ }\mathrm{GeV}/c$ momentum impinging on a stationary proton target. This data set allows for a systematic partial-wave analysis in 100 bins of three-pion mass, $0.5<{m}_{3\ensuremath{\pi}}<2.5\text{ }\text{ }\mathrm{GeV}/{c}^{2}$, and in 11 bins of the reduced four-momentum transfer squared, $0.1<{t}^{\ensuremath{'}}<1.0\text{ }\text{ }(\mathrm{GeV}/c{)}^{2}$. This two-dimensional analysis offers sensitivity to genuine one-step resonance production, i.e. the production of a state followed by its decay, as well as to more complex dynamical effects in nonresonant $3\ensuremath{\pi}$ production. In this paper, we present detailed studies on selected $3\ensuremath{\pi}$ partial waves with ${J}^{PC}={0}^{\ensuremath{-}+}$, ${1}^{++}$, ${2}^{\ensuremath{-}+}$, ${2}^{++}$, and ${4}^{++}$. In these waves, we observe the well-known ground-state mesons as well as a new narrow axial-vector meson ${a}_{1}(1420)$ decaying into ${f}_{0}(980)\ensuremath{\pi}$. In addition, we present the results of a novel method to extract the amplitude of the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ subsystem with ${I}^{G}{J}^{PC}={0}^{+}{0}^{++}$ in various partial waves from the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ data. Evidence is found for correlation of the ${f}_{0}(980)$ and ${f}_{0}(1500)$ appearing as intermediate ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ isobars in the decay of the known $\ensuremath{\pi}(1800)$ and ${\ensuremath{\pi}}_{2}(1880)$.

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