Abstract
We report on the results of a novel partial-wave analysis based on $50\cdot 10^6$ events from the reaction $\pi^- + p \to \pi^-\pi-^\pi^+ + p_\text{recoil}$ at 190 $\text{GeV/c}$ incoming beam momentum using the COMPASS spectrometer. A separated analysis in bins of $m_{3\pi}$ and four-momentum transfer $t'$ reveals the interference of resonant and non-resonant particle production and allows their spectral separation. Besides well known resonances we observe a new iso-vector meson $a_1(1420)$ at a mass of 1420 $\text{MEV\c}^2$ in the $f_0(980)\pi$ final state only, the origin of which is unclear. We have also examined the structure of the $0^{++}$ $\pi\pi$-isobar in the $J^{PC}=0^{-+}, 1^{++}, 2^{-+}$ three pion waves. This clearly reveals the various $0^{++}$ $\pi\pi$-isobar components and its correlation to the decay of light mesons.
Highlights
Light meson spectroscopy has been performed for about 50 years using various tools and production mechanisms, each one possessing its own virtue and sensitivity to particular quantum numbers
We report on the results of a novel partial-wave analysis based on 50 · 106 events from the reaction − + p → − − + + precoil at 190 GeV/c incoming beam momentum using the COMPASS spectrometer
Various beams are being used but in this work we focus on diffractive pion dissociation leading to a 3 final state
Summary
Light meson spectroscopy has been performed for about 50 years using various tools and production mechanisms, each one possessing its own virtue and sensitivity to particular quantum numbers. Various beams are being used but in this work we focus on diffractive pion dissociation leading to a 3 final state. This reaction populates iso-vector states with all possible quantum numbers J P , but limited to C = +1. With 10 to100 times more events as compared to previous works we gain new sensitivity to states with very small production cross section, open up the possibility to study our reactions in terms of t as kinematic variable and take a look inside one of the prominent isobars, namely the S-wave
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