Observation of exotic meson production in the reaction ${\pi }^{-}p\to {\eta }^{\prime }{\pi }^{-}p$ at 18 GeV/c

Abstract

An amplitude analysis of an exclusive sample of 5765 events from the reaction ${\pi }^{-}p\to {\eta }^{\prime }{\pi }^{-}p$ at 18 GeV/c is described. The ${\eta }^{\prime }{\pi }^{-}$ production is dominated by natural parity exchange and by three partial waves: those with $J^{PC}=1^{-+},2^{++},$ and $4^{++}$. A mass-dependent analysis of the partial-wave amplitudes indicates the production of the $a_2(1320)$ meson as well as the $a_4(2040)$ meson, observed for the first time decaying to ${\eta }^{\prime }{\pi }^{-}$. The dominant, exotic (non-$q\bar{q})$ $1^{-+}$ partial wave is shown to be resonant with a mass of $1.597\pm 0.010_{-0.010}^{+0.045}$ GeV/c^2 and a width of $0.340\pm 0.040\pm 0.050$ GeV/c^2 . This exotic state, the ${\pi }_1(1600)$, is produced with a $t$ dependence which is different from that of the $a_2(1320)$ meson, indicating differences between the production mechanisms for the two states.

5 pages with 4 figures