Speaker
Description
Lattice QCD and models predict the lightest hybrid meson with explicit gluonic degrees of freedom to have spin-exotic quantum numbers $J^{PC}=1^{-+}$ and a mass below $2\,\text{GeV}$. Experiments have reported two candidates with such exotic quantum numbers, the $\pi_1(1400)$ in the $\eta\pi$ channel and the $\pi_1(1600)$, which has been observed in various final states, including $\eta^\prime\pi$. However, two spin-exotic states at roughly the same mass are not expected. This puzzle was recently resolved by an analysis by JPAC performed on COMPASS data of the $\eta^{(\prime)}\pi$ final states.
The analysis presented in this talk uses the full $\eta^{(\prime)}\pi$ data set measured at the COMPASS experiment. As in the earlier JPAC analysis, we employ a coupled-channel $N/D$ amplitude model that incorporates theoretical constraints such as unitarity and analyticity. Intensities and relative phases of the $J^{PC}=2^{++}$ $D$-wave and the exotic $J^{PC}=1^{-+}$ $P$-wave are simultaneously fitted. By analytically continuing the amplitudes to the complex plane, we extract the pole positions of all relevant resonances. We find a single pole in the $P$-wave, consistent with previous results, and two poles in the $D$-wave associated with the two well known states $a_2(1320)$ and $a_2^\prime(1700)$. The high statistics of the full $\eta^{(\prime)}\pi$ data set also allow for a future $t^\prime$-dependent analysis to further improve the separation of resonant and non-resonant contributions.
