Observation of $\eta (2600)$ and Threshold Enhancements in the $\Lambda \bar{\Lambda }$ System

Abstract

Using $2712.4\pm 14.3$ million $\psi (3686)$ events collected with the BESIII detector, the $\Lambda \bar{\Lambda }$ system produced in $\psi (3686)$ radiative decays is studied. A model-independent partial wave analysis reveals a significant threshold enhancement structure dominated by the ${}^1{S}_0$ and ${}^3{P}_0$ partial waves, corresponding to $J^{PC}=0^{-+}$ and $0^{++}$, respectively. In addition, a new pseudoscalar resonance, designated as $\eta (2600)$, is observed in the ${}^1{S}_0$ partial wave with a mass value consistent with the previously reported $X(2600)$ state, which represents the heaviest light meson observed to date. These results enhance our understanding of baryon-antibaryon threshold dynamics and the pseudoscalar light hadron spectroscopy.

16 pages, 6 figures