### Speaker

### Description

The BESIII experiment at the Beijing Electron Positron Collider (BEPCII) has collected the world's largest data sample at the $J/\psi$ resonance including 10 billion events, which offers an unique opportunity to study hadron spectroscopy and search for light exotic states. In 2003, BESII reported the first observation of a $p\bar{p}$ mass threshold enhancement $X(p\bar{p})$ in $J/\psi \rightarrow \gamma p \bar{p}$. In addition, a structure was observed in $J/\psi \rightarrow \gamma \eta^\prime \pi^+ \pi^-$ and $J/\psi \rightarrow \gamma K_s K_s \eta$ in 2005 and 2015 called $X(1835)$. Both were determined to have spin-parity $J^P = 0^-$. While the masses of the $X(p\bar{p})$ and $X(1835)$ are in agreement with each other, their widths are significantly different, in fact $\Gamma_{X(p\bar{p})} < 76$ MeV (90% C.L.) and $\Gamma_{X(1835)} = 245$ MeV. Additionally, detailed studies of $J/\psi \rightarrow \gamma \eta^\prime \pi^+ \pi^-$ show a significant abrupt change in slope of the $X(1835)\rightarrow \eta^\prime \pi^+ \pi^-$ line shape at the $p\bar{p}$ mass threshold. An overview of recent developments of the mentioned observations as well as approaches to understand the line shape will be presented in this talk.

Graduate Student | Yes |
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