The X (1835) and X(pp¯) puzzle at BESIII

The BESIII experiment at the Beijing Electron Positron Collider (BEPCII) has collected the world's largest data sample of J/ψ events (10 billion), which offers a unique opportunity to study hadron spectroscopy and search for light exotic states. In 2003, BESIII reported the first observation of a pp¯ mass threshold enhancement X (pp¯) in J/ψ → γpp¯. In addition, a structure was observed in J/ψ → γη′π+π− and J/ψ → γKsKsη in 2005 and 2015 called X(1835). Both were determined to have spin-parity JP = 0−. While the masses of the X(pp¯) and X(1835) are in agreement with each other, their widths are significantly different. Additionally, detailed studies of J/ψ → γη′π+π− show a significant abrupt change in slope of the X(1835) η′π+π− line shape at the pp¯ mass threshold. An overview of recent developments of the mentioned observations as well as approaches to understand the line shape was given during the conference.