title: Puzzle of anomalously large isospin violations in η(1405/1475)→3π.
abstract: The BES-III Collaboration recently reported the observation of anomalously large isospin violations in J/ψ→γη(1405/1475)→γπ(0)f(0)(980)→γ+3π, where the f(0)(980) in the ππ invariant mass spectrum appears to be much narrower (~10 MeV) than the peak width (~50 MeV) measured in other processes. We show that a mechanism, named as triangle singularity (TS), can produce a narrow enhancement between the charged and neutral KK¯ thresholds, i.e., 2m(K(±))~2m(K(0)). It can also lead to different invariant mass spectra for η(1405/1475)→a(0)(980)π and KK ¯*+c.c., which can possibly explain the long-standing puzzle about the need for two close states η(1405) and η(1475) in ηππ and KK¯π, respectively. The TS could be a key to our understanding of the nature of η(1405/1475) and advance our knowledge about the mixing between a(0)(980) and f(0)(980).
authors.affiliation: Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Jia-Jun
authors.initials: JJ
authors.lastname: Wu
authors.forename: Xiao-Hai
authors.initials: XH
authors.lastname: Liu
authors.forename: Qiang
authors.initials: Q
authors.lastname: Zhao
authors.forename: Bing-Song
authors.initials: BS
authors.lastname: Zou