Studyη’→π~+π~-π~+π~-and η’→π~+π~-π~0π~0Decay At Besiii

Since the discovery of η’ in1960s,extensive theoretical and experimental efforts have been devoted towards achieving a better understanding of its properties and interactions with other particles due to its special role in chiral perturbation theory.Although the main η’ decays have been well studied,the decay mode η’→4π is still an open field due to lack of data.The four-pion decays have not yet been observed,and the best upper limits until now come frm the CLEO collaboration:B(η’→π+π-π+π-)<2.4×10-4and B(η’→π+π-π0π0)<2.6×10-3at the90%confidence level(C.L.).Three decades ago,a theoretical calculation using the broken-SU6×03quark model yielded a branching ratio of1.0×10-3fOr B(η’→π+π-π+π+(0)π-(0)).Recently Guo,Kubis and Wirzba,using a combination of chiral perturbation theory and a vector-meson dominance model,obtained the following prediction:B(η’→π+π-π-π+π-)=(1.0±0.3)×10-4and B(η’→π+π-π0π0)=(2.4±0.7)×10-4.In this Letter,we report the first observation of η’→π+π-π+π- and η’→π+π-π0π0decays coming from J/ψ→γη’ radiative decay events using a sample of1.3×109J/ψevents (2.25×108events in2009and1.09×109in2012) accumulated at the BESIII experiment.In experiment,MC simulations are used to determine detection eficiencies or study backgrounds. However,event generators with precision are essential to do a reliable MC simulation to remove the systematic uncertainty as much as possible.So we developed generators of some η/η’For the first time,we observed the decay modes η’→π+π-π+π- and η’→π+π-π0π0 with a statistical significance of18.3σ and5.7σ,respectively,and measured their branching fractions: B(η’→π+π-π+π-)＝(8.41±0.68(stat.)±0.63(sys.))×10-5and B(η’→π+π-π0π0)=(1.90±0.36(stat.)±0.19(sys.))×10-4, which are consistent with theoretical predictions based on a combination of chiral perturbation theory and vector-meson dominance.