Search For η’→γγη At BESⅢ

The Beijing Spectrometer III(BESⅢ),operating at the Beijing Electron Positron ColliderⅡ(BEPCII),is a general purpose detector,which is designed for(?)-charm physics.At present,a sample of 1.31×109J/ψevents was accumulated at the BESIII detector,and with the advantages of low background,simple topology of final states and high detection efficiency,the world’s largest samples of J/ψ events offer a unique opportunity to study light meson spectroscopy and light meson decays via charmonium decays.The η’ meson provides a unique place for understanding the distinct symmetry-breaking meehanisms in low-cnergy Quantum Cnrodynamics(QCD)and its decays play an important role in exploring the effective theory of QCD at low energy.Based on the frameworks of the chiral-loop,Liner σ Model(Lσ)and the Vector Meson Dominance(VMD)model,the branch-ing fraction of decays η’→γγη was predicted to be 2.6×10-4,and the dominant contribution is from the VMD.The branching fraction of J/ψ→γη’(5.15±0.16)×10-3,is quite large.At present,a sample of 1.31×109 J/ψ events was collected with the BESIII detector,corresponding to 6.75×106η’events,which offers an opportunity to explore the vU rare decays.In this thesis,we performed a search for η’→γγη via J/ψ→γη’for the first time.No evident η’ signal is observed in the γγη invariant mass spectrum.Using the Bayesian approach,the upper limit on the branching fraction of η’→γγη is determined to be 1.18×10-4 at the 90%confidence level.Of interest is the upper limit is lower than the theoretical prediction by a factor of 2.Most recently another sample of 3.7×109 J/ψ events was collected at the BESIII detector,which allows us to make a further investigation on the decay ofη’→γγη.The new result is then expected to have a strong constraint on the theoretical models.