Study Of Two-photon Decays Of The Pseudoscalar Mesons

Decays of pseudoscalar meson to two-photons are regarded as an ideal laboratory to test QCD. In flavor-SU(3) theory, the π0,η and η’ mesons belong to the same pseudoscalar nonet.The 77 and η’ mesons are commonly understood as mixtures of the pure SU(3) flavor octet and singlet states: η=η8cosθp - η1sinθp, η’= η8cosθp + η1sinθp, where θp is the mixing angle of the mesons. The η-η’ mixing angle can be extracted from the ratio of B的(J/ψ→γη’)/Br(J/ψ→βη).In chiral perturbation theory,the pseudoscalar mesons is the appearance of an additional degree of freedom.The decay of π0 → γγ was observed with a much shorter lifetime(lifetime = 8.4 × 10-17s).A routine calculation using PCAC fails,it predicts a much longer lifetime. People develop a mechanism called "the triangle anomaly", which can solve this problem. Study the two-photon decays of π0,η and η’can provide experimental evidence for the development of the triangle anomaly mechanism.There are some evidences for the pseudoscalar states of η(1440) that in this mass region exists two structures,η(1405) and η(1475). Usually, one of them is considered as a meson and the other one is glueball candidate. The measurements of two-photon partial width can also help clarify the nature of the η(1760) and X(1835). Otherwise,the width of two-photon decays of η(1405),η(1475),η(1760) and X(1835) is an important parameter for Vector Meson Dominance(VMD) model,which is crucial for the test of VMD model.Using the sample of 4.52×10ψ’ events collected with the BESIII detector in 2009 and 2012, the decays of J/ψ→γπ0(η,η’) → 3γ, J/ψ → γη → 3γ, J/ψ → γη’ → 3γ are studied via ψ’ → π+π- J/ψ. The branching ratios are determined to be Br(J/ψ → γπ0)=(3.47 ± 0.12) × 10-5, Br(J/ψ → γη)=(1.08±0.01) × 10-3, Br(J/ψ → γη’)=(5.51±0.10) × 10-3 and the η-η’ mixing angle is determined to be θP=(-21.77±0.44(stat.) ± 1.96(syst.))°. No η(1405),η(1475),η(1760) and X(1835)signal is evident in the mass spectrum of γγ, and the upper limit of branching ratios are given at 90% confidence level, Br(J/ψ → γη(1405) → 3γ)< 2.86 × 10-6, Br(J/ψ → γη(1475) → 3γ)< 2.40 × 10-6, Br(J/ψ → γη(1760) → 3γ)< 7.88 × 10-6 and Br(J/ψ → γX(1835) → 3γ)< 3.81×10-6.The branching ratio of J/ψ → γπ0(η,η’) obtained from this work is consistent with the result of CLEO, based on the large statistic sample,the branching ratio accuracy obtained in this work has been improved.