EXPERIMENTAL STUDY ON J / ψ â†' γγφ IN BES â…¢

Quantum Chromodynamics(QCD) successfully predicts the conventional mesons and baryons in quark model. However, besides conventional mesons and baryons, QCD itself does not exclude the existence of the non-conventional states such as multi-quark states, glueballs, hybrids, etc. The J/ψ radiative decays are believed to be the ideal lab to search for these exotic states. In the years 2009 and 2012, the Beijing Spectrometer III(BESIII) experiment running at the Beijing Electron Positron Collider II(BEPCII) has accumulated 1.31 billion J/y events. This is the largest J/ψ data sample in the world, which provides a new opportunity to study the J/ψ radiative decays and search for the non-conventional states.In this thesis, using the large J/ψ data sample accumulated at the BESIII experiment,we study the radiative process J/ψâ†' γ γ ? and examine the possible exotic states in the γ? invariant mass. We analyze the possible backgrounds and find that the low mass peak of γ ? mainly comes from η(1440) while the f1(1420) contribution is small. By fitting theγ ? mass peak, we determine the mass and width of η(1440).The branching fraction(BR)of J/ψ â†' γ η(1440), η(1440)â†' γ ? is measured to be(6.3±0.7±1.2)×10-6. Compared to the Particle Data Group(PDG), which only gives an upper limit 8.2×10-5 for the BR, our result is more accurate and provide important experimental information for understanding the η(1440) structure. Because the contribution from f1(1420) is small, the upper limit of BR(J/ψâ†' γ f1(1420), f1(1420)â†'γ?) at 90% confidence level is determined to be 2.1×10-6. Furthermore, we observe a broad structure in the high mass region of γ?.The mass and width of the peak are measured to be(1823.9±12.0±9.1)Me V and(143.1± 4.0 ± 15.0)Me V, and the statistical significance is 4.7σ The mass and width are consistent with those of the previously discovered particle X(1835), thus the structure observed here may be the same particle as X(1835). Assuming the structure is X(1835),the branching fraction of J/ψ â†' γX(1835), X(1835)â†' γ? is measured to be(2.8±0.4±0.8)×10-6. Our measurements provide a deeper understanding for the nature of the structure and production of the X(1835).