| Flavor-changing neutral current(FCNC)is a rare decay mode in the Standard Model(SM).The study on FCNC could be used to verify the Standard Model and search for the New Physics.FCNC processes are forbidden at the tree level but could occur at higher loop level.They are highly suppressed according to the GIM mecha-nism.The decay mode D0→ γγ in this thesis is a FCNC process with charm number changed(AC = 1)and quark electric charge conserved(c→ u).D0→ γγ decay is predicted to be dominated by the long distance effect in the SM.The calculation by Vector Meson Dominance(VMD)and HQχPT both predict the branching fraction to be at 10-8 level separately.In the context of the minimal supersymmetric standard model(MSSM),gluino exchange can enhance the SM rate by 2 orders of magnitude。The precise measurement of the branching fraction of D0→ γγ provides a good opportunity to verify the SM and search for the New Physics.The BES III detector has already collected data at ψ(3770)peak energy with the integrated luminosity of 2.92 fb-1,which is the largest ψ(3770)sample of the world at present.It will provide a good opportunity for the research of charm physics.In this thesis,the D0→ γγ analysis is based on this data sample.The D0→ π0π0 process is also studied because it is an important background for D0→ γγ and it is used for normalizing the branching fraction.D0 → γγ is a rare decay mode.Blind analysis is introduced for the reduction of bias.Also for a clean signal shape,the background must be suppressed.Both single tag and double tag methods are used in order to find the best solution.After the final selection for D0→ γγ,no significant signal is observed.With the single tag method,by calculating B(D0→ γγ)/B(D0→ π0π0)and the branching fraction of D0 →π0π0 in PDG,the branching fraction for D0→ γγ could be determined.In this thesis,the upper limit is set as:the upper limit for D0→ γγ is:B(D0→ γγ)<4.9 x 10-6,@90%C.L.(8)With double tag method,the final upper limit of D0 → γγ is:B(Do→ γγ)<3.8 x 10-6,@90%C.L.(9)This result is better than the single tag method,and is chosen to be the final result.This result could still not reach the precision predicted by the SM and is con-sistent with the SM.The result is at the same level as the BABAR experiment where D0→ γγ was measured via γ(4S)decay.The integrated luminosity in BES Ⅲ is far below BABAR,but the results are comparable because of the low background level ofψ(3770)→ DD decay.The D0 → π0π0 mode is also studied and the branching fraction is:B(D0→π0π0)=(8.24 ± 0.21 ± 0.30)x 10-4(10)where the first and second errors are statistical and systematic.This result is consistent with the one from the BABAR experiment and is the most precise to date.The strong and weak interactions could be easily separated in semileptonic decay modes and the strong interaction could be described by the form factor.They are good places for the measurement of CKM matrix elements.The measurement of branching fraction of D+→ ωe+ve,φe+ve is helpful for a complete understanding of charm semileptonic decays.Also it will throw light on the mixing between ω and φ.In the thesis the absolute branching fractions of D+→ωe+ve,φe+ve are mea-sured using 2.92 fb-1 data at ψ(3770)energy.The D tagging method is used in the analysis.The signal is reconstructed from the recoiling side of the tagged D meson.The neutrino is not able to be detected,and the final signal number is achieved by fitting the Umiss distribution.The results for the absolute branching fraction of D+→ ωe+ve is:B(D+→ ωe+ve)=(1.63 ± 0.11 ± 0.08)x 10-3(11)The uncertainties are statistical and systematic.The precision is much better than the one from CLEO experiment.No significant signal is observed after the selection of D+ → φe+ve,and the upper limit of the branching fraction is set as:B(D+→φe+ve)<1.3 x 10-5,@90%C.L.(12)This result is also better than the latest results from CLEO experiment. |
