Study Of The Pair-production For Double-strange Baryon Ξ Near Its Kinematic Threshold And Through The Decay Of Charmonium χ_cJ At BESⅢ

With the discovery of Higgs boson,all the fundamental particles predicted by the Standard Model of particle physics were confirmed by experiment.Although the Standard Model has achieved great success,there are still many problems to be solved within its theoretical framework.For example,the inner structure of baryon and the non-perturbative effect of strong interaction at low momentum transfer.The BEijing Spectrometer(BESⅢ)worked at a transition energy region with the interplay of nonperturbative QCD and perturbative QCD.This energy regime covers abundant resonance structures and baryon-pair production thresholds,which provide an ideal platform to investigate the aforementioned problems.In this thesis,two related experimental studies are presented based on the datasets collected with the BESⅢ detector.Baryon has inner structure,this has been confirmed by experiment,e.g.the anomalous magnetic moment of nucleon,the anomalous differential cross section in protonelectron elastic scattering.If taking into account the distribution of charge and magnetic moment in baryon,the inner structure of baryon can be parameterized by the electromagnetic form factors,which can describe the non-point like behavior in experiment.The study of form factor is very important for the understanding of baryon properties and the QCD theory of strong interaction.The space-like form factor can be studied in electron-nucleon scatting,but this is difficult for hyperon case.In the 1960s,physicist proposed that time-like form factor can be studied by measuring the cross section of baryon-antibaryon production from electron-position annihilation.Relevant experimental studies observed anomalous behavior of the baryon-pair production cross section in the near threshold region.Thus,BESⅢ experiment collected data at energies close to a series of baryon-pair thresholds.Using the data with total integrated luminosity of 363.9 pb-1 collected at eight energy points from 2.644 to 3.080 GeV,the Born cross sections of e+e-→Ξ-Ξ+ process and the effective form factors of Ξ-baryon are measured for the first time.Due to limited statistics,the non-zero cross section near threshold is not significant with statistical significance less than 1σ,so the upper limit for the cross section near threshold is calculated at 90%confidence level.In addition,the measured cross sections are consistent with the neutral Ξ0 baryon case within uncertainty in the same energy region.The measured effective form factors at threshold are not consistent with the point-like expectation,this is different to the nucleon cases,and indicates the special role of strange quark in baryon.Thus,it is necessary to further understand the strange quark as well as the inner structure and dynamics of baryon.The experimental results in this thesis will be references for the relevant studies.Baryon-pair can be produced from charmonium decay,this process is an important tool to study the QCD.The Color-Octet Mechanism(COM)is a QCD-based method,which has been well applied in the inclusive decay of P-wave charmonium.Recent study points out that the exclusive decay of P-wave charmonium should include the contribution of COM as well.Currently the COM can be applied to the processes of meson-pair final state.But for the baryon-pair cases,the COM can not give a universal prediction.In addition,the helicity selection rule,obtained from the perturbative QCD,prohibits charmonium χc0 decay to baryon-pair,however this suppressed process has already been observed in experiment.Currently higher order correction is considered in some theoretical calculations to explain this phenomenon,but discrepancy still exist in some processes between data and theoretical calculation.Thus more experimental result is desired.Charmonium χcj is the P-wave triplet,the abundant production of χcJ can be obtained through the ψ(3686)→γχcJ process.Based on(448.1 ± 2.9)× 106ψ(3686)decay events collected with the BESⅢ experiment,the branching fractions and angular distributions for the processes χcJ→Ξ-Ξ+ and Ξ0Ξ0 are measured preciously.The processes χc1→Ξ0Ξ0 and χc2→Ξ0Ξ0 are observed for the first time,and the statistical significance are 7σ and 15σ,respectively.The measured branching fractions of other processes are consistent with previous results,and the precision of these results are improved significantly.In addition,with respect to the angular distribution measurement,the results for the processes χc0→Ξ-Ξ+ and Ξ0Ξ0 are consistent with the isotropic distribution expected for the decay of a scalar meson,and these results can be treated as a cross check for the remaining four angular distribution measurements.These experimental results will be references for the understanding of hadron property and decay.