Measurements Of B(X_cJ??^-???⁺)and Study Of E⁺e^-??⁰???⁰ At BES?

The thesis comprises investigations of two topics related to charmonium spec-troscopy and strange hyperon-antihyperon physics at BESIII.Despite the successes of the Standard Model(SM),the non-perturbative dynamics of strong interaction is un-doubtedly one of the most challenging areas of contemporary science.The interaction of quarks and gluons in the high energy experiments are well explained by the Quan-tum Chromodynamics(QCD),but still perturbation theory fails to demonstrate where the distance among the quarks comparable to the size of nucleon.As a theory of the strong interaction,QCD has been well tested in the high energy region.However,in the lower energy region,nonperturbative effects are dominant,and theoretical calcula-tions are very complicated.The BESIII spectrometer is an ideal platform to investigate the dynamic of the strong force using charmed quarks,hyperons,and gluon-rich mat-ter that have potential to connect the transition region between perturbative and the non-perturbative QCD regimes.The charmonium resonance ?(3686)has a mass in the transition region between the perturbative and nonperturbative regimes.Therefore,studying ?(3686)hadronic and electromagnetic decays will provide knowledge of its structure and may shed light on perturbative and nonperturbative strong interactions in this energy region Moreover,timelike hyperons form factors are also important ob-servables for probing the inner structure of hadrons and profound understanding to the strong interaction.Base on the facts,quarkonium systems,especially cc states,are regarded as a unique laboratory to study the interplay between perturbative and nonperturbative effects in quantum chromodynamics(QCD).Experimental studies of charmonium decays have ability to test QCD and QCD-based effective field theory calculations.Besides J/?and ?(3686)decays,the decays of the XcJ(J?0,1,2)are also valuable to probe a wide variety of QCD phenomena.The XcJ states belong to the charmonium P-wave spin triplet,and therefore cannot be produced via a single virtual-photon exchange in electron-positron annihilations as are the J/? and ?(3686).Until now the understand-ing of these states has been limited by the availability of experimental data.Recent theoretical work indicates that the Color Octet Mechanism(COM)could have large contributions to the decays of the P-wave charmonium states.Experimental studies of the XcJ(J=0,1,2)states are important for testing models of non-perturbative QCD.The world's largest data set of ?(3686)events collected with the BESIII detector,pro-vides a unique opportunity for detailed studies of XcJ decays,since they are copiously produced in ?(3686)radiative transitions each with branching fractions of about 9%.Using 448.1×106?(3686)events collected with BESIII,we measured the branch-ing fractions of the decays XcJ??-?+ via radiative transition of ?(2S)???cJ and obtained B(?cJ??-?+)?(51.3±2.4±4.1)×10-5,(5.7±1.4±0.6)×10-5,and(4.4±1.7±0.5)×10-5,for J?0,1,2,respectively,where the first un-certainties are statistical and the second systematic.The decays of ?cJ??-?+ with the final states nn?+?-are studied for the first time and it provide the important test of isospin symmetry such as flavor SU(3)in the strong interaction.Hyperons are also ideal probes for studying the strong interaction in the transition region between the nonperturbative and perturbative QCD regimes.In addition,two-body hy-peron weak decays play an important role in the study of symmetry properties in particle physics.Since nucleons constitute the major part of the visible mass of the universe are proven to be non-point like a particle.Besides nucleons,all hyperons in the SU(3)spin 1/2 octet and spin 3/2 decuplet are energetically accessible within the BEPCII energy range.The hyperons,as the SU(3)-flavour partners of the nucleons that contain one or more strange quarks,are extremely interesting because they offer another dimension to study the nucleon structures,and the larger mass of the strange quark provides valuable insights into the environmental sensitivity to the distribution of quarks inside a hadron.Experimental measurements on ? hyperons,especially the ?0 is important to measure in timelike region that provide important input for understanding baryonic structures.The Bom cross section of e+e-??0?0 are determined with high precision near thresh-old to(?)?3.08 GeV.From c.m.energies(?)?2.50 to 3.08 GeV,the Born cross sections of e+e-??0?0 are measured by means of a single baryon-tag technique.Besides,the process of e+e-??0?0 is also studied exclusively with approximately 1.2 MeV above threshold to(?)=2.396 GeV.Non-zero cross sections near threshold are observed by using Vr method.The effective electromagnetic form factors of the ?0 hyperon are determined accordingly.The measurements provide precision experimen-tal inputs for understanding baryonic structures and the observed new feature of form factors brings new challenges for theory.The analysis used 460 pb-1 of e+e-annihi-lation data over a wide range of center-of-mass energies between(?)?2.3864 to 3.08 GeV with the BESIII spectrometer.