Cross Section Measurement Of E~+e~-??~0?~0J/? And Partial Wave Analysis At BES?

Heavy quark-antiquark states are an ideal laboratory to understand the nonper-turbative QCD and its interplay with perturbative QCD.Heavy quarkonium plays a prominent role in investigations of QCD dynamics both within and beyond the Standard Model.The spectrum of charmonium states with masses below open charm thresh-old has been well-established for several decades.However,the spectrum of states above open charm threshold is more complicated.Besides the conventional mesons and baryons,QCD also predicts the existence of exotic states,such as glueball,hybrids and multiquark states.Charmonium physics provides an excellent laboratory for study-ing these exotic states and understand the QCD dynamics.The last few years many charmonium-like states,such as Y(4260)and Zc,have been discovered experimentally,which might be the existence of tetraquarks,pentaquarks,molecules,hybrids and other exotic configurations of quarks and gluons.A series of theoretic models were applied to explain these exotic states,and predict their new decay modes and properties.However,the understanding of these states is primarily at the level of conjecture.A compelling and unified understanding of the new states has not yet emerged.Thus,it is necessary to provide more precise measurements of these charmonium-like states and search for their new decay modes.Based on the 12.4 fb-1 e+e-collision data collected by the BESIII detector at BEPCII collider,the cross section of e+e-??0?0J/? is measured precisely at the center-of-mass energies from 3.8077 to 4.5995 GeV.The cross section of neutral chan-nel is consist with the results of charged channel e+e-??+?-J/? with a factor of 0.5.A fit to the cross section line shape confirms the existence of Y(4220)and gives an evidence of Y(4320)with significance of 4.5?.The mass and width of Y(4220)are measured to be M =(4221.3±1.7±1.2)MeV/c2 and ? =(46.1±3.2±2.5)MeV,which shows a good consistent with the results of charged process.Partial Wave Analysis(PWA)is applied to measure the spin and parity of 2g(3900),which is determined to be JP = 1+ with a statistic significance of 6.4? over other quantum number hypotheses.The pole position of Z0c(3900)is measured to be M =(3895,1±4.7±11.4)-i(28.4±4.0±13.7)MeV/c2 with the parameterization of Flatte-like function for Z0c(3900).Meanwhile,the proportion of e+e-??0Z0c??0?0J/? over e+e-?0?0J/? is extracted.However,it is difficult to figure out the relationship of Y states and the production of Z,states due to low statistics and lack of data.More data taken will be helpful to further understand the properties of Y and Z states.In addition,a new method to detect the neutral tracks is studied.Particle identifi-cation for neutral tracks is quite important in physics analysis based on neutral particles.However,the detection method for neutral track is very limited.The general identifica-tion method for(anti-)neutron and photon is based on shower information from EMC at BES? experiment,which has relatively low efficiency.Although the Time-of-Flight detector is designed to measure the flight time of charged tracks,it is possible to mea-sure the time-of-flight of neutral track,which will provide a new method to identify(anti-)neutron and photon,and improve the detection efficiency for neutron and anti-neutron.In this thesis,an algorithm is created to measure the flight time of neutral tracks according to the hit information of TOF at BESIII for the first time.Based on the control samples J/??pn?-+c.c.and J/???+?-?0 selected from data taken at(?)= 3.097 GeV,the flight times of(anti-)neutron and photon are reconstructed,and the corresponding reconstruction efficiencies are studied.