Study Of E~+e~-?K~+K~- At BES? And Energy Spread Measurement Based On Threshold Effect

The standard model is currently the most successful theoretical model for describ-ing the composition and interaction of substances in the microscopic world.Quantum chromodynamics(QCD)in the model is the basic theory that describes strong interac-tions,which is asymptotically free.Strong interactions can be calculated by perturbation in the high energy region.Meanwhile,in the lower energy region,the strong coupling coefficient increases and the perturbation method will no longer be applicable.It is very important to experimentally study the behavior of QCD at low energy.Even above the non-perturbation energy region,the phenomenon of inconsistency between hadron be-havior and QCD prediction was observed experimentally,such as ? electromagnetic form factor and transition form factor.In addition,QCD also predicts the existence of many exotic states,such as glueballs,miscellaneous states,and multiquark states.Although many experiments have observed some resonances with properties similar to those of exotic states,whose detail properties require further research.The behavior of pion meson,as the lightest hadron,at low energies has been exten-sively studied.Similarly,the behavior of kaon meson has been studied in some experi-ments,among which the best accuracy is the K+K-pair production process measured from BaBar experiment through the initial state radiation process.The results show a discrepancy from the QCD predictions,and there is a sign of new resonance in the pro-duction spectrum.However,the uncertainty of the result is quite large in high energy region,which need further study.In this paper,based on experimental data at 2—3 GeV collected by BESIII detector at BEPCII,the generation cross section of e+e-? K?K?process has been studied,and the K+/K-electromagnetic form factor is extracted.In the study,the accuracy of the performance parameters of the accelerator and the detector will affect the systematic uncertainties of the analysis results.Physical analysis needs the study of these parameters.In this paper,based on the data collected by the detector,several methods for measuring the energy spread of the accelerator beam are proposed according to the characteristics of some physical processes.In the study of the e?e-? K+K-process,the background from cosmic rays and multi-body processes are suppressed by using the energy measured by the Electro-magnetic Calorimeter and the time of flight measured by Time-Of-Flight detector.The excellent momentum measured by the Main Drift Chamber is used to separate the signal from other two-body processes.A quite good signal to noise ratio is obtained for the signal process.Here,we avoid using the particle identification algorithm which is com-monly used in BES? analysis,but extract the signal using characteristic momentum based on the feature of the two-body process.This method can effectively avoid the system uncertainty caused by the particle identification algorithm and improve the ac-curacy of the cross section measurement.At the same time,Monte Carlo simulation has been used to study the detection efficiency of the signal process and the influence from the initial state radiation and vacuum polarization.Then,the cross section can be calcu-lated using the number of measured signal events,the efficiency,the correction factor,and the luminosity measured via the Bhabha process.In the cross-section line shape of the e+e-?K+K-process,we find a new resonance at 2.2 GeV.By fitting the line shape,the parameter of the resonance is measured to be m = 2239.2±7.1 ±11.3 MeV/c2 and ? =139.8 ± 12.3 ± 20.8 Me V.From the cross section of the e?e-? K?K-pro-cess,we can extract the form factor of K+/K-.In a energy range higher than 2.4 GeV in the line shape of the form factor,we verify the trend of the energy-dependence of the form factor predicted by the quantum chromodynamics.In the study of energy spread measurement,we propose to measure the energy spread by using the influence of energy spread on the near-threshold production of baryon pairs and on the spectrum resolution of some quantities.In these two methods,the Monte Carlo simulation is used to provide the relationship between the characteristic physical quantity and the energy spread.From the relation,we can measure the energy spread.Both energy spreads given by the two methods at 4575 MeV with ?+c?--c pair production are around 2 MeV.This result is consistent with the design of the BEPCII and agrees with the extrapolation of the measured value from the J/? energy region to 4575 MeV.