Measurement Of Cross Section For E~+e~-??~-(?)~+ Near Threshold At BES?

Particle physics is one of the foundational subjects to study the elementary building blocks of matter and their interactions.It utilizes high energy particles acquired from accelerators to collide with a fixed target or other high energy particles from the opposite direction.A high-precision detector will record the final state particles from the reaction.Historically,the first accelerators used simple technology of static high voltage to accelerate charged particles.In 1929,Ernest Lawrence invented the cyclotron which was only 4 inches in diameter.In 1983,the Fermi Laboratory had developed a large scale accelerator device and pushed the beam energy to 1 TeV.Now,the world's largest particle physics center,CERN(European Organization for Nuclear Research)has the highest energy particle accelerator LHC in which over 7000 scientists and engineers from more than 80 countries are working on.The particle physics has become a field of"big science" requiring large scale scientific facilities and worldwide collaborations.In China,the particle physics,especially the experimental particle physics,started rather later.The BEPC(Beijing Electron Positron Collider)started construction in October 1984 and started the first collision in 1988.As the first high energy accelerator in China,BEPC has made a series of world-leading research achievements and made outstanding contributions to the study of light hadron spectroscopy and the ?-charm physics.In this paper,the Born cross section and effective form factor of positron and electron annihilation into ?-?+ are studied by using the data of 7 energy points with a total luminosity of 629.0 pb-1 obtained from BESIII detector of the Beijing Electron Positron collider.Prior to this work,the BESIII collaboration has measured the cross section near the threshold of pp,??,?c+?c-and ?±?(?).And in pp,??,?c+?c-,the near-threshold line shapes are found to be inconsistent with the theory calculations.This work could be seen as a continuation of the work of the collaboration in physical analysis near the baryon pair threshold.Baryon pair productions exhibit complicated and strange near-threshold behaviors.It is helpful to measure more baryon pair productions near the threshold.In this paper,we first give a brief introduction to the knowledge of elementary particle physics,including the Standard Model,hadronic structure,and the experiments of electron and electron annihilation to baryon pairs.Then,the analysis process of the paper is introduced in detail,including event selection,background analysis,Born cross section and effective form factor measurement,fitting and systematic uncertainty of this analysis.In order to improve the detection efficiency,we adopt the single tag method.In addition,because of the low statistics,the paper uses the sideband technique to count the signal number and the background number.Finally,the cross section were fitted using the formula near the perturbation QCD threshold,and no significant threshold enhancement was observed near the e+e-? ?-?+threshold.In the future,it is hoped that BESIII could collect more and higher luminosity data near the ? threshold,reduce statistical uncertainty,verify the results of our analysis,or find new behavior that we didn't see due to low statistics.