| The development of the high energy physics is the result of pervasion and effect between theory and experiment each other. Of course the theory is important, but it must come from practice, can be tested by experiment and instruct practice inversely. In general, the high energy physics experiment is first to use accelerator to generate high energy particle beam, then let particle beam hit the target or two bunches of particle beam collide and detect the generated particles after collision, at last search the discipline by analyzing the products. During the last two decades, colliding-beam machines have become important in the field of particle physics, become the leading high energy accelerator and digging new particles is most from e+e- collider. The Beijing Electron Positron Collider (BEPC) works in 2-5GeV region, although the energy is low, it has its own advantage in the study of charm quart, τ lepton, low energy R value. This energy region includes series of resonance regions and generating threshold regions, so it has the characteristics, such as little systematic error and background, high measurement precision etc. The BEijing Spectrometer (BES) is an only large general purpose solenoidal detector at the BEPC, on which the physics is to study e+e- collide physics in 2-5GeV region , the emphasis is in the τ-charm physics and the topics include J/ψ physics, ψ (2S) physics, T physics, Ds physics, D physics and hadron's generation mechanism as well as the test of QCD in 2-5GeV. With the cooperation of many domestic and overseas scientists, there are many internationally high level important results, such as "Precise measurement of the mass of the τ lepton", "R value measurement in 2-5GeV", and other studies in J/ψ,ψ (2S), Ds physics. These results bring universal regard and comprehensive attention. From 1990 to the present, BES has accumulated 67 million J/ψ events, 17 million ψ(2s) events, 100 thousand T pairs, 22pb-1events and 2-5 GeV R value scan data. In the paper we study the time-like region form factor of the proton using the 2-3 GeV R value scan data.Nucleon is not pointlike, and the direct evidence is from Dirac equation. If assuming that nucleon is pointlike, from the equation the magnetic moment of the proton is one unit and of the neutron is zero, but their values measured from ex-Vperiment are anomalous. By far there is no perfect theory to describe its complex structure and we can only empirically use electromagnetic form factor to describe its distribution of inner charges and magnetic moments. The nucleon has four form factors. According to proton and neutron's electron and magnetic moment respectively, there are proton's electronic form factor GpE(q2) and magnetic form factor GpM(q2); neutron's electronic form factor GnE(q2) and magnetic form factor GnM/(q2). The early measurements of proton form factor were in the space-like region, and there are a lot of results, which show that empirical dipole formula can work well for proton form factors. While in the time-like region, it is some difficult to measurement proton form factors, because the annihilation cross section is small so that the each experiment group's data is very limited. Especially for 5.9GeV2 < s < 9GeV2, there is no data at present. Running with updated BESII from BESI was firstly devoted to careful scan of the R value covering the center-of-mas's energy of 2-5 GeV. Here we use R scan data of 2-3GeV to study the proton form factor of time-like region.Firstly, BEPC and BES are introduced briefly. BEPC is designed specially for the study of T-charm energy region physics. It is composed of 202-meter-long electron linear accelerator and 240-meter-circumference storage ring. In the linear region the electron and positron are accelerated to 1.1 ~ 1.4GeV, then injected to storage ring in the opposite direction and accelerated to needed energy to collide. At the same time four high frequency cavities supply the energy to beam current continuously,which is lost because of synchronous radiation, and keep be...
|
PDF Full Text Request