Precision Measurement Of R Value In The Continuum Region At BES?

In particle physical experiments,the R value(the lowest order cross section for e?e-?hadrons in units of the lowest order QED cross section for e?e-??+?-)is an important input of the QCD(Quantum Chromodynamics)theory in Standard Model.The precision measurement of R value is always an important topic.Especially in the low energy region,the theory of perturbation QCD is not applicable.There is more demand to measure the R value.It is an input to calculate some important physical pa-rameters,such as the electromagnetic coupling constant,anomalous magnetic moment of the ? and so on.The calculation of these physical quantities is largely limited by the uncertainty of the contribution from hadronic process.It is a great challenge to measure the R value in the low energy region.BESII experiment raise the precision of R value measurement to an average un-certainty of 6.6%in the center of mass energy region between 2 and 5 GeV.The scan of R value gives the clear structure of heavy chamonium resonance.Based on the new detector,we hope to raise the precision of R value measurement via more statistic of data sample and improved measurement method in BESIII experiment.The goal is to give a result with the uncertainty about 3.0%.The measurement is mainly based on the data sample collected at BESIII experi-ment for R scan in the early years.We do more detailed research and inspection to get each parameter taking part in the R value measurement,including the hadronic event selection,the model of hadronic event generator,the measurement of integrated lumi-nosity,the calculation of initial state radiation factor and so on.During the research,we use the mixed hadronic event generator by mixing the improved LUARLW gener-ator with many measured exclusive channels.This will simulate various final hadronic processes more accurately.For the first time,the automatic fitting is used to optimize the parameters of generator in the global space,and the distributions is well compared with that of the experimental data.This help us to get the detection efficiency more accurate and reliable.On the other hand,new generators with larger precision for two photon processes(e?e-? e?e-+X)are used to estimate the remaining background in experimental data.These studies make the error of each measurement well controlled,and get the total uncertainty of the R value less than 3.0%.Using this strategy and method,we hope to make more detailed scanning of the R value in this region by using the large R scan data sample collected by BES? experi-ment.