BEPCII Luminosity Monitor & Upper Limit Of The Yield Of DiOmega In Central Au-Au Collision At S_nn～(1/2)=200MeV With HIJING

The Beijing Electron Positron Collider(BEPC) is consisted of the linear accelerator, the transportation line, the storage ring. BEPC supply e^-e⁺ collision beams for Beijing Spectrometer(BES) and electron beam for Beijing Synchrotron Radiative Facility(BSRF) altemately. For continuously playing a leading role in the tau-charm physical study, we need higher luminosity electron positron collider and more precision detector system. Hence, it is the time to upgrade the BEPCâ… machine and BESâ…¡detector system, to make its performance be greatly improved based on its past successful operation.The BEPCâ…¡use the most advanced configuration of two beam pipes for electron and positron respectively after upgrade. The luminosity will increased up to about 10³³cm^-2·s^-1, while it's energy up to 3.77GeV in center of mass frame. A fast and real time luminosity monitor system will be used in the BEPCâ…¡. The luminosity of each bunch pair is measured by counting photons produced in radiative Bhabha scattering at the interaction point. The merit of the radiative Bhabha scattering is its high counting rate. At BEPCâ…¡, a total of 93 pairs of e^- and e⁺ bunches will collide with each other at the IP. The time interval of adjacent bunch is 8ns. Through measuring the relative luminosity of each bunch pair, the status of each bunch pair will be clearly shown to the beam operator and modifications can be made to keep the bunches in the best status. The total luminosity of 93 pairs of e^- and e⁺ bunches could also be measured if a proper calibration is done. The main achievements of this dissertation include:·The whole process which happened in the detector is simulated by using GEANT4. The design parameters are optimized.·The physical and mechanical design of the luminosity monitor are accomplished.·The luminosity detector is tested by electron beam of BEPC and cosmic ray. Cosmic ray test shows that the responses of the detector to MIP are: the average photoelectron number collected by the photo-cathode of each photomultiplier is 6.9±3.7; time resolution is 0.25ns; the efficiency of MIP is over 98ï¼…(the HV of the PMT is 800V, the threshold is 3mV(equivalence to 2 photoelectron collected by the cathode of PMT)). The results of the electron shows that the responses of the system on electrons are: the average photoelectron number collected by the photo-cathode of each photomultiplier increase when the energies of injecting electrons increase; the average photoelectron number collected by the photo-cathode of each photomultiplier when 400MeV electrons injects is 14.4±8.7 which is more than the cosmic ray test; time resolution is 0.21ns which is similar to the cosmic ray test. According to the comparison of the simulation and the test of the electron beam, the result of the simulation is calibrated. With the loss counting probability and the monitor accuracy are set to 1ï¼…, the proper threshold of the front-end circuit should be the value which corresponds 45.0 photoelectrons are collected by the cathode of the PMT after calibration.·The analysis of the preliminary results of the luminosity monitor running with BEPCâ…¡indicates that it can give the relative luminosity of each bunch pair in real time. The status of each bunch pair which include the time and space information are also can be acquired. These results can be used to provide the fast prescription for the luminosity debugging of the BEPCâ…¡and the choice of the best status.