With the blow of a whistle at 8 a.m. on 30 April, the Beijing Electron Positron Collider (BEPC) finished running and the installation of its major upgrade, BEPCII, began. By the end of October the first stage, including the upgrade of the linac injector and the removal of the Beijing Spectrometer (BES) from the interaction region, will have been carried out. The upgrade will be finished by the end of 2006 and physics running should be resumed by the spring of 2007. To minimize interruption to the users of the Beijing Synchrotron Radiation Facility, the upgrade is planned in three stages, with synchrotron radiation runs in between.
BEPC has been running in the energy region of the tau and charm for more than 15 years, with many notable experimental results. However, to meet the challenges in the precision measurements of the charm energy region, a thorough upgrade is necessary if the facility is to continue productive studies and to lead the world in this research. The Chinese government approved the BEPCII programme, which has a budget of 640 million Chinese yuan ($77 million) and a construction period of three years.
To meet the challenging goal of continuing world-leading studies of charm physics, a double-ring design has been chosen. A storage ring will be added in the existing tunnel so that the electrons and positrons can travel separately in their own rings. The number of positron and electron bunches will be increased from 1 to 93 in each ring, with a large horizontal collision angle of ±11 mrad. In addition, other new technologies have been adopted – such as a superconducting radiofrequency system, superconducting micro-beta quadrupoles and a low-impedance vacuum chamber – so that the performance of BEPCII will be improved by a factor of 100, for a design luminosity of 1033 cm-2s-1 at a centre-of-mass energy of 3.77 GeV. As the circumference of BEPCII is only 240 m and the straight section of the interaction region is rather short, many technical challenges will have to be overcome to meet the design goals.
At the same time, the Beijing Spectrometer is being upgraded to improve its measurement precision and reduce systematic errors, as well as to adapt to the high event rate of BEPCII. The upgraded BESIII includes a CsI calorimeter, a superconducting solenoid magnet and a main drift chamber with small cells and helium-based gas.
The removal of BES and the upgrade of the linac mark the beginning of the BEPCII installation. After the upgrade, the positron injection rate of the linac will reach 50 mA per minute, an improvement of a factor of 10, with full energy injection up to 1.89 GeV. Further milestones will involve the dismantling of the old storage ring and the installation of the new double ring, from April 2005 to January 2006, followed by the moving of BESIII into the interaction region
in October 2006.
The upgraded BEPC should be able to maintain its leading role in charm-physics research, with new results in the search for glueballs, quark-gluon hybrids and exotic particles, precision measurements of the R-value, precision measurements of the Cabibbo-Kobayashi-Maskawa matrix element, the study of the charmonium spectrum and charmonium decay properties, and so on. The hope is that BEPCII will provide a new platform for productive and fruitful physics, not only for Chinese physicists but also those from around the world.
