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--------------------------------------------------------------------- BES Steps into a New Stage of Upgrade

BES Steps into a New Stage of Upgrade

On June 1, 1995, BES(Beijing Spectrometer) completed its final data- taking preupgrade.

BEPC has operated for five years since 1990. During this period, about $9\time\10^{6}$ J/psi events, $3\time\10^{6} $\psi{'}$ events and $25pb^{-1}$ of $D_{s}$ data at 4.03GeV center-mass-energy were taken. Moreover, the famous accurate measurement of tau lepton mass has been done at $\tau^{+}\tau^{-}$ production threshold.

With all the data collected by BES, many useful and valuable physics accomplishments are achieved through careful and arduous data analysis. We have measured the resonance width of J/psi, the branch ratio of $D_{s}'$s leptonic pair decay, pure leptonic decay and its decay constant $f_{D_{s}}$, the absolute branch ratio of $D_{s}\rightarrow\phi\pi$ and the branch ratio of $\psi^{'}\tightarrow\tau^{+}\tau^{-}$ for the first time in the world. The problem of $\rho\pi$ puzzle is exhaustively studied and a new constraint process was discovered. Besides, in the studies of some interesting mesons, such as $f_{0}(975)$, l(1440), $f_{2}(1720)$, $\xi(2230)$ and so on, good results have been gained, includig the discovery of new decay modes, new structure and new quantum numbers. The efforts in measurements of tau mass and tau neutrino mass especially have the lead in the world.

Apart from the results having been published before, the new results obtained recently in data analysis will be delivered at #FH1995 International Symposium on Lepton-Photon Interactions in Beijing.

While the symposium is prepared, BES goes into a new period of its overall upgrade, from improvement of the separate components to the whole spectrometer.

The main goal of BES upgrade is to make it response more quickly to adapt to much more higher counting rate experiments, resist disturbance more effectively to work in a higher radiative environment and have a better detecting performance to fit more strict physics requirements. In a word, the upgraded BES should have much better performance and more physics accomplishments must be attained in three to five years providing the luminosity of BEPC goes up remarkably.

The upgrade of BES began in 1993, among the main items are:

Redesign and reconstruct a Main Drift Chamber, where many improvements have been done so far.
Redesign and reconstruct a new Time of Flight(TOF) Counter with new materials and components, great improvement could be expected in time resolution.
Reconstruct the Small Angle Luminosity Monitor and replace the Central Drift Chamber (position resolution is 200$\mu$m) with a Vertex Chamber (its space resolution is 60 $\mu\m).
Use large scale programmable gate array technique to rebuild trigger system to meet the requirements of the new main drift chamber and vertex detector.
Use VME-CAMAC I/O and computer $\alpha$3600 to replace VAX785 and VAX-CAMAC I/O to improve the speed of data acquisition.

In addition, many improvements have been made in gas system, n-pentane system, magnet power system, environment monitoring system, especially in shower counter and electronics system.

The upgrading work for computer environment and data analysis software is proceeding well. On one hand, the computing ability, namely, computing speed, disk space and I/O, is greatly enhanced. On the other hand, the UNIX operating system is being widely used.

The main upgrading work concerning physics analysis software are listed below:

To build a HP-UX system platform.
To transplant and build a BES systematical software in HP-UX, including user's environment, filter routines, DPG environment and Codeman system.
To construct a Robot 8mm-tape library.
To reconstruct a new vertex chamber, to transplant and develop calibration routines.
To reconstruct a shower counter and a TOF counter as well as the related calibration routines.
To develop Monte Carlo simulation routines of each subdetector such as shower counter, vertex chamber, main drift chamber and luminosity monitor.
To reconstruct GSOBER, the Monte Carlo simulation frame based on GEANT3.

All the efforts mentioned above are approaching completion now. TOF counter is under assembly and test. The ventilation and test of MDC will be ready at the end of June. The tests of online system have also been done and the event have been recorded and displayed picturely.

At present, assembly of new detectors and disassembly of BES are simultaneously under way. The mounting of new detectors will begin in October and all the improvements will have been finished by the end of the year. The improved BES will have its performance tests early next year. A brand new Beijing Spectrometer will appear soon before us.

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June 1995 BEPC News