Charmed particles in Beijing

1 December 2000


The world’s largest data sample of J/psi particles produced directly from electron-positron annihilation has been accumulated with the BESII spectrometer at the BEPC collider in Beijing. The discovery of this particle in 1974 heralded a revolution in particle physics. In the remarkable progress made since then, the J/psi, composed of a charmed quark bound to a charmed antiquark, continues to provide a useful benchmark.

The BES collaboration successfully completed a scan of R (the ratio of hadron to muon pair production) over 85 scan points in the important collision energy region of 2-5 GeV in 1999. Precision R values in this energy region are crucial for the accurate determination of the quark-gluon coupling constant alpha at the Z mass and the interpretation of the muon (g-2) measurement at Brookhaven, which are essential for precision tests of the Standard Model and for narrowing the mass window for Higgs particle searches.

After finishing the R scan, the collaboration turned its attention to other charmonium (charmed quark-antiquark bound states) physics with the goal of accumulating 5 x 107 J/psi events, about six times higher than the world’s existing largest J/psi sample.

The detector was turned on in mid-November 1999. Data-taking started in December 1999 and ended in mid May with about 2.2 x 107 J/psi events accumulated as planned.

In addition, special data runs were taken at 3.0 GeV and at the J/psi resonance peak for the study of quantum electrodynamics background and the trigger efficiency. Data runs were also taken at the peak of the psi(2S) resonance to help in the determination of the total number of J/psi events.

The J/psi run shows that with BESII and the upgraded BEPC, both the hadronic rate and the integrated luminosity accumulated per day have been increased by a factor of two to three compared with BESI. Also, the upgraded barrel time-of-flight system with a time resolution of about 180 ps significantly improves particle identification.

All the accumulated data has been reconstructed. Preliminary physics analysis shows that the data quality is excellent and that the detection efficiency is higher than the J/psi data collected with the BESI detector.

BES continued accumulating J/psi events in the autumn and hopes to reach the total of about 5 x 107 events before next summer.

With this world’s largest J/psi event sample, the BES collaboration can systematically study light hadron spectroscopy; excited baryonic states such as N*, L* and S*; search for glueballs, chiral partners and exotic states; and probe lepton flavour violation and CP violation using J/psi decays. The collaboration is very excited about the physics that can be done with this unique huge sample.

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