Beijing tests complete precision measurement

30 August 1999


The BES II spectrometer at the Beijing electron­positron collider (BEPC) has completed a measurement of hadron production rates over the 2­5 GeV energy range which is valuable input for Standard Model calculations.

Three vital input parameters in the electroweak sector of the Standard Model are a, the electromagnetic coupling strength (which depends on energy), the Fermi constant of weak decay and the mass of the Z boson ­the neutral carrier of the weak force.

To test the Standard Model, the electromagnetic coupling strength has to be evaluated at the Z resonance. The LEP measurements of the Z mass are of such high quality that now the error on the coupling strength is a limiting factor in tests of the Standard Model. Its accurate determination of a is critical for the indirect determination of the mass of the Higgs particle. A more accurate value narrows the mass window for Higgs particle searches.

Of particular importance in the extrapolation of a is the hadronic contribution to the vacuum, such as virtual quark­antiquark pairs, which cannot be calculated reliably but can be related to a factor known as R ­ the ratio of hadron to muon pair production in electron­positron annihilation. Uncertainties in the measured values of R in the 2­5 GeV energy range contribute to the error in a.

After the first R scan in spring 1998, the BES collaboration performed a finer scan in the 2­5 GeV energy region ­ almost the extremes of energy region that the BEPC can cover. The scan began in February and finished in early June. Data were taken at 85 energy points. To subtract background, separated beam runs were performed at 26 energy points, and single beam runs for electron and positron beams were carried out at 7 energy points. Special runs were taken at the J/psi resonance to determine the trigger efficiency and calibrate the detector. These runs show that the 12-tracking-layer vertex chamber, rebuilt from the SLAC Mark III endplates and beryllium beam pipe, has a spatial resolution of about 100 µm.

The figure shows the on-line values from the new R scan. Note that the detection efficiency, the background subtraction, as well as the radiative corrections have not been taken into account. The plot includes the R values for 6 energy points measured last year. The upgraded BEPC, as well as the good co-operation and hard work of the BEPC staff, were essential for the success of the scan, which continued even through the traditional Chinese Spring Festival.

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