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LEP II Data selection

When the decision was made to upgrade the LEP I accelerator to LEP II, there were mainly two goals to achieve. The energy should be upgraded from 45 GeV to reach an energy above 100 GeV and the luminosity should be increased by introduction of so-called minibunches. This means that each bunch in the accelerator was to be split up to 4 minibunches in order to pack more electrons into the ring.

The minibunch scheme was tried out with some success for 45 GeV data in 1995, but when the energy was increased in 1996 big problems occurred. This can be seen in Fig. 4.5, where the long flat line for 1996 corresponds to about 60 days without any stable beams. Finally the minibunch scheme was abandoned for high energy data, but kept for the calibration runs with Z0 data. Clearly the luminosity improved in 1997, and was outstanding for the years 1998 to 2000.

Figure 4.5: The integrated luminosity over year 1990-2000.
Figure 4.6: The integrated luminosity per energy interval.

The minibunch scheme was quite difficult for the VSAT, as the minibunch spacing was shorter than the rise time of the calorimeter signal. Some new electronics were developed [22], but this never really worked as intended and suffered from big problems. So for the VSAT point of view it was good luck that the minibunch scheme was abandoned.

In 1996 the VSAT detector was experiencing some saturations in its fad and strip planes [15], which was fix in the shutdown 1996-1997. In the beginning of 1998 year data taking, it was clear that there should be no more minibunch running at high energy. The rise time problem could then be cured by delaying the VSAT trigger, and since then the energy reconstruction works without any problems. Although a method has been developed to reconstruct the energy for 1996 and 1997 [14], these years are kept out of this analysis. There are three reasons for this:

• In order to recover the correct energy the data needs to be reprocessed in the central DELPHI processing farm, which has not been done.

• The luminosity for 1996 is totally negligible and the 1997 running would add less than 10% to the data, which is not worthwhile with the extra work needed.

• VSAT was moved closer to the beampipe in 1998, and a coherent analysis can not be made with data before that.

The data from 1998 to 2000 can be grouped into six different energy intervals. The whole of 1998 was taken on 94.5 GeV and in 1999 the energy was fixed on four energy levels (96, 98, 100 and 101 GeV). In year 2000 the LEP accelerator was pushed to extreme limits and there was no fixed ( $\pm$1.5GeV) energy within the LEP fills, and an average of 103 GeV was obtained over the whole year. The integrated luminosity delivered to the experiments can be seen in Fig. 4.6. From the VSAT point of view, the luminosity from 1998 should be multiplied with about a factor of 1.8. This is the cross section increase the VSAT gets for $\gamma$$\gamma$ physics due to the movement of the VSAT modules, and it is clear that data before 1998 (or a beam energy less than 94.5 GeV) is almost of no significance.