The most straight forward way to calculate the expected size of the off-energy background is to measure the probability to have an off-energy electron in any of the modules. The expected number of background events is then the probability multiplied with the number of hadronic triggers. The easiest way to measure the off-energy electron probability is to measure the coincidence rate with a Bhabha event.
For this purpose both the coincidence of a STIC or a VSAT Bhabha with an off-energy electron was measured [17]. The STIC Bhabha trigger will only use DELPHI events, whereas the VSAT Bhabha trigger covers separate VSAT events. The two individual measurements will provide a double crosscheck and assure a good result. Unfortunately the background conditions vary strongly and rapidly over the year. When the final data is selected the average probability obtained by the STIC and VSAT Bhabha measurement therefore might not be totally accurate.
The off-energy electron probability is therefore also estimated
with a number of other methods (section 5.2). Table
3.1 shows off-energy electron probabilities that were
found to best fit all measurements for the energy intervals used
for LEP II. The probability to have two off-energy electrons in
the VSAT is simply constructed as the joint probability of the two
modules. The coincidence with a single tag
-event is
also calculated in a similar fashion.
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