VSAT events by type - definition.

There are main three main types of events in VSAT:

• Bhabha (BH)
  events as a real sign of interaction of between beam particles - it is just an elastic scattering of electrons on positrons (or vice versa). It is a event of two simultaneous hits in opposite modules which forms a diagonal or arm (1 and 4 or 2 and 3). The energy should be above the upper energy threshold. Some other requirements as the exactly opposite coordinates and momentum are under check only in off-line due to the lack of time and complexity of the decision for the on-line trigger.
• Single Electrons (SE)
  considering as a clear background subtracted mainly on-line. Single electron events it is just simply hit on one of four VSAT modules with energy greater than lower threshold. In the same time some of this events are taken into data with special mark to be a good sample of background for further analysis . The down-scaling parameters which define the part of SE to be recorded can vary for outside (higher background rate due to the circumference force) and inside (lower background rate) modules.
• False Bhabhas (FB)
  or Accidental Bhabhas - coincidence of two Single Electrons "simultaneous" events which should be subtracted in off-line. Like in Bhabha case two hits should be above the higher thresholds in the opposite (diagonal) modules but the time differs by 4 BCO (Beam Cross Over) - i.e. by four bunches. This type of events was used as one more sample of more hidden background. Since the 1998 this type of events was excluded since high trigger rate, well working alghoritm to evaluate FB rate from SE sample and different cuts which can wash out the FB from Bhabha sample.

The number of Bhabha events will give one quite good estimation about Luminosity (i.e. number of e⁺e^- collisions) in DELPHI.

The number of Single electron events is used as background rate criteria.

The False Bhabhas is also a type of background but similar to Bhabha and it have importance only for off-line data processing.

Rates of events described above are in Rates Page.