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VSAT in DELPHI

The VSAT detector consists of four identical modules placed symmetrically in two pairs on each side of the beam pipe $\pm$ 7.7 meters from the interaction point (Fig. 2.3). This placement is optimal for studying Bhabha events, as the two outgoing electrons from Bhabhas head off back to back and should give a signal in both modules in any of the two diagonals in the detector. In 1998 the LEP beampipe radius at VSAT was decreased from 60 to about 55 mm in size and the VSAT modules are since then placed around 5.8 cm from the beamline. To squeeze down the transverse beamsize at the DELPHI interaction point, there are two focusing quadrupoles in front of the VSAT modules. Outgoing particles will defocuse in x making the VSAT angular coverage to be 3-8 mrad instead of 7.7-10.4 mrad (without the quadrupole the beampipe would in fact shadow the particle trajectory).

Figure 2.3: The position of the VSAT modules.
Figure 2.4: The FAD and strip plane layout of a VSAT module

Each of the modules consists of eleven 5x5 cm² Full Area Silicon Detectors (FADs), interspaced with tungsten layers (Fig. 2.4). When an electron hits the detector it cascades into a particle shower that travels along the detector and is absorbed by the tungsten plates. The FADs are used for energy measurements and VSAT has an energy resolution of about 4% at 45 GeV. Since the detector was optimized for 45 GeV, the gain of the electronics was adjusted  [12] so it could continue to make high precision measurements (4-5% resolution) of the energies above 100 GeV (Fig. 2.5).

Three strip plane layers with 1 mm pitch have been interspaced between the tungsten layers at shower maximum for x- and y-position measurements (precision is about 170 $\mu$m at 45 GeV). The primary purpose of this is to make detector leakage corrections. When the particle shower travels through the detector some part of it will leak out through the sides, and the closer to the edge the shower is, the more energy will be lost. Energy leakage correction is made by correcting the data with exponential curves both in y and x (Fig. 2.6).

Figure 2.5: The energy distribution of Bhabha events before and after the cuts in energy and in x- and y-position. Some background still remains in the Bhabha sample, which has to be subtracted afterwards.
Figure 2.6: The leakage correction curve (solid line) as a function of x-position (shown as a histogram). The drop of the raw energy (dots) at the far edge is due to interaction with a flange in front of the VSAT modules.

The x- and y-position measurements are necessary in order to isolate the Bhabha peak (Fig. 2.5) from the off-energy electron background and to remove both of them from the $\gamma$$\gamma$-collision signal. The VSAT detector is also used as a fast luminosity and background monitor for the DELPHI experiment and the LEP central operation. It therefore has a local trigger system in order to monitor more events. The VSAT can since 1999 store 25 events in a local buffer before it is read out by a DELPHI trigger  [13]. In the unlikely event that the buffer becomes full, VSAT will trigger the whole DELPHI experiment. For LEP II running, there are three different types of events that can trigger the VSAT detector:

• Every time there is a DELPHI trigger, VSAT will be read out as well, no matter what is recorded in the VSAT modules. In the raw data this represents around 25-30% of all the readout triggers. In the primary stage of offline processing, events with no signal in any of the VSAT modules are removed, reducing the DELPHI+VSAT events to about 5% of the total VSAT data.

• If there are simultaneous signals in two of the diagonal VSAT modules the Bhabha trigger is activated and the event is read out. This is the main VSAT trigger and represents about 80% of the data.

• Every time there is a signal in any of the VSAT modules the T1 trigger is activated. This is however heavily downscaled, due to the high rate of off-energy background that hits the detector, so only a small fraction of this is read out. The single electron signal is important for background studies and occupies around 15% of the data sample.

Both the beampipe upgrade in 1998 and the increase of LEP energy from 45 GeV to above 100 GeV put new requirements of the VSAT electronics and trigger system [14]. The problems encountered were however solved [15] and VSAT continued to give high quality data till the end of LEP data taking and dismantling in November year 2000.