DELPHI Logo Very Small Angle Tagger (VSAT) at DELPHI
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The VSAT is a detector in DELPHI, located close to the beam at 7.7m from the interaction point. The physical processes being monitored are the elastic scattering of electrons and positrons, Bhabha scattering, used to determine the luminosity , and two photon collisions .

VSAT detector layout The detector consists of 4 small electromagnetic calorimeter modules located symmetrically around the LEP beam pipe. The detector modules contain tungsten absorbers (FAD), each interspaced with silicon planes for the energy measurement. The electromagnetic shower coordinates, in the horizontal and vertical directions, are given by silicon strip planes close to the shower maximum.

The signals are amplified, and transported to electronics where they are temporarily stored until a decision is taken if it should be kept or not. The decision is formed by requiring that the energy deposited in one or more module must exceed a given threshold. The signals are digitized in case of positive decision.

Example of bhabha event Due to its placement near the beam pipe and hence high counting rate, the VSAT detector provides useful online monitoring of beam parameters and luminosity. Information is sent to the central DELPHI monitoring system and to the LEP control room. This information contains estimates of the beam situation around the DELPHI interaction point. The picture to the left shows a bhabha scattering as recorded in VSAT. The signal from each silicon plane is drawn on the box, and the signals from the three silicon strip planes inside the box.

In order to obtain good results with small errors, are detector specific parameters, e.g. status of individual channels,continously monitored Any changes in the parameters are recorded for later usage in the analysis of the data.

People Involved

Ulf Mjörnmark
13 october 1997

e+e- annihilation as seen by the DELPHI detector
©1997 Particle Physics, Lund University