Even though the LEP ring has one of the best vacuums in the world, some beam gas particles still reside inside the beam pipe(static pressure). Besides this there are also radiation pressure coming from the beam itself in the form of synchrotron radiation(dynamic pressure). Beam particles interact with both of these contaminations and loose energy by bremsstrahlung radiation. These off-energy particles will experience directional changes as they are passing through the LEP magnets and the so called off-momentum background is produced.
Particles with big energy loss and momentum changes will leave the beam path faster and as a result the high energy background comes from far away. Only background produced less than 300 meters away from DELPHI will reach the experiment [10]. The vacuum and optics situation in the LEP ring produced two types of off-momentum background in the VSAT. The inner modules was radiated with a background coming from a region close to DELPHI, whereas the background in the outer modules had its origin in a bad vacuum pressure region far away. As a result the background in the outer modules has higher energy(fig 4.1), better confinement to the horizontal plane(fig 4.2) and much higher trigger rate(fig 4.1).
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