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Introduction

LEP machine background and noise in the electromagnetic calorimeters can affect many DELPHI analyses. Particularly sensitive are the analyses of two photon interactions [1] and events with a single photon in the final state [2] since these analyses select or trigger on energy in the calorimeters. However, analyses which veto on energy in the calorimeters e.g. STIC also need to take this background into account.

In this note several different studies of calorimeter background have been compiled. At low angles, i.e. in VSAT and STIC, the most troublesome background comes from off-energy electrons caused by bremsstrahlung from beam particles on rest-gas molecules. This background has been simulated in DELPHI [3]. The simulation has given a better understanding of the production and origin of the background but cannot be used for quantitative estimates, since it needs as input the vacuum pressure in LEP, which is not known in detail.

The only way of estimating the off-energy background is to use real data. Any sample of events which is not expected to give electrons in VSAT can be used to estimate the probability of an off-energy electron in VSAT. A few basic questions need to be answered: What is the rate of the background (normalized to luminosity) and how does it vary with time ? What is the probability that an off-energy electron is recorded together with a genuine physics event ? How can the background best be rejected ?

The off-energy background in VSAT has been estimated with three different data samples: VSAT Bhabha events, STIC Bhabha events and muon events. All the analyses were done with 1998 high energy data.

Three different event samples were also used for the study of off-energy electrons in STIC: the STIC single arm events, the STIC Bhabha events and random triggered events. Data from both 1998 and 1999 have been studied.

In the other DELPHI calorimeters such as FEMC, HPC and HAC, there is no off-energy electron background but noise can cause spurious showers. This problem was studied with the 1999 random triggered events.