Event selection to reduce pileup in data from a pixel calorimeter prototype

Abstract

To learn more about matter at high density, the installation of a forward calorimeter (FoCal) is proposed as an addition to the ALICE detector at CERN. A small prototype for this calorimeter is build, the mTower. In February 2020, measurements with electron beams of a few GeV have been performed at the DESY test beam facility in Hamburg with this prototype to test its performance. The measurements suffer from pileup, where multiple electrons enter the detector during one event. To assess the performance, measurements of single electrons in the prototype have to be selected. A set of 5 criteria has been developed to make this selection of events without pileup. These 5 criteria reduce the amount of pileup substantially, but there is still an asymmetry present in the final distributions of the number of hits, which suggests that not all of the events with pileup are removed. However, it is not possible to reduce the pileup further with the current available data. The selection criteria are applied to the current data set, resulting in a sufficiently clean sample. The distributions of the number of hits are shown for several electron beam energies. For future test beam measurements, the amount of pileup should be reduced by for example lowering the electron beam rate or by changing the trigger setup to create a new source of data which can be used to identify pileup.