The ATLAS Level-2 Trigger

The LVL2 trigger provides the next stage of event selection after the hardware-based LVL1 trigger. It uses the Region of Interest (RoI) guidance received from LVL1 to seed the validation and enhancement of the LVL1 trigger using selected full granularity event data. The LVL2 trigger consists of three stages: (i) The selection of the E/M clusters using the full calorimeter granularity, (ii) The track searching in the Inner Detector (ID) and (iii) The decision making based on the cuts on the reconstructed parameters.

For a quick (and rather simplified) information, have a look at the poster I prepared.

My First Year Report gives a better and more detailed view of the Level-2 Trigger and summarises the work I’ve done during the first seven months of my PhD.

The e/gamma Analysis Package.

The e/gamma Analysis Package calculates the selection efficiencies for electrons (or positrons) and for photons at each Trigger Level (i.e at LVL1 Calorimeter, at LVL2 Calorimeter, at LVL2 ID and at the combined ID and Calorimeter, at the Event Filter ID, at the Event Filter Calorimeter and at the combined Event Filter ID and Calorimeter). Starting from the production ntuples, the selected cuts for each level are applied and finally the electron identification efficiencies, the jet rejection factors and the rates are provided . It is based on the original code written by M. Diaz-Gomez et al.

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Performance studies on the EM Calorimeter.

At the Level-2 Trigger, the EM Calorimeter has a crucial role. The transverse energy deposited on the EM clusters and the position and the shape of these clusters are some of the parameters which are important for selecting the electrons and photons from the background (completely dominated by di-jet events). One the other hand, understanding the resolution of the EM clusters in terms of η and z, contributes in improving the tracking efficiency, when the calorimeter is combined with the Inner Detector (ID).

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The size of the Region of Interest (RoI).

The Region of Interest (RoI) is a very significant and fundamental mechanism from which the HLT starts. The RoI is the geographical object which is formed by the combined signatures of the Calorimeter and the Muon Spectrometer and defines the area where HLT will search at. As in most cases un-interesting events are rejected in an early stage and in a short period of time, the RoI mechanism is a powerful way for fulfilling the HTL requirements for quick and efficient response. Not only the selected RoIs are of major importance, but the size of them contributes in a large extent.

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