High Level Trigger role in extending physics reach of the ATLAS experiment at the LHC

The trigger system is an indispensable ingredient of every high energy physics experiment. To study rare processes high luminosities are required which in turn result in a high rate of background. Only the early filtering of events containing interesting signatures makes it possible to carry measurements without unsurmountable burden of analysing already known processes. In the proton-proton, proton-lead and lead-lead collisions at the LHC the ratio of signal to background is about 10−7 − 10−9 depending on the process and centre of mass energy. At the LHC experiments, the trigger system is responsible for implementing bulk of this rejection, preserving about 10−5 (p-p) of collision events. In the filtered sample the interesting signal events are preserved with a high efficiency and backgrounds suppressed significantly. The ATLAS trigger system consists of the hardware electronics system, the Level 1, capable of handling 40 MHz of collisions rate of the LHC and the software High Level Trigger part that is responsible for a precise events selection. For every collision the trigger system is responsible for selecting events from many hundreds of channels thus maximising physics output of the experiment. Due to finite capacity of the detector readout buffers the hardware system is designed to take the decision about an event in 2.5 μs. It uses for that purpose coarse granularity information from the Calorimeters and fast detectors of Muon Spectrometer to reduce the rate to about 100 kHz. The High Level Trigger acts on preselected events and uses full granularity information from Calorimeters, Muon Spectrometer and Inner Detector trackers. For the filtering a resource saving reconstruction in regions indicated by hardware is performed. The flexible system allows also, where justified by efficiency and/or purity, to perform complete event reconstruction. About a thousand of fast and precise algorithms orchestrated by the High Level Trigger steering component are engaged in the decision taking process. As of 2015 the High Level Trigger reduces rate of events which are saved for analysis to 1000 kHz in average processing time of 300 ms per event. In this monograph the High Level Trigger is described. However for the sake of completness also the hardware trigger and surrounding data acquisition components are covered briefly. The purpose and constraint of the system are discussed first and referred to later in the part describ- ing the design and implementation. Bulk of the content contains discussion of performance of triggering strategies from the perspective of the how they are implemented online. Remarks on the steady improvements and planned upgrades of the hardware trigger system and the High Level Trigger are also presented.