Triggering in ATLAS in Run 2 and Run 3

\abstract{The ATLAS experiment at the LHC records about 1~kHz of physics collisions, out of the LHC design bunch crossing rate of 40~MHz. To achieve a high selection efficiency for physics events while reducing the significant background rate, a two-level trigger system is used. The event selection is based on physics signatures, such as the presence of energetic leptons, photons, jets or missing energy. In addition, the trigger system can exploit algorithms using topological information and multivariate methods to carry out the filtering for the many physics analyses pursued by the ATLAS collaboration. In Run 2, around 1500 individual selection paths, the trigger chains, were used, with specified rate and the output bandwidth assignments. The optimization of the trigger menus, the lists of the trigger chains enabled in the data taking, were performed by a dedicated framework using the real $pp$ collisions data. The optimization of the Run-3 trigger menus is currently ongoing. For Run 3, HLT software will be fully upgraded with a multithreading technique. Some improvements on the detector and electronics will be propagated to the trigger system. In this article, for example, the upgrade of the electron trigger project is presented briefly.