Level 1 Trigger System for the Belle II Experiment

The super-KEKB B factory, currently under construction at the KEK High Energy Accelerator Research Organization in Japan, has a goal of producing 50 ab^-1 of integrated luminosity, thus allowing the Belle II experiment to study rare decays of B mesons, D mesons, and τ leptons. Such large statistics on these decays provide an experimental probe of physics beyond the Standard Model. An online trigger system is indispensable to Belle II to reduce the number of beam background events associated with high electron and positron beam currents, as well as to enhance physics-oriented events. For this purpose, we have designed the Belle II online trigger system with two kinds of primary Level 1 trigger components: a track trigger and an energy trigger. The track trigger is composed of 2-dimensional and 3-dimensional tracking algorithms, and the energy trigger implements algorithms based on total energy, isolated clusters, and identification of Bhabha events. In addition, precise event trigger timing and muon tagging information are provided by the time-of-propagation detector and iron flux return muon detector, respectively.