Design and Performance of the Belle II High Level Trigger

The data acquisition system of the Belle II detector is designed for a sustained first-level trigger rate of up to 30 kHz at the design luminosity of the SuperKEKB collider. The raw data read out from the subdetector frontends is delivered in realtime into an online High Level Trigger (HLT) farm, consisting of up to 20 computing nodes housing around 5000 processing cores, where it is reconstructed by the full Belle II reconstruction software in realtime. Based on this online reconstruction, events are filtered before being stored to disk for later offline processing and analysis. In this manuscript we will present the newly developed data flow throughout the HLT system utilizing the open-source library ZeroMQ, which was first used in the beam run period in fall 2019. Further, we show the performance of the HLT reconstruction and how the current setup scales with the anticipated future data rates when SuperKEKB increases its luminosity to the design value.