The Belle II Online Reconstruction and Software Trigger

Nils Braun ; Ryosuke Itoh ; Thomas Hauth ; Chunhua Li

08 July 2018
CHEP 2018

Abstract: The Belle II detector is currently commissioned for operation in early 2018. It is designed to record collision events with an instantaneous luminosity of up to 8⋅10^35 cm−2/s−1 which is delivered by the SuperKEKB collider in Tsukuba, Japan. Such a large luminosity is required to significantly improve the precision on measurements of B and D mesons and Tau lepton decays to probe for signs of physics beyond the standard model. After a fast, hardware-based level 1 trigger stage, a high-level trigger (HLT) system is used to combine the measurement signals from the sub-detectors. This HLT is comprised of a x86-based computing cluster of up to 20 units with 320 cores per unit for highly parallel event processing. A software-based reconstruction is performed to fully reject or prescale background processes and to ensure that relevant signal channels are stored for later analysis. Furthermore, the regions of the innermost pixel detector which should be read out are determined using found tracks in the outer detectors and fed-back to the ONSEN read-out system. The challenging aspects of a software trigger at Belle II is the large beam-induced background due to the much increased instantaneous luminosity compared to Belle. This contribution will report on the design of the online reconstruction software, the properties and performance of the event selection and of first results and insights gained in the 2018 data taking with the Belle II HLT and online reconstruction.

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 Record created 2018-07-08, last modified 2018-07-08

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