Demonstrator of the Belle II online tracking and pixel data reduction on the High Level Trigger system

We present a computer-farm based data reduction system for the pixel detector of the future Belle II experiment. Belle II is a major upgrade of the Belle detector that is currently ongoing in KEK (Tsukuba, Japan). Belle II will collect data from SuperKEKB, an asymmetric e⁺ e^- collider whose target luminosity (8 × 10³⁵ Hz/cm²) is a factor 40 larger than the present world record currently held by KEKB. The higher trigger rate (10 kHz) and worse background conditions (3% average occupancy on the pixel sensors) are a major challenge for both the detectors and the Data Acquisition System (DAQ) in particular for the inner part of the Belle II tracking system that consists of two cylindrical layers of DEPFET pixels sensors (PXD) placed just outside the beam pipe and very close to the Interaction Point (IP). The long integration times of the PXD together with the high background rate expected from pairs production occurring at the IP will lead to a large amount of data (more than 20 GB/s) that must be reduced by at least a factor 10 to satisfy the constraints coming from the available store space and bandwidth. A large amount of the background hits can be rejected by defining a set of Region Of Interest (ROI) on the PXD sensors and then recording just the data from the pixels inside the ROI. The ROIs are defined on an event by event basis by extrapolating back onto the PXD the charged tracks detected in the outer silicon tracker (a 4 layer double sided silicon strip detector) and reconstructed on line in real time. A demonstrator of this architecture was under beam test earlier this year in Desy (Hamburg, Germany). The demonstrator was operated on an electron beam whose momentum was in the 2 - 6 GeV/c range with a typical trigger rate of a few kHz in a magnetic field of strength up to 1 T. The demonstrator consists of one PXD sensor and 4 SVD sensors arranged in a 5 layers configuration mimicking the Belle II VXD. The det- ctors read out was a scaled down version of the Belle II DAQ + High Level Trigger (HLT) chain. The demonstrator was used to detect the particles, reconstruct in real time the trajectories, identify the ROI on the PXD plane and select the PXD hits belonging to it. In this contribution, we describe the requirements and the architecture of the final system together with the results obtained with the demonstrator.