  The future Belle II experiment will employ a computer-farm based data
reduction system for the readout of its innermost detector, a DEPFET-technology
based silicon detector with pixel readout. A large fraction of the background
hits can be rejected by defining a set of Regions Of Interest (ROI) on the
pixel detector 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 trackers (a 4 layer
double-sided silicon strip detector surrounded by a wire chamber). The tracks
are reconstructed in real time on the High Level Trigger (HLT). The pixel
detector is then read out based on the ROI information. A demonstrator of this
architecture was under beam test earlier this year in DESY (Hamburg, Germany).
The demonstrator was operated in 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 pixel sensor and 4 silicon
strip sensors arranged in a 5 layers configuration mimicking the Belle II
vertex detector. The detector readout was a scaled down version of the full
Belle II DAQ + HLT chain. The demonstrator was used to detect the particles,
reconstruct in real time the trajectories, identify the ROIs on the PXD plane
and record the PXD data within. We describe the requirements and the
architecture of the final system together with the results obtained with the
demonstrator.
