The Evolution of the Region of Interest Builder for the ATLAS Experiment at CERN

ATLAS is a general purpose particle detector, at the Large Hadron Collider (LHC) at CERN, designed to measure the products of proton collisions. Given the high interaction rate (40 MHz), selective triggering in real time is required to reduce the rate to the experiment's data storage capacity (1 kHz). To meet this requirement, ATLAS employs a hardware trigger that reduces the rate to 100 kHz and software based triggers to select interesting interactions for physics analysis. The Region of Interest Builder (RoIB) is an essential part of the ATLAS detector Trigger and Data Acquisition (TDAQ) chain where the coordinates of the regions of interest (RoIs) identified by the first level trigger (L1) are collected and passed to the High Level Trigger (HLT) to make a decision. While the current custom VME based RoIB operated reliably during the first run of the LHC, it is desirable to have a more flexible RoIB and more operationally maintainable in the future, as the LHC reaches higher luminosity and ATLAS increases the complexity and number of L1 triggers. We are responsible for migrating the functionality of the multi-card VME based RoIB into a commodity PC with a PCI-Express card. In the ATLAS TDAQ testbed at CERN, the RoIB evolution achieved a rate above 100 kHz with 12 channels using fragments of 400 bytes.