The evolution of the region of interest builder for the ATLAS experiment at CERN

The ATLAS detector uses a real time selective triggering system to reduce the high interaction rate from 40 MHz to its data storage capacity of 1 kHz. A hardware first level (L1) trigger limits the rate to 100 kHz and a software high level trigger (HLT) selects events for offline analysis. The HLT uses the Regions of Interest (RoIs) identified by L1 and provided by the Region of Interest Builder (RoIB). The current RoIB is a custom VMEbus based system that operated reliably since the first run of the LHC. Since the LHC will reach higher luminosity and ATLAS will increase the complexity and number of L1 triggers, it is desirable to have a more flexible and more operationally maintainable RoIB in the future. In this regard, the functionality of the multi-card VMEbus based RoIB is being migrated to a PC based RoIB with a PCI-Express card. Testing has produced a system that achieved the targeted rate of 100 kHz.