The upgrade of the CMS trigger system

The LHC accelerator at CERN in Geneva is being upgraded to increase its energy and luminosity, which requires that the CMS (Compact Muon Solenoid) detector and its trigger system are also upgraded to make full use of higher collision rates. The trigger upgrade will proceed in several stages. At present the hardware-based Level-1 trigger is being upgraded to improve the resolution and thus allow for efficient data taking at higher collision rates while keeping the present Level-1 trigger rate of 100 kHz. The new system will be running in parallel to the existing Level-1 trigger for commissioning in 2015, when LHC starts running again, and will take over the full trigger functionality in 2016. A fundamental change in the trigger is planned for the time when the CMS silicon tracker is replaced in 2022. While at present the tracker does not send data to the Level-1 trigger and is only read out when a positive Level-1 trigger decision is received, the new tracker will be integrated into the Level-1 trigger. For that second upgrade stage, a significant increase of the Level-1 trigger rate by a factor of up to ten is planned.