The CMS High Level Trigger

The CMS experiment has been designed with a two-level trigger system: the Level 1 Trigger, implemented using FPGA and ASIC technology, and the High Level Trigger (HLT), running a streamlined version of the CMS offline reconstruction software on a cluster of commercial rack-mounted computers, comprising thousands of CPUs. The design of a software trigger system requires a tradeoff between the complexity of the algorithms running online, the output rate, and the selection efficiency. The complexity is limited by the available computing power, while the rate is constrained by the offline storage and processing capabilities. The main challenge faces during 2012 is the fine-tuning and optimisation of the algorithms, in order to cope with the increasing LHC pile-up without impacting the physics performance. We present a review of the performance of the main triggers used during the 2012 data taking, ranging from simple single-object selections to more complex algorithms combining different objects, and applying analysis-level reconstruction and selection. We discuss how the increasing LHC pile-up is affecting their performance, and how these effects are being mitigated.