The CMS experiment uses a two-level triggering system consisting of the Level-1, instrumented by custom-design hardware boards and delivering an output rate of the order of 100 kHz, and the High Level Trigger, a stream- lined version of the offline reconstruction software running on a computer farm, which sends a rate of several kHz to permanent storage. This system has been evolving continuously since the startup of the LHC. While this system is still in use during the present LHC running period (“Run-3”), new features and algorithms are being developed to take care of higher data loads due to increasing luminosity and pileup but also of new experimental signatures to be investigated (in particular, displaced decay vertices stem- ming from relatively long-lived particles created in proton-proton collisions). Machine-learning techniques are being implemented in the trigger electron- ics and will occupy a more important place in the future. To avoid missing completely unexpected signatures from New Physics, studies are underway to employ anomaly detection using autoencoders.
A major upgrade of the triggering system will happen within the frame- work of the upgrade of the collider to the “High-Luminosity LHC (HL- LHC)”, which will deliver a luminosity of 5 − 7.5 × 1034cm−2s−1, corre- sponding to 140–200 pileup events. An important difference from the present system will be the fact that after the upgrade, information from the silicon strip tracker will be available already for the Level-1 Trigger. This will allow CMS to use so-called “particle flow” objects, i.e. signals seen not only in one subdetector but put together from all available subdetectors, resulting in much sharper efficiency turn-on curves for trigger objects. Also, trigger rates will rise by a significant factor both at Level-1 (to 750 kHz) and at the High-Level Trigger, and the latency - the processing time available for arriving at the Level-1 trigger decision - will increase significantly, allowing for the use of more sophisticated algorithms at Level-1.
zoom: https://cern.zoom.us/j/65711598156?pwd=U2ZhRUZoajU2WFpGM1FoSy8zak1adz09
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