Design of the CMS calorimeter trigger upgrade from Phase i to Phase II of the LHC

The CMS experiment implements a sophisticated two-level triggering system composed of the Level-1, instrumented by custom-design hardware boards, and the software High Level Trigger. In 2017, the LHC delivered proton-proton collisions at a centre-of-mass energy of 13 TeV with a peak instantaneous luminosity larger than 2×10³⁴ cm^-2s^-1, more than twice the peak luminosity reached during Run I and far larger than the design value. The CMS Level-1 calorimeter trigger was upgraded during the long shutdown 1 between 2013 and 2015, to improve its performance at high luminosity and large number of simultaneous inelastic collisions per crossing (pile-up). All the electronic boards have been replaced, tested and commissioned with data. Smarter, more sophisticated, and innovative algorithms are now the core of the first decision layer of CMS: the upgraded trigger system implements dynamic clustering techniques, pile-up subtraction and isolation requirements for electrons and tau leptons. In addition, the new global trigger is capable of computing complex variables such as those involving the invariant mass of trigger objects. The trigger selections used for a wide variety of physics signals during Run II are presented, ranging from simple single-object selections to more sophisticated algorithms combining different objects and applying analysis-level reconstruction and selection. The design and operation of the Phase I calorimeter trigger will be reviewed. The technological choices made influenced the path towards the Phase II upgrade system necessary for the LHC run at a center-of mass energy of 14 TeV with luminosity of 5-7×10³⁴ cm^-2s^-1, corresponding to 140-200 pile-up events. The addition of the tracker information at Level-1 and the enhanced calorimeter granularity will be used to maintain the trigger object thresholds at a similar level as the present system.