Upgrade of the ATLAS Level-1 Calorimeter Trigger

The Level-1 Calorimeter Trigger (L1Calo) of the ATLAS experiment has been operating well since the start of LHC data taking, and played a major role in the Higgs boson discovery. To face the new challenges posed by the upcoming increases of the LHC proton beam energy and luminosity, a series of upgrades is planned for L1Calo. The initial upgrade phase in 2013–14 includes substantial improvements to the analogue and digital signal processing to allow more sophisticated digital filters for energy and timing measurement, as well as compensate for pile-up and baseline shifting effects. Two existing digital algorithm processor subsystems will receive substantial hardware and firmware upgrades to increase the real-time data path bandwidth, allowing topological information to be transmitted and processed at Level-1. An entirely new subsystem, the Level- 1 Topological Processor, will receive real-time data from both the upgraded L1Calo and Level-1 Muon Trigger to perform trigger algorithms based on entire event topologies. During the second upgrade phase in 2018–19, major parts of L1Calo will be rebuilt in order to exploit a tenfold increase in the available calorimeter data granularity compared to that of the current system. The calorimeter signals will be received via optical fibres and distributed to two distinct processing systems. Those systems apply sliding window algorithms and quasi-offline algorithms to achieve object reconstruction and identification. The algorithms are implemented on high-density electronics boards which make use of recent developments in high speed data transmission and FPGA technology. The expected performance improvements are presented together with the upgraded hardware and firmware implementations. The status of the prototypes, integration and commissioning efforts are also reviewed.