An Upgraded ATLAS Central Trigger for 2015 LHC Luminosities

The LHC collides protons at a rate of 40 MHz and each collision produces ∼1.5~MB of data from the ATLAS detector. The ATLAS trigger system is implemented in three levels and selects only the most interesting collision events to reduce the event storage rate to about 400 Hz. The first level is implemented in custom electronics and reduces the input rate to ∼75 kHz with a decision latency of ∼2.5 us. It is also responsible for initiating the read-out of data from all the sub-detectors in ATLAS. Based primarily on information from calorimeters and muon trigger detectors, the Central Trigger Processor (CTP) produces the Level-­1 trigger decision. After a very successful first run, the LHC is now being upgraded to operate with increased luminosity and a center-of-mass energy of up to 14 TeV. To cope with the higher luminosities, the Level-1 trigger system will have to perform a more refined selection in order to not lose interesting physics data while keeping the total Level-1 rate below 100~kHz. In this presentation, the current Level-1 Central Trigger systems will be reviewed, the motivation for their upgrade will be outlined and the plans for how to meet the requirements for post-2014 physics runs at the LHC will be described.