The ALICE Trigger system has to handle the very different environments of both proton-proton and lead-lead collisions at the CERN LHC. The system consists of the Central Trigger Processor (CTP) and 24 Local Trigger Units (LTU) that act as a uniform interface to sub-detector front-end electronics. The CTP generates three levels of hierarchical hardware triggers - L0, L1 and L2. After a successful L2 trigger, sub-detectors are readout and the event is sent to the High Level Trigger (HLT) for further processing and selection. At any time, the 24 sub-detectors of the ALICE experiment are partitioned into up to 6 independent clusters. Trigger selection includes the Past-future Protection - a fully programmable hardware mechanism of controlling event pile-up. A trigger physics class is the basic processing structure throughout the CTP logic and represents a particular physics signature, with a defined set of trigger input requirements. The CTP allows for up to 50 of these independently programmable classes to be active at any given time.
The L0 trigger must reach time-critical sub-detectors within 1.2 μs after an interaction. The L1 trigger follows at 6.5 μs and, finally, the L2 trigger is delivered at 100 μs after an interaction. The latency of the L2 trigger is governed by the drift time of the TPC. The short latency for L0 requires the CTP to be as close as possible to the sub-detectors. Even so, the CTP must send the L0 signal within 100 ns of receiving the last L0 input in order to achieve the required latency.
The LTUs serve as an interface between the CTP and the sub-detector readout electronics. One of the features of the LTU is its ability to run in stand-alone mode where can it fully emulate the CTP protocol. This mode has proved to be essential for sub-detector development and testing.
For Run II, the current L0 CTP board has been replaced by new LM/L0 board. This new LM/L0 board incorporates a new pre-trigger (which is issued before the L0 trigger), increases the number of trigger physics classes from 50 to 100, increases the number of detector clusters from 6 to 8, and significantly increases the number of allowable trigger inputs. The improved trigger is working well and gives ALICE improved efficiency and more flexibility for the exciting physics potential of Run II.