12 Triggering and High-Level Data Selection
545
L1 Global trigger added a topological trigger (L1Topo) to allow geometrical or
kinematic association between trigger objects received from the L1 Calorimeter or
Muon Triggers [23].
The CMS L1 Global Trigger sorts ranked trigger objects, rather than histogramming objects over a fixed threshold. This allows all trigger criteria to be applied and
varied at the Global Trigger level rather than earlier in the trigger processing. All
trigger objects are accompanied by their coordinates in (η, φ) space. This allows
the Global Trigger to vary thresholds based on the location of the trigger objects.
It also allows the Global Trigger to require trigger objects to be close or opposite
from each other. In addition, the presence of the trigger object coordinate data in the
trigger data, which is read out first by the DAQ after a Level-1 trigger accept (L1A),
permits a quick determination of the regions of interest where the more detailed HLT
analyses should focus. The Global L1 Trigger transmits a decision to either accept
(L1A) or reject each bunch crossing. This decision is transmitted through the Trigger
Throttle System (TTS) to the Timing Trigger and Control system (TTC). The TTS
allows the reduction by prescaling or blocking L1A signals in case the detector
readout or DAQ buffers are at risk of overflow. For Run-1, the Global L1 Trigger
allowed up to 128 algorithms to contribute to the overall trigger decision. For Run2, this was upgraded to a modular design capable of up to about 500 algorithms that
was typically running about 300 [16].
12.2 Higher-Level Selection
12.2.1 Introduction
The design of the Higher-Level Selection of events after Level-1 takes place in
a number of “trigger levels”. Generally, collider experiments use at least two
additional trigger levels, referred to as the Level-2 and Level-3 trigger. Some
experiments have a Level-4 trigger. The higher the number the more general purpose
(or commercial) the implementation, with the Level-3 and Level-4 triggers being
composed of farms of standard commodity computers. The physical implementation
of the Level-2 trigger varies substantially between experiments from inclusion in
the Level-3 farm of processors to an independent farm of processors to customized
dedicated processing hardware. The Level-2 trigger has to operate at the output rate
of the Level-1 trigger, generally with a subset of the higher resolution and fullgranularity available to the full reconstruction code available at Level-3 and higher.
Typically, the Level-1 output rate ranges between 1 and 100 kHz depending on the
experiment. The Level-2 trigger is generally limited in execution time so that the
full event data cannot be unpacked and processed. Instead, the higher resolution and
full-granularity data is unpacked in “regions of interest” determined by the Level-1
trigger data.
545
L1 Global trigger added a topological trigger (L1Topo) to allow geometrical or
kinematic association between trigger objects received from the L1 Calorimeter or
Muon Triggers [23].
The CMS L1 Global Trigger sorts ranked trigger objects, rather than histogramming objects over a fixed threshold. This allows all trigger criteria to be applied and
varied at the Global Trigger level rather than earlier in the trigger processing. All
trigger objects are accompanied by their coordinates in (η, φ) space. This allows
the Global Trigger to vary thresholds based on the location of the trigger objects.
It also allows the Global Trigger to require trigger objects to be close or opposite
from each other. In addition, the presence of the trigger object coordinate data in the
trigger data, which is read out first by the DAQ after a Level-1 trigger accept (L1A),
permits a quick determination of the regions of interest where the more detailed HLT
analyses should focus. The Global L1 Trigger transmits a decision to either accept
(L1A) or reject each bunch crossing. This decision is transmitted through the Trigger
Throttle System (TTS) to the Timing Trigger and Control system (TTC). The TTS
allows the reduction by prescaling or blocking L1A signals in case the detector
readout or DAQ buffers are at risk of overflow. For Run-1, the Global L1 Trigger
allowed up to 128 algorithms to contribute to the overall trigger decision. For Run2, this was upgraded to a modular design capable of up to about 500 algorithms that
was typically running about 300 [16].
12.2 Higher-Level Selection
12.2.1 Introduction
The design of the Higher-Level Selection of events after Level-1 takes place in
a number of “trigger levels”. Generally, collider experiments use at least two
additional trigger levels, referred to as the Level-2 and Level-3 trigger. Some
experiments have a Level-4 trigger. The higher the number the more general purpose
(or commercial) the implementation, with the Level-3 and Level-4 triggers being
composed of farms of standard commodity computers. The physical implementation
of the Level-2 trigger varies substantially between experiments from inclusion in
the Level-3 farm of processors to an independent farm of processors to customized
dedicated processing hardware. The Level-2 trigger has to operate at the output rate
of the Level-1 trigger, generally with a subset of the higher resolution and fullgranularity available to the full reconstruction code available at Level-3 and higher.
Typically, the Level-1 output rate ranges between 1 and 100 kHz depending on the
experiment. The Level-2 trigger is generally limited in execution time so that the
full event data cannot be unpacked and processed. Instead, the higher resolution and
full-granularity data is unpacked in “regions of interest” determined by the Level-1
trigger data.