16 Integration of Detectors into a Large Experiment: Examples from ATLAS. . .
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a time of ∼ms to completely read in the event. This is therefore the rate at which
the elements of the farm system can operate on events. The two time-scales are very
different, and this is where the deep buffers present in the readout system serve to
minimise the coupling between the stages.
The design of the DAQ system is very modular, thereby allowing for a staged
installation. The event builder has been conceived with the possibility of a phased
installation from the very beginning. The operation of the ATLAS experiment has
begun with a DAQ system serving only a reduced bandwidth of approximately
20–40 kHz. The deferrals were necessary because of funding pressures, whereas
a staged installation of the DAQ was viewed as less damaging to the physics
programme, since the initial instantaneous luminosity of the LHC is far below the
design value.
16.6.3.2 High-Level Trigger
As mentioned previously, the HLT is a software filtering process executed on
standard commercially available processors. The software is drawn from the offline
reconstruction software of the experiment. Both levels of the HLT are executed
within the offline framework, but in contrast to the event filter which uses the
same algorithms as the offline, the L2 trigger processors run more dedicated
code (in particular with faster data-preparation algorithms). The trigger software
is steered differently from the offline and initiates the reconstruction from the
physics candidate objects identified by the previous levels (L1 or L2 trigger). The
overall rejection factor is achieved by applying, in software, a number of successive
reconstruction and selection steps.
As an example, the HLT electron trigger is typically driven by a L1 electron/photon candidate, which is identified as a high-energy isolated electromagnetic
(EM) energy deposition in the calorimeters. At the output of the L1 trigger, the rate
is dominated by QCD jets. The first task in reconstructing the electron in the HLT
is to rerun the clustering algorithm with access to the full granularity and resolution
of the EM calorimeter and to obtain a new, more accurate, measurement of the
transverse energy (E T ) of the EM cluster. Given the rapidly falling cross section,
this already provides a rejection factor of ≈2 with respect to the input event rate.
Further shower-shape and isolation cuts are also applied at this point. The events
surviving the EM calorimeter requirements are subsequently subjected to a search
for a charged-particle track in the tracking detectors. The matching between track
and cluster is a powerful requirement, which yields at least a factor of 10 rejection
against jets while maintaining a very high efficiency.
Events selected by the HLT are forwarded to mass storage and from there to the
offline system for reconstruction and physics analysis. Given the unprecedented rate
of online rejection, another very important task of the HLT is to provide detailed
information on the events which have been rejected at each stage of the filtering
process.
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a time of ∼ms to completely read in the event. This is therefore the rate at which
the elements of the farm system can operate on events. The two time-scales are very
different, and this is where the deep buffers present in the readout system serve to
minimise the coupling between the stages.
The design of the DAQ system is very modular, thereby allowing for a staged
installation. The event builder has been conceived with the possibility of a phased
installation from the very beginning. The operation of the ATLAS experiment has
begun with a DAQ system serving only a reduced bandwidth of approximately
20–40 kHz. The deferrals were necessary because of funding pressures, whereas
a staged installation of the DAQ was viewed as less damaging to the physics
programme, since the initial instantaneous luminosity of the LHC is far below the
design value.
16.6.3.2 High-Level Trigger
As mentioned previously, the HLT is a software filtering process executed on
standard commercially available processors. The software is drawn from the offline
reconstruction software of the experiment. Both levels of the HLT are executed
within the offline framework, but in contrast to the event filter which uses the
same algorithms as the offline, the L2 trigger processors run more dedicated
code (in particular with faster data-preparation algorithms). The trigger software
is steered differently from the offline and initiates the reconstruction from the
physics candidate objects identified by the previous levels (L1 or L2 trigger). The
overall rejection factor is achieved by applying, in software, a number of successive
reconstruction and selection steps.
As an example, the HLT electron trigger is typically driven by a L1 electron/photon candidate, which is identified as a high-energy isolated electromagnetic
(EM) energy deposition in the calorimeters. At the output of the L1 trigger, the rate
is dominated by QCD jets. The first task in reconstructing the electron in the HLT
is to rerun the clustering algorithm with access to the full granularity and resolution
of the EM calorimeter and to obtain a new, more accurate, measurement of the
transverse energy (E T ) of the EM cluster. Given the rapidly falling cross section,
this already provides a rejection factor of ≈2 with respect to the input event rate.
Further shower-shape and isolation cuts are also applied at this point. The events
surviving the EM calorimeter requirements are subsequently subjected to a search
for a charged-particle track in the tracking detectors. The matching between track
and cluster is a powerful requirement, which yields at least a factor of 10 rejection
against jets while maintaining a very high efficiency.
Events selected by the HLT are forwarded to mass storage and from there to the
offline system for reconstruction and physics analysis. Given the unprecedented rate
of online rejection, another very important task of the HLT is to provide detailed
information on the events which have been rejected at each stage of the filtering
process.