12 Triggering and High-Level Data Selection
547
The algorithms deployed in the HLT are dynamic, reflecting continuing improvements in the offline reconstruction that represent the functions that the HLT is
attempting to approach within the constraints of processing time. The descriptions
below of algorithms in the LHC experiments represent a snapshot at the time of
Run-1 processing. These considerably evolved during Run-1 into Run-2, although
the general techniques shown continue to be applied.
12.2.2 Tracking in Higher Level Triggers
The principal new information in the higher level triggers is tracking information.
Either it is introduced for the first time in the event selection process or it is greatly
refined over rudimentary tracking used in the Level-1 trigger. There are two major
sources of tracking information. A pixel detector provides the most inner tracking
and some vertex information. Outside of the pixel detector, silicon strip and then
in some cases drift chambers, fibers, or straw tube detectors provide additional
information at larger radius. For example, ATLAS [9] uses space points found in
the pixels and silicon central tracker (SCT) to find the z-vertex location, fit tracks
into the Transition Radiation Tracker (TRT) and measure the φ and p T of the track
above a p T of 0.5 GeV/c. In the latter part of Run-2, ATLAS commissioned the
Fast TracKer (FTK), a dedicated Associative-Memory hardware processor which
delivers tracks with p T > 1 GeV/c for every L1A to the HLT within 100 μs [25].
In CMS, two types of tracking are employed. Charged particle tracks are first
quickly reconstructed using pixel hits and then more laboriously but more accurately
reconstructed with additional hits from the silicon strip tracker. Generally, tracking
is “seeded” by the confirmed Higher Level Trigger objects, which themselves are
“seeded” by Level-1 trigger objects.
12.2.3 Selection of Muons
The first algorithms executed in Level-2 on Level-1 selected muons are refinements
of the reconstruction of the tracks in the muon chambers. In the case of ATLAS,
where only the RPC (Barrel) and TGT (Forward) chambers provide information
for the L1T, the precision hit information from the Monitored Drift Tubes (MDTs)
is added to the RPC and TGC determined candidates. This provides good track
reconstruction in the muon spectrometer. Since the ATLAS Muon Chambers are
mostly in air, there is little multiple coulomb scattering. The found tracks are
extrapolated for combination with tracks found in the Inner Detector. Matching
between muon tracks measured independently in the muon system and those in the
Inner Detector selects prompt muons and rejects fake and secondary muons. The
isolated muon triggers also use information from the calorimeter towers surrounding
the found muon track.
547
The algorithms deployed in the HLT are dynamic, reflecting continuing improvements in the offline reconstruction that represent the functions that the HLT is
attempting to approach within the constraints of processing time. The descriptions
below of algorithms in the LHC experiments represent a snapshot at the time of
Run-1 processing. These considerably evolved during Run-1 into Run-2, although
the general techniques shown continue to be applied.
12.2.2 Tracking in Higher Level Triggers
The principal new information in the higher level triggers is tracking information.
Either it is introduced for the first time in the event selection process or it is greatly
refined over rudimentary tracking used in the Level-1 trigger. There are two major
sources of tracking information. A pixel detector provides the most inner tracking
and some vertex information. Outside of the pixel detector, silicon strip and then
in some cases drift chambers, fibers, or straw tube detectors provide additional
information at larger radius. For example, ATLAS [9] uses space points found in
the pixels and silicon central tracker (SCT) to find the z-vertex location, fit tracks
into the Transition Radiation Tracker (TRT) and measure the φ and p T of the track
above a p T of 0.5 GeV/c. In the latter part of Run-2, ATLAS commissioned the
Fast TracKer (FTK), a dedicated Associative-Memory hardware processor which
delivers tracks with p T > 1 GeV/c for every L1A to the HLT within 100 μs [25].
In CMS, two types of tracking are employed. Charged particle tracks are first
quickly reconstructed using pixel hits and then more laboriously but more accurately
reconstructed with additional hits from the silicon strip tracker. Generally, tracking
is “seeded” by the confirmed Higher Level Trigger objects, which themselves are
“seeded” by Level-1 trigger objects.
12.2.3 Selection of Muons
The first algorithms executed in Level-2 on Level-1 selected muons are refinements
of the reconstruction of the tracks in the muon chambers. In the case of ATLAS,
where only the RPC (Barrel) and TGT (Forward) chambers provide information
for the L1T, the precision hit information from the Monitored Drift Tubes (MDTs)
is added to the RPC and TGC determined candidates. This provides good track
reconstruction in the muon spectrometer. Since the ATLAS Muon Chambers are
mostly in air, there is little multiple coulomb scattering. The found tracks are
extrapolated for combination with tracks found in the Inner Detector. Matching
between muon tracks measured independently in the muon system and those in the
Inner Detector selects prompt muons and rejects fake and secondary muons. The
isolated muon triggers also use information from the calorimeter towers surrounding
the found muon track.