262
C. W. Fabjan and D. Fournier
crossing, and is comparable to the level of the single hadronic particle resolution
[125, 126].
6.6 Triggering with Calorimeters
The ability of calorimeters to provide rapidly (order 100 ns) information on the
energy distribution of the collisions products is one of the major assets of this
technique. In the very rich trigger ‘menu’ of the LHC experiments all but muon
physics is based on calorimetric triggers at the first trigger level L1. The calorimeter
trigger provides a selectivity of ~10 −3 and reduces the 40 MHz bunch collisions
rate accordingly. A ‘Sliding Window’ technique is used to search for local energy
topologies in the η × ϕ transverse energy distribution. The optimum window
size depends on the particle type (photons, electrons or jets), on their threshold,
the depth of the calorimeter included in the sum and possibly luminosity. More
complex topologies requiring isolated energy clusters (e.g. triggering on isolated
photons or electrons) are also used. The L1 trigger is implemented with dedicated
hardware processors. The trigger decision time or “latency” of its response is fixed,
and is typically a few μs. The information contained in all detectors is “pipelined”
during this time, in such a way that no dead time is generated by the L1 trigger. In
subsequent stages, called “high-level-trigger” (HLT) selection criteria and energy
thresholds are sharpened with software-based algorithms. The treatment during
these phases is asynchronous, and many processors (up to thousands) work in
parallel. One of the main challenges with the trigger systems is to allow recording W
and Z leptonic decays (i.e. with transverse momenta thresholds below ~30 GeV) for
calibration purposes, and for electroweak physics, without saturating the bandwidth
of the data acquisition systems. As luminosity increases, refinements are necessary
to meet this requirement. MET and B-tagging are part of the overall menu of the
HLT, in which of the order of one thousand different conditions are examined in
parallel. Triggers on hadronic decay modes of τs, which rely on narrow hadronic
jets in the calorimeters are also implemented in HLT. See Ref [127] as example for
ATLAS.
In LHCb, which addresses heavy flavour physics in the pseudorapidity range
between 2 and 5, the transverse momentum thresholds are much lower, typically
3 GeV for both the electron and the hadron trigger. Such low thresholds are made
possible due to the lower luminosity operation of the experiment (typically 0.4
10 33 cm −2 s −1 ) and the high data acquisition rate (up to 1 MHz). See Ref [128]
for details.
C. W. Fabjan and D. Fournier
crossing, and is comparable to the level of the single hadronic particle resolution
[125, 126].
6.6 Triggering with Calorimeters
The ability of calorimeters to provide rapidly (order 100 ns) information on the
energy distribution of the collisions products is one of the major assets of this
technique. In the very rich trigger ‘menu’ of the LHC experiments all but muon
physics is based on calorimetric triggers at the first trigger level L1. The calorimeter
trigger provides a selectivity of ~10 −3 and reduces the 40 MHz bunch collisions
rate accordingly. A ‘Sliding Window’ technique is used to search for local energy
topologies in the η × ϕ transverse energy distribution. The optimum window
size depends on the particle type (photons, electrons or jets), on their threshold,
the depth of the calorimeter included in the sum and possibly luminosity. More
complex topologies requiring isolated energy clusters (e.g. triggering on isolated
photons or electrons) are also used. The L1 trigger is implemented with dedicated
hardware processors. The trigger decision time or “latency” of its response is fixed,
and is typically a few μs. The information contained in all detectors is “pipelined”
during this time, in such a way that no dead time is generated by the L1 trigger. In
subsequent stages, called “high-level-trigger” (HLT) selection criteria and energy
thresholds are sharpened with software-based algorithms. The treatment during
these phases is asynchronous, and many processors (up to thousands) work in
parallel. One of the main challenges with the trigger systems is to allow recording W
and Z leptonic decays (i.e. with transverse momenta thresholds below ~30 GeV) for
calibration purposes, and for electroweak physics, without saturating the bandwidth
of the data acquisition systems. As luminosity increases, refinements are necessary
to meet this requirement. MET and B-tagging are part of the overall menu of the
HLT, in which of the order of one thousand different conditions are examined in
parallel. Triggers on hadronic decay modes of τs, which rely on narrow hadronic
jets in the calorimeters are also implemented in HLT. See Ref [127] as example for
ATLAS.
In LHCb, which addresses heavy flavour physics in the pseudorapidity range
between 2 and 5, the transverse momentum thresholds are much lower, typically
3 GeV for both the electron and the hadron trigger. Such low thresholds are made
possible due to the lower luminosity operation of the experiment (typically 0.4
10 33 cm −2 s −1 ) and the high data acquisition rate (up to 1 MHz). See Ref [128]
for details.