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D. Froidevaux
prepared before presenting it to the central trigger processor. This intermediate stage
carries some significant functionality: the muon trigger to central trigger processor
Interface resolves overlaps between chamber sectors in the barrel and between barrel
and end-cap chambers and forms the muon candidate multiplicities for the trigger
decision.
The final decision on the event is obtained by the central trigger processor itself,
using either information from only the muon trigger or in association with other
objects in the event (e.g. the presence of a high-p T electron).
16.6.2.2 Calorimeter Trigger
The L1 calorimeter trigger provides essentially all the L1 trigger streams for
the experiment (electrons, photons, QCD jets, τ −jets, missing E T ) except for
the muons. The architecture of this trigger contains three elements, namely the
generation of the trigger primitives, a local calorimeter trigger which processes
information from limited parts of the detector, and a global calorimeter trigger
which combines all the information from the local processors, prior to sending the
summaries to the central trigger processor. Data from the calorimeters are combined
to form trigger towers of approximate size 0.1 × 0.1 in η − φ space. Analogue sums
are formed on the detector and sent through analogue transmission to the counting
room.
The information is then digitised and processed to determine the transverse
energy E T in each trigger tower. As discussed previously, most of the ATLAS
calorimeters have pulse shapes which extend well beyond a single crossing, so the
signals are processed to assign each energy deposition to the correct bunch crossing.
Once the transverse energies and the bunch crossing are determined, the algorithms
in the local calorimeter trigger take over. The basic features can be summarised as
follows:
• Electrons and photons are searched for as peaks in the E T deposited in a limited
η − φ region (neighboring towers) of the EM calorimeter. The corresponding
hadronic energy is required to be small, relatively to the EM calorimeter energy.
Additional isolation requirements, e.g. by demanding that neighbouring towers
do not have energy larger than a certain threshold, may be imposed.
• Jets are formed by adding the energy in a large η − φ region consisting of an
array of 4 × 4 trigger towers/elements. The algorithm provides flexibility in the
measurement of the jet energy through the use of a sliding window, but therefore
requires an additional processing step to settle jet overlaps and eliminate doublecounting.
• τ -jets are formed by demanding very narrow energy depositions in the electromagnetic and hadronic calorimeters. Isolation requirements may also be applied.
• The missing transverse energy (as well as the total transverse energy in the event)
is estimated from the sum of the transverse energies of all the calorimeter cells.
D. Froidevaux
prepared before presenting it to the central trigger processor. This intermediate stage
carries some significant functionality: the muon trigger to central trigger processor
Interface resolves overlaps between chamber sectors in the barrel and between barrel
and end-cap chambers and forms the muon candidate multiplicities for the trigger
decision.
The final decision on the event is obtained by the central trigger processor itself,
using either information from only the muon trigger or in association with other
objects in the event (e.g. the presence of a high-p T electron).
16.6.2.2 Calorimeter Trigger
The L1 calorimeter trigger provides essentially all the L1 trigger streams for
the experiment (electrons, photons, QCD jets, τ −jets, missing E T ) except for
the muons. The architecture of this trigger contains three elements, namely the
generation of the trigger primitives, a local calorimeter trigger which processes
information from limited parts of the detector, and a global calorimeter trigger
which combines all the information from the local processors, prior to sending the
summaries to the central trigger processor. Data from the calorimeters are combined
to form trigger towers of approximate size 0.1 × 0.1 in η − φ space. Analogue sums
are formed on the detector and sent through analogue transmission to the counting
room.
The information is then digitised and processed to determine the transverse
energy E T in each trigger tower. As discussed previously, most of the ATLAS
calorimeters have pulse shapes which extend well beyond a single crossing, so the
signals are processed to assign each energy deposition to the correct bunch crossing.
Once the transverse energies and the bunch crossing are determined, the algorithms
in the local calorimeter trigger take over. The basic features can be summarised as
follows:
• Electrons and photons are searched for as peaks in the E T deposited in a limited
η − φ region (neighboring towers) of the EM calorimeter. The corresponding
hadronic energy is required to be small, relatively to the EM calorimeter energy.
Additional isolation requirements, e.g. by demanding that neighbouring towers
do not have energy larger than a certain threshold, may be imposed.
• Jets are formed by adding the energy in a large η − φ region consisting of an
array of 4 × 4 trigger towers/elements. The algorithm provides flexibility in the
measurement of the jet energy through the use of a sliding window, but therefore
requires an additional processing step to settle jet overlaps and eliminate doublecounting.
• τ -jets are formed by demanding very narrow energy depositions in the electromagnetic and hadronic calorimeters. Isolation requirements may also be applied.
• The missing transverse energy (as well as the total transverse energy in the event)
is estimated from the sum of the transverse energies of all the calorimeter cells.