12 Triggering and High-Level Data Selection
549
hits are reconstructed and the track p T is checked for consistency with the Super
Cluster energy. For both electron and photon triggers, tracks are seeded from pairs
of hits in the pixel layers in a rectangular η × φ region around the direction of the
reconstructed electron or photon, where these seeds are required to be consistent
with the nominal vertex spread (photons) or closest approach of the electron path
to the beam line (electrons). Then for electrons a threshold is applied to the p T
sum of the tracks within a cone around the electron direction and on the number
of tracks for the photon. In ATLAS [11] the electromagnetic cluster is identified
as an electron by association with a track in the Inner Detector, which is found by
independent searches in the SCT/Pixel and TRT detectors in the region identified
by the L1T RoI. For electron candidates, matching in both position and momentum
between the track and cluster is required.
12.2.5 Selection of Jets and Missing Energy
The primary processing of the jet candidates at Level-2 begins with the L1T
jet candidates, which are used as seeds for the Level-2 jets. The first step is
to recalculate the jet energy for these candidates using the full granularity and
calorimeter energy resolution information, which is not available to the Level-1 jet
energy calculation. In ATLAS, the Level-2 jet finding searches in the RoIs produced
by the Level-1 calorimeter logic. In CMS, jets are reconstructed using an iterative
cone algorithm with cone size R =
Δη 2 + Δφ 2 = 0.5 that sums over all projected
electromagnetic and hadronic calorimeter cells with energy greater than a threshold
set above the level of noise (0.5 GeV). In addition, to be declared a jet, at least one
seed tower must have E T > 1 GeV. After summation, the jet energy is adjusted by
an η-dependent correction for the calorimeter response.
Missing energy is calculated by summing all towers with E T above a noise
threshold. For CMS, this threshold is 0.5 GeV. No energy corrections are applied
to Missing E T . Since Missing E T is susceptible to noise because it is summing
over many channels, an alternative is often considered. This is Missing H T , which
is Missing E T calculated by summing over the jets in the event rather than the
calorimeter cells. Since there are fewer cells involved in the computation of missing
H T , there is less noise included in this sum.
It is typical to ask for two or more jets in the HLT algorithms. It is also common
to combine two or more jets with missing E T or H T . Also, topological constraints
are often employed such as requiring forward jets or acoplanarity between multiple
jets or jets and Missing E T .
549
hits are reconstructed and the track p T is checked for consistency with the Super
Cluster energy. For both electron and photon triggers, tracks are seeded from pairs
of hits in the pixel layers in a rectangular η × φ region around the direction of the
reconstructed electron or photon, where these seeds are required to be consistent
with the nominal vertex spread (photons) or closest approach of the electron path
to the beam line (electrons). Then for electrons a threshold is applied to the p T
sum of the tracks within a cone around the electron direction and on the number
of tracks for the photon. In ATLAS [11] the electromagnetic cluster is identified
as an electron by association with a track in the Inner Detector, which is found by
independent searches in the SCT/Pixel and TRT detectors in the region identified
by the L1T RoI. For electron candidates, matching in both position and momentum
between the track and cluster is required.
12.2.5 Selection of Jets and Missing Energy
The primary processing of the jet candidates at Level-2 begins with the L1T
jet candidates, which are used as seeds for the Level-2 jets. The first step is
to recalculate the jet energy for these candidates using the full granularity and
calorimeter energy resolution information, which is not available to the Level-1 jet
energy calculation. In ATLAS, the Level-2 jet finding searches in the RoIs produced
by the Level-1 calorimeter logic. In CMS, jets are reconstructed using an iterative
cone algorithm with cone size R =
Δη 2 + Δφ 2 = 0.5 that sums over all projected
electromagnetic and hadronic calorimeter cells with energy greater than a threshold
set above the level of noise (0.5 GeV). In addition, to be declared a jet, at least one
seed tower must have E T > 1 GeV. After summation, the jet energy is adjusted by
an η-dependent correction for the calorimeter response.
Missing energy is calculated by summing all towers with E T above a noise
threshold. For CMS, this threshold is 0.5 GeV. No energy corrections are applied
to Missing E T . Since Missing E T is susceptible to noise because it is summing
over many channels, an alternative is often considered. This is Missing H T , which
is Missing E T calculated by summing over the jets in the event rather than the
calorimeter cells. Since there are fewer cells involved in the computation of missing
H T , there is less noise included in this sum.
It is typical to ask for two or more jets in the HLT algorithms. It is also common
to combine two or more jets with missing E T or H T . Also, topological constraints
are often employed such as requiring forward jets or acoplanarity between multiple
jets or jets and Missing E T .