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W. H. Smith
In CMS, all of the muon chamber systems participate in the L1T. The L1T muon
candidates are used to seed the reconstruction of tracks in the muon chambers in
the Level-2 algorithm. First, an initial pattern recognition is performed on muon
segments along the trajectory, then a second more precise fit using all hits on these
segments is used to determine the muon parameters. Since the CMS chambers
are surrounded by steel, the propagation of track parameters to adjacent muon
stations must take into account material effects such as multiple Coulomb scattering,
and energy losses due to ionization and bremsstrahlung in the muon chambers
and the iron. To avoid excessive processing times, these are estimated from fast
parameterizations. Muons passing this first reconstruction are then input to the
Level-3 reconstruction that uses hits in the silicon tracker within a rectangular η × φ
region. Pairs or triplets of hits in the innermost layers of the tracker form trajectory
seeds that are required to be compatible with the η × φ region and the primary vertex
constraints. These are then grown into tracks of about seven hits and optionally
combined with the reconstructed hits from the Level-2 algorithm. In Level-2, the
isolation variable is calculated from the weighted sums of energies deposited in
the ECAL and in the HCAL in the region around the muon track. For the Level-3
isolation variable, only charged-particle tracks near the vertex of the candidate muon
are selected for inclusion. This excludes tracks from pile-up of contributions from
other pp collisions (which occur at another vertex location), making this isolation
less sensitive to pile-up than calorimetric isolation.
12.2.4 Selection of Electrons and Photons
The first algorithms executed in Level-2 on Level-1 selected electrons and photons
are refinements of the clustering algorithms. For example, in ATLAS [9], the
energy deposited in windows of the electromagnetic LAr calorimeter cells and
the energy-weighted position information, as well as the leakage energy into the
hadronic calorimeter are calculated. CMS [10] also reconstructs energy in clusters of
electromagnetic calorimeter cells corresponding to the Level-1 calorimeter triggers,
adding a margin around the trigger region to ensure complete collection of energy.
These clusters are then formed into “Super Clusters” which are groups of clusters
along a road in the φ direction, chosen due to bending in the magnetic field. These
clusters are then required to be isolated in the electromagnetic calorimeter. The
hadronic calorimeter energies are then reconstructed and the energies in the hadronic
tower behind the cluster and the adjacent towers are required to be small with respect
to the electromagnetic cluster energy.
The second tier of algorithms performed on electrons and photons confirmed
by the first algorithms are tracking algorithms. The first or more local steps of
these are generally called Level 2.5 algorithms. This involves establishing track
isolation around the electromagnetic cluster and for electron triggers, associating
the electromagnetic cluster with a track. For CMS electron triggers, the energy and
position of the Super Cluster is used to search for hits in the pixel detector. These
W. H. Smith
In CMS, all of the muon chamber systems participate in the L1T. The L1T muon
candidates are used to seed the reconstruction of tracks in the muon chambers in
the Level-2 algorithm. First, an initial pattern recognition is performed on muon
segments along the trajectory, then a second more precise fit using all hits on these
segments is used to determine the muon parameters. Since the CMS chambers
are surrounded by steel, the propagation of track parameters to adjacent muon
stations must take into account material effects such as multiple Coulomb scattering,
and energy losses due to ionization and bremsstrahlung in the muon chambers
and the iron. To avoid excessive processing times, these are estimated from fast
parameterizations. Muons passing this first reconstruction are then input to the
Level-3 reconstruction that uses hits in the silicon tracker within a rectangular η × φ
region. Pairs or triplets of hits in the innermost layers of the tracker form trajectory
seeds that are required to be compatible with the η × φ region and the primary vertex
constraints. These are then grown into tracks of about seven hits and optionally
combined with the reconstructed hits from the Level-2 algorithm. In Level-2, the
isolation variable is calculated from the weighted sums of energies deposited in
the ECAL and in the HCAL in the region around the muon track. For the Level-3
isolation variable, only charged-particle tracks near the vertex of the candidate muon
are selected for inclusion. This excludes tracks from pile-up of contributions from
other pp collisions (which occur at another vertex location), making this isolation
less sensitive to pile-up than calorimetric isolation.
12.2.4 Selection of Electrons and Photons
The first algorithms executed in Level-2 on Level-1 selected electrons and photons
are refinements of the clustering algorithms. For example, in ATLAS [9], the
energy deposited in windows of the electromagnetic LAr calorimeter cells and
the energy-weighted position information, as well as the leakage energy into the
hadronic calorimeter are calculated. CMS [10] also reconstructs energy in clusters of
electromagnetic calorimeter cells corresponding to the Level-1 calorimeter triggers,
adding a margin around the trigger region to ensure complete collection of energy.
These clusters are then formed into “Super Clusters” which are groups of clusters
along a road in the φ direction, chosen due to bending in the magnetic field. These
clusters are then required to be isolated in the electromagnetic calorimeter. The
hadronic calorimeter energies are then reconstructed and the energies in the hadronic
tower behind the cluster and the adjacent towers are required to be small with respect
to the electromagnetic cluster energy.
The second tier of algorithms performed on electrons and photons confirmed
by the first algorithms are tracking algorithms. The first or more local steps of
these are generally called Level 2.5 algorithms. This involves establishing track
isolation around the electromagnetic cluster and for electron triggers, associating
the electromagnetic cluster with a track. For CMS electron triggers, the energy and
position of the Super Cluster is used to search for hits in the pixel detector. These