Measurement of the Z boson cross-section in - Harvard University ...

Chapter 4: Data Collection and Event Reconstruction 120
The default jet reconstruction in ATLAS uses topological clusters and the anti-kT
jet algorithm.
Electron reconstruction in the calorimeter
Electron reconstruction can be track-seeded or calorimeter-seeded; the track-
seeded approach has been described earlier in the context of inner detector reconstruc-
tion. In the calorimeter, electron-induced showers are identified using EM clusters
formed with the sliding window technique 14 . Electron showers are in general broad
compared to photon showers, since electrons have a larger probability of interaction
than photons in the material upstream of the calorimeter, and because they emit
bremsstrahlung photons while bending in φ inside the inner detector. Several cluster
collections are therefore built with different window sizes for electron and photon
identification.
Electron reconstruction starts by building a seed-list of EM clusters with ET > 3
GeV. For each seed, a matching inner detector track is looked for within a ∆η × ∆φ
window of 0.05 × 0.10. If such a track is found, the algorithm looks for an associated
conversion, and checks that the ratio E/p of the cluster energy to the track momentum
is smaller than 10. If no conversion is flagged and the E/p criterion is satisfied, an
electron candidate is created. If no matching track is found or if the matched track
belongs to a conversion pair, a photon candidate is created.
At this point, various refined identification criteria can be applied to an electron
candidate, incorporating calorimeter and inner detector quantities, to distinguish jets
14 Topological clusters are not used in electron and photon reconstruction by default.