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

Chapter 4: Data Collection and Event Reconstruction 121
and δ-electrons 15 from signal electrons. A cut-based electron identification scheme is
used by default. Three standard sets of cuts have been defined: loose, medium and
tight. This approach provides flexibility in physics analyses; for example, the user
can choose electrons that pass the loose cuts if the analysis demands high electron
identification efficiency but is insensitive to a relatively large fake rate.
The loose cuts use simple calorimeter quantities, namely, shower shape variables
in the middle layer of the EM calorimeter and the amount of leakage energy into the
hadronic calorimeter. The medium cuts improve the identification by adding cuts on
the strips in the first EM calorimeter layer (see Chapter 2) and on track quantities.
The tight cuts use additional quantities such as the number of hits in the vertexing
layer, the number of TRT hits and the ratio of high threshold TRT hits to all TRT
hits. The tight cuts provide very high jet rejection. Details of the cuts can be found
in the electron identification chapter of the ATLAS CSC note ([17], p. 73).
In addition to the cut-based approach, multivariate approaches using a likelihood
discriminant, boosted decision tree and neural network have been implemented in
ATLAS. Analyses involving electrons with the early 2010 data used cut-based electron
definitions only.
Photon reconstruction in the calorimeter
As mentioned above, photon candidates are created from EM clusters that do not
have an associated track or is matched with a track in a conversion pair. At this
point, three photon identification techniques are available: a cut-based method, a
15 A δ-electron or δ-ray is an energetic electron that has been knocked out of an atom by a charged
particle.