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

Chapter 4: Data Collection and Event Reconstruction 118
Figure 4.8 shows the various steps in jet reconstruction starting from the two types
of clusters. For sliding window clusters, the first step is a ‘resummation’ step. Because
of cell signal fluctuations due to noise, some towers can end up with a net negative
energy. The resummation step combines such towers with nearby positive-energy
towers to ensure that all towers that are input to jet finding have physically meaningful
four-momenta. The output of the jet finding algorithms are jets at the raw energy
scale, which are then calibrated with a cell-signal based weighting function 13 . Further
corrections for pile-up, electronic noise and algorithm effects are applied in order to
reproduce the hadron-level jet shape and energy. Finally, all jets with pT < 7 GeV
are discarded. Calibration to the interaction-level parton requires further corrections
for the physics environment, and is usually done during specific physics analyses.
Topological clusters at the raw energy scale yield jets at the same scale, which
are then calibrated to the hadronic scale as before and the usual corrections applied
to recover the particle-level jet. Clusters that have been locally calibrated to the
hadronic scale lead to jets at this scale, which have to be further corrected for the
fact that local calibrations and correction factors have been derived from single pion
response [17]. After all corrections, jets with pT < 7 GeV are discarded as before.
One powerful way to improve the energy resolution of jets is to match a calorimeter
cluster with a charged inner detector track pointing to it. If the track momentum
resolution is better than the calorimeter energy resolution, the cluster ET is replaced
with the track PT . This technique is known as energy flow reconstruction.
13 In a non-compensating calorimeter, such as the ATLAS calorimeters, low cell signal densities
indicate energy deposition by charged hadrons. The observed energy can be calibrated to the actual
hadronic energy by weighting the cell signal with a factor of the order of the electron/pion signal
ratio [101].