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

Chapter 4: Data Collection and Event Reconstruction 114
characterized as EM, hadronic or noise clusters based on their position and shape.
They are then weighted with a calibration function depending on their location and
energy. Next, a correction for energy loss in dead material in or near the cluster is
applied. Finally, a correction is made for signal losses due to the clustering procedure.
Note that all tower and topological clusters are defined as pseudo-particles with
four-momenta, computed using the reconstructed energy and the cluster direction.
This is important, for example, for jet finding algorithms, discussed below.
Jet reconstruction
The goal of jet reconstruction is to first find a calorimeter-level jet, then estimate
the energy and shape of the particle-level (i.e. , hadron-level) jet, and eventually
reproduce the kinematics of the original parton at the interaction-level. Because
of the widely varying nature of the physics processes that result in jets, there is
no optimal way to reconstruct jets from the hadronic final states of all interesting
topologies. In general, any jet finding mechanism should be infrared-safe, that is, the
presence of soft particles between two particles belonging to the same jet should not
influence the inclusion of the two particles into the jet. It should also be collinear-
safe, i.e. , it should reconstruct a jet correctly independent of the fact that a certain
amount of energy may be carried by one particle or by two collinear particles. From
a detector viewpoint, jet finding should be independent of detector technologies, and
should not be affected by changing conditions such as underlying event activity and
instantaneous luminosity.
Two jet finders are commonly used in ATLAS, namely, the seeded fixed cone jet