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

Chapter 4: Data Collection and Event Reconstruction 112
are built: EM clusters, used for electron and photon identification, and combined
clusters, using information from both EM and hadronic calorimeters, and used for jet
finding. The clustering proceeds in three steps.
In the first step, the available η − φ space is divided up into a grid of towers of
size ∆η × ∆φ. For EM clusters, ∆η =∆φ =0.025, while for combined clusters,
∆η =∆φ =0.1 [107]. Within each tower, the energy of cells in each longitudinal
layer is summed into a tower energy. In the second step, a window of a fixed size is
moved across each element of the tower grid in steps of ∆η and ∆φ, and the transverse
energies of all towers in the window is summed. If the window transverse energy is a
local maximum and above a threshold, a cluster seed is formed. The threshold is 3
GeV for EM clusters and 15 GeV for combined clusters. At this point, for combined
clusters, the seeds are directly transformed into clusters; cluster quantities such as
position and energy deposition per layer are calculated based on the cells encompassed
by the sliding window. For EM clusters, however, there is an additional step in which
clusters of different sizes are built depending on the hypothesized particle type and the
location of the cluster in the calorimeter. In general, the cluster must be large enough
so that it contains most of the energy deposited by the particle. But including more
cells in a cluster also means potentially including more noise, so the actual number
is a trade-off between these two factors.
Topological clustering technique
Topological clustering attempts to reconstruct three-dimensional ‘energy-blobs’ in
the calorimeter. It starts with a seed cell, and adds to the cluster any neighboring
cell if its energy is significantly above the noise level. The resulting clusters have a