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

Chapter 4: Data Collection and Event Reconstruction 111
performance than the DNA method, but is much slower.
4.2.2 Reconstruction in the Calorimeters
Information from the calorimetry is used for identifying and measuring the energies
of electrons, photons, and hadronic and τ jets, as well as missing transverse energy.
The strategies used for reconstructing the various objects will be discussed in this
section, starting at the cell level.
An incoming particle creates a shower in the calorimeters, depositing its energy in
a number of cells both in the longitudinal and lateral directions 10 . When an event is
accepted by a Level-1 calorimeter trigger, the analog signal from each calorimeter cell
is digitized and sent to a set of digital signal processors (DSPs). The DSPs convert
the signals into an energy deposition value for each cell, taking into account electronic
and pile-up noise 11 . To estimate the energy in a shower from the cell energies, the
cells are first grouped into clusters and the total energy in each cluster summed up.
ATLAS uses two different clustering techniques, namely, sliding window clustering
and topological clustering.
Sliding window clustering technique
The sliding window method is based on summing energies of cells within a fixed
window in η − φ space. Two types of sliding window clusters (also called towers)
10 In this context, the longitudinal direction is the direction of shower development, i.e. , along
the path of the incoming particle. Similarly, the lateral direction is that perpendicular to the path
of the particle.
11 ‘Pile-up noise’ in the calorimetry can arise from multiple ‘pile-up’ events in the same bunchcrossing
in which the primary interaction occurred, or from interactions in bunch-crossings close in
time to that of the primary interaction. The latter is possible because the response time of the
calorimeters is longer than the 25 ns bunch-crossing interval.