The reconstruction of the SSD hits is performed in two steps implemented in
two different modules.
The first module
SSD Cluster Finder (SCF) scans the
strip signals on both sides of each SSD wafer in order to build clusters.
This module :
These steps are schematically described here. A
detailed description of the module can be found here.
- reads and sort the strip signals from the spa_strip table
- searchs clusters by :
- scanning the strips and looking for a central strip with an amplitude
above a given threshold
- scanning the neighboring strips in the list looking for consecutive
strips with respect to the central strip.
- creates a cluster list and add any new cluster
- scans all the clusters and tries to split the cluster by looking for a
local minimum in the cluster profile.
- updates and sorts the cluster list and finally stores the cluster in
the scf_cluster table.
The second step is performed by the
SSD Cluster Matcher
(SCM) module with two main goals : to associate the clusters found on
the sides of each wafer and to determine the main parameters (position,energy
loss) of the resulting space point.
Silicon microstrip wafers are not real bidimentional detectors and
consequently clusters found on the n-side and the p-side of the detector
need to be correlated based on some criteria. Depending on the particle
multiplicity and their positions in the wafer, such correlation can be non
trivial and can lead to ambiguous associations. Hopefully, for a given
particle, the charges collected on both sides of the wafer are produced
by the same energy deposite and a strategy based on a charge matching can
be applied to help to correctly select the true associations.
Depending on the number of clusters on each side (latter on referenced as
"packages") involved in this association problem, several configurations
can be encountered. They are listed
here and the cases
1, 1-2, 2-2,
3-3 are illustrated.
The following steps are performed in the module :
- reading of the clusters from the scf_cluster table
- in each wafer, the cluster matching is started by looking for and creating
new packages by mean of geometrical correspondances
- an attempt to solve each package is then made by determining which
configurations are possible. The charge matching is then used to assign
relative probabilities to each configuration and a given weight to the
resulting space points.
- Each space point is then caracterized (x-y-z position,u-v position,
energy loss and quality factor) and add to a list
- The space points are stored in the scm_cluster and scs_spt tables.
The list of tables used and methods defined in SCM to
perform these steps can be found here. An example of
how the geometrical correspondance is applied and how the packages are found
is illustrated here. Solutions to apply
the charge matching and to assign relative probabilities are shown for the
cases 2-2 and
The performances of the hit
reconstruction modules will be evaluated with the help of the
SSD Cluster Evaluator (SCE) module. The impact of
their efficiencies on the tracking performance will be also studied. This
module still needs to be written.
Last modified: Wed Jan 19 16:57:07 MET