Description of ALICE ITS Code (Rev. 0.3 — May, 16th 2008 — M. Masera)
The goal of this document is to give a concise description of the ITS software. For more general information the reader is referred to the ALICE offline documentation.
The reader is assumed to have a basic knowledge of AliRoot and of the Inner Tracking System Detector.
The documentation is organized in the following sections:
SPD. This section gives information on SPD simulation, local reconstruction, calibration and specific run types. Author: D. Elia.
SDD. As above. Author: F. Prino.
SSD. As above. Author: E. Fragiacomo.
Geometry. In this section a short description of the implementation of the ITS geometry used for simulation and reconstruction is given.
ITS tracking. This section provides information on the trackers used to prolong track candidates from the TPC (AliITStrackerMI class) and to find and fit tracks with the ITS alone (AliITStrackerSA). Information on cosmic muon reconstruction is also provided. To be written.
Primary vertex measurement. This section describes the vertexers (AliITSVertexerZ and AliITSVertexer3D) used to provide a first estimete on the postition of the primary vertex position based on the local reconstruction of the 2 pixel layers. Author: M. Masera. To be written.
Charged particle multiplicity.
(Author: T. Virgili)
The charged particle multiplicity is determined with the
AliITSMultReconstructor class, which measures the number of
clusters and tracklets.
A tracklet consists of two ITS clusters, one in the first pixel layer and one in the second. The clusters are associates if differences in Phi (azimuth) and Zeta (longitudinal coordinate) are inside a fiducial region, defined by appropriate cuts These cuts depend on the interacting system (p-p or Pb-Pb) and can be set via SetPhiWindow and SetZetaWindow methods. In case of multiple candidates, the one with minimal azimuthal difference is chosen.
It is also possible to control if two clusters in layer 2 can be associated to the same cluster in layer 1 or not. This is defined by means of the AssociationChoice (TRUE means double associations exluded; default = TRUE) method.
The method GetNTracklets returns the number of tracklets and the relative information: theta and phi of the tracklet (as measured by layer 1), Delta phi (phi difference between layer 1 and 2), the label of the particle(s) producing the clusters.
The unassociated clusters can be retrieved by the method GetNSingleClusters.