Vertex Detector Offline Documentation
A number of VTX routines interface with GEANT routines to initialize
VTX geometry for the Monte Carlo. The VTX driver
which is called from the SLD routine
SGGINI is usually called after the first "GO" statement in IDA. The default
VTX geometry is stored in the template
VXBASE. There are many
different versions of VXBASE which can be loaded in order to
simulate various incarnations of VXD2 and VXD3. These versions
are controlled by poking
VXBASE.VXDVERS. The correct
version must be poked before the first event has been run. In other
circumstances, (e.g. running DISPLAY),
should be declared EXTERNAL and called after poking
VXBASE.VXDVERS in order to
assure that the correct geometry is loaded. The correct version of
VXBASE is loaded by using
a unique CONSTANT file depending on which VXD version is needed.
See the source code for
(which is called from VXINIT.PREPMORT)
to see how these various constant files are loaded.
The templates necessary for setting up geant are:
The GEANT simulation uses the design VXD geometry. This is necessary
since everything in GEANT is always done in a symmetric way. GEANT
simulation, does not include random smearing of CCD's or actual charge
deposition on various pixels. That is the job of the
Simulation of Charge Deposition and
Because the GEANT simulation uses the ideal position
of the Vertex Detector CCD's and doesn't actuall simulate charge
deposition or triggering, it is necessary to process the Monte
Carlo hits through a simulation and digitization stage.
The simulation is controlled by the templates
Contains the location, size and orientation of all VTX elements.
Contains information which GEANT needs like name, mother volume, shape, etc.
Contains information on various material needed by the vertex detector.
Contains definitions of all material used in GEANT.
There are two steps in the simulation of VXD hits.
The first is the Digitization stage. This is done in
which is called from GUDIGI.PREPMORT. The primary purpose of VTXDIG.PREPMORT
is to take Monte Carlo Hits from the MCHITS.TEMPLATE bank and to extrapolate
them to the plane of the actual CCD. The location of the hit on or near the
CCD is determined and the amount of charge depoisted is determined
All information from VTXDIG is stored in the temporary VXD bank
The second step in the simulation of hits is the AEB (Aleph Event Buffer)
stage. The main driver routine for this is
which is called from GUSAEB.PREPMORT. VTXAEB.PREPMORT is used to smear
the charge from a Monte Carlo hit into adjacent pixels, to add beam related
background and electronic noise to the CCD's, and to simulate the
Cluster Arithmetic Processor which is responsible for sparsifying
the data using an Extended Row Filter and kernel
thresholds. VTXAEB.PREPMORT is also used
for overlaying random trigger data events with Monte Carlo simulated events.
The output of VTXAEB.PREPMORT is the raw data in a Monte Carlo packed
format stored in
A flow chart of VTXAEB is available.
The reconstruction is done almost exactly the same for data and
Monte Carlo. However, the input raw data bank is in a different format
for Monte Carlo
(VXRAW.TEMPLATE) and for data
The reconstruction is controlled by the template
A flow chart of the Reconstruction Process
is available as well as
a chart showing the Reconstruction Data Structure.
The Reconstruction process consists of five major
steps controlled by different processors.
Flow charts are available for these processors.
Here is a list of templates which provide control parameters, switches,
and constants for much of the offline code. The default parameters
for most of these files are stored in *.CONSTANT files with the
same name as the TEMPLATE file. However,
VXREAL are part of the
run dependent constant system and are controlled by VTXKON.CONSTANT.
also has many versions
and the appropriate CONSTANT file is loaded by poking
- The first step
is the primary extrapolation in which
the CDC tracks are extrapolated to all layers of the VXD.
The track parameters and errors on each CCD plane are stored.
- In the second step
pixels are grouped together in clusters. The standard reconstruction
only does this clustering on CCD's which have been intersected by
a track extrapolation including its errors. Only about half the CCD's
have this track extrapolation, so this speeds up the time necessary for
- The third step
is called the linking stage. In this step the CDC track
is constrained to go through a cluster within the primary error
ellipse. The track is refit with this constraint and extrapolated
to inner layers. A smaller secondary error ellipse is formed and
a search is made for clusters inside this error ellipse. Any track
with at least one primary cluster and one secondary cluster is a
track candidate. A secondary extrapolation is performed for every
cluster inside every primary error ellipse on all layers for every
track. This leads to many track candidates consisting of a CDC
track and possible VXD clusters. Subsets of possible links are eliminated
and all possible links
(VXVEC.TEMPLATE) are stored in
- The fourth step
consists of looking for track candidates with only a single associated
VXD cluster. This is done only for tracks without a multiple VXD
cluster track candidate. Tracks are constrained to go through the
interaction point, and clusters near the track-CCD intersection are considered
good track candidates.
- In the fifth and final step,
all possible track candidates are sorted
by their quality, and the best candidate(s) are stored in the DST.