B S T   a n d   F S T   r e c o n s t r u c t i o n
Last updated     by  Thomas Naumann


1. Hit finding
2. 2D pattern recognition
3. 3D reconstruction
4. Monitoring
5. Alignment

The BST and FST reconstruction software is coded in the
H1REC CMZ file in the patches CT_BSTREC and CT_FSTREC.

It consists of four steps:

1. H i t  f i n d i n g

The raw data from the SIFE banks are clustered to hits packed
in the BRSE and BRUE banks for the BST r and u detectors and
in the FRSE banks for the FST detectors. The clustering reduces
the number of hits per event from more than 100 to about 20
for the r detectors.

The signal-to-noise ratio is about 13 for the r detectors
and 25 for the u detectors.

2. 2 D   p a t t e r n   r e c o g n i t i o n

The local mapping functions BSTMAPR, BSTMAPU and FSTMAP transform
the packed digital hits into 2D space points in the local BST and FST
coordinate systems. They only use the coordinates measured by a
given detector type (r,u,v) and allow track finding in the
local r-z, u-z and v-z coordinate systems.

The pattern recognition starts from the triplet sagitta defined as

   sag = (r1+r3) / 2 - r2

for hits with coordinates ri in 3 subsequent detector planes i.
It is zero for straight and a constant for circular tracks.
Based on the sagitta 2D track candidates in r-z, u-z or v-z
are combined and fitted as straight lines.

The widths of the unfiltered combinatorical sagitta distributions
are 18 um for the r detectors and 26 um for the u detectors.
The tails of the sagitta distributions are due to nonzero curvatures.

Before pattern recognition the distributions of hits over the u and r
detectors contain noisy channels or regions. After pattern recognition
they reflect the signal distribution which is a product of
physics and detector geometry.

The detector resolutions are derived from the residual distributions of the
hit coordinates w.r.t. the straight line fit. They are a convolution of the
intrinsic resolution of the Si detectors and alignment. For the r detectors
a resolution of 18 um is reached:

This resolution is a mixture of the resolutions of single and double hits.
An upper limit of 13 um for the intrinsic detector resolution comes from
the resolution of the best aligned sectors.

3. 3 D   r e c o n s t r u c t i o n

First, 2D track candidates are combined: r-z and u-z for the BST and
u-z and v-z for the FST. This means combining r-z and u-z hits for the
BST and u-z and v-z hits for the FST to 3D xyz points.

Now, the global mapping functions BSTALI and FSTALI apply the
global alignment transformations to the 3D space hits.
They can be written out as standard hit banks BRXX and FRXX and e.g.
be visualized in the H1 event display at debug level 1.

Then, circles are fitted in x-y. Using the curvature, straight lines
are fitted in s-z with s being the arc length of the helix.
The event vertex can be included to the both fits. The default is to
include the CSKV vertex to the x-y circle fit but to avoid this in s-z.

The resulting 5 parameters of the helix, the covariance matrix and
the chi2 and ndf's of the x-y and s-z fits are stored in the
BJKT and FJKT track banks for BST and FST. Both banks use the H1
coordinate system. Transformations to vertex coordinates are left to the user.

BST2 in general only measures r-z. To get the hit and track
parameters as precise as possible in alignment and analysis
external phi information is taken. The mapping function
BSTMAPF performs both local and global transformations and uses
phi information.

4. M o n i t o r i n g

The ideal status is documented in the Monte Carlo reference plots.
During online reconstruction the following quantities are monitored:

4.1  Hit related detector properties

Signal/noise, occupancies (raw+fitted), efficiencies (internal+external)

4.2  Track related alignment and reconstruction properties

sagitta, residuals, vertex pointing