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

`Contents:`

`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:`

`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.`

`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`