Alignment of the
ALICE Si tracking detectors
A.Dainese
INFN Padova
for the ALICE Collaboration
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
1
Layout






The ALICE Inner Tracking System (ITS)  see also V. Manzari
ITS alignment strategy
Cosmics for alignment
Validation of survey measurements
Results from software alignment algorithms
Summary and outlook
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
2
Inner Tracking System (ITS)
Silicon Pixel Detector (SPD):
• ~10M channels
• 240 sensitive vol. (60 ladders)
Silicon Drift Detector (SDD):
• ~133k channels
• 260 sensitive vol. (36 ladders)
Silicon Strip Detector (SSD):
• ~2.6M channels
• 1698 sensitive vol. (72 ladders)
SSD
SPD
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
SDD
Andrea Dainese
ITS total:
2198 alignable
sensitive
volumes
 13188 d.o.f.
3
yloc
ITS detector resolutions &
target alignment precisions
zloc
xloc
detector local c.s.: xloc~rfglob, yloc~rglob, zloc = zglob
SPD
SDD
SSD
(r = 4 & 7 cm)
(r = 14 & 24 cm)
(r = 39 & 44 cm)
nom. resolutions
xloc  (yloc)  zloc [mm3]
12  (0)  100
35  (0)  25
20  (0)  830
residual mis. (shifts)
xlocyloczloc [mm3]
10  10  20
20  20  20
15  15  100
Residual: expected misalignment left after applying the
realignment procedure(s). Target ~0.7resol.  ~20%
degradation of the resolution
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
4
Impact of ITS misalignment
on ITS+TPC tracking resolutions

Effect of misalignment on track impact parameter to the
primary vertex (d0), pt, vertex resolutions studied
d0 resolution
no misal
misal 10 mm
misal 20 mm
misal 30 mm
pt resolution
Target
residual
misalignment
effect of misalignment:
 large worsening
 factor 2 at high pt
 plus 5 mm
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
no misal
misal 10 mm
misal 20 mm
misal 30 mm
effect of misalignment
above 10 GeV/c
Andrea Dainese
5
ITS alignment with tracks:
general strategy

Data sets: cosmics + first pp collisions (and beam gas)


use cocktail of tracks from cosmics and pp to cover full detector surface and
to maximize correlations among volumes
Start with B off, then switch on B (pp)  possibility to select high-momentum
(no multiple scattering) tracks for alignment


General strategy:
1) validation of survey measurements with cosmics
2) start with layers easier to calibrate: SPD and SSD
o good resol. in rf (12-20mm), worse in z (120-830mm)
3) global ITS alignment relative to TPC (already internally aligned)
4) finally, inclusion of SDD, which need longer calibration (interplay
between alignment and calibration)
Two independent track-based alignment methods:
 global: Millepede (default method)
 local: iterative method based on residuals minimization
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
6
Triggering and tracking the cosmics

Trigger: SPD FastOR



Coincidence between top outer SPD layer
and bottom outer SPD layer
rate: 0.18 Hz
AND
ITS Stand-Alone tracker adapted for cosmics


pseudo-vertex = point of closest approach between two “tracklets” built
in the top and bottom SPD half-barrels
Search for two back-to-back tracks starting from this vertex
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
7
Cosmic data sample

Statistics collected (after reco): ≈105 events with B=0 in 2008
Layer 1
(SPD)
Layer 4
(SDD)
Layer 1
(SPD)
Layer 4
(SDD)
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Layer 5
(SSD)
Andrea Dainese
8
Validation of Strips (SSD) survey
with cosmics

SSD survey: 2 local shifts (x,z) + 1 rotation (q)



Modules on ladders (critical: small stat on single modules with cosmics)
o precision ~5 mm
Ladders on support cones (important starting point for alignment)
o precision ~15 mm
Validation with cosmics. Three methods:
1.
2.
3.
Extra clusters from acceptance overlaps  distance between two
clusters from same track on contiguous (overlapping) modules on
same ladder
Fit one track on outer layer, one on inner layer  distance and angles
between the two tracks
Fit track on one SSD layer (2 points)  residuals on other SSD layer
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
9
Validation of SSD survey
with cosmics (1)

Extra clusters from acceptance overlaps  distance between two
clusters from same track on contiguous (overlapping) modules on same
ladder
module
on ladder
misalignment
xy)=25 mm (48)
point)=25/√2=18 mm
misal)<5 mm (27)
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
xy)=26 mm (36)
point)=26/√2=18 mm
misal)<5 mm (15)
Andrea Dainese
10
Validation of SSD survey
with cosmics (2)

Fit one track on top “half barrel”, one on bottom  distance
and angles between the two tracks
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
11
Validation of SSD survey
with cosmics (3)

Fit track on one SSD layer (2 points)  residuals on other
SSD layer
overall module
misalignment
point)~25 mm
misal)~10 mm
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
12
ITS alignment with Millepede


Determine alignment parameters of “all” modules in one go, by
minimizing the global c2 of track-to-points residuals for a large set of
tracks (cosmics + pp)
The alignment of the ITS follows this hierarchical sequence:
ITS Schematic
SPD
layout
SPD SECTORS (10)
optionally: SPD STAVES (60)
SPD HALF-STAVES (120)
SPD LADDERS (sensitive
modules 240)
SPD BARREL W.R.T. SSD
BARREL
SSD LADDERS (72)
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
13
Checking the quality of realignment
y




Select muons with DCA to (0,0) < 1 cm
Main variable: track-to-track xy at y=0
Acceptance overlaps  “extra” clusters
Alignment monitoring tool
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
x
14
Millepede SPD realignment:
xy at y=0

Track-to-track matching (2 points per track in the pixels)
Sim, ideal geom:
DATA B=0
not realigned
realigned
43 mm
48 mm
track-to-track xy [cm]
Expected
spread
 spatial=14 mm  misal~9 mm
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
 spatial=11 mm (Sim)
Andrea Dainese
15
Millepede realignment:
SPD “extra” clusters
Sim, ideal geom:
DATA B=0:
not realigned
realigned
15 mm
20 mm
Expected
spread
 spatial=14 mm misal~9 mm
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
 spatial=11 mm (Sim)
Andrea Dainese
16
Cross checks with B-on runs

Stand-alone tracking in SPD (two points) and look at “extra”
clusters to track distance (unaffected by curv.)
B=+0.5T
B=0
172 mm
181 mm
OK
B=-0.5T
202 mm
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
17
Millepede SPD realignment:
getting right the hierarchy
non-hiearchical
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
hiearchical
Andrea Dainese
18
Millepede SPD-SSD realignment:
xy at y=0

Pixels+Strips: two tracks (top and bottom) with 4 pts each
Sim, ideal geom:
DATA B=0
realigned
19 mm
30 mm
track-to-track transverse distance at y=0 [cm]
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
19
Millepede SPD-SSD realignment:
xy at y=0

Pixels+Strips: two tracks (top and bottom) with 4 pts each
Simulation:
ideal geometry
target of alignment
DATA B=0
realigned
promising!
already close
to target
30 mm
track-to-track transverse distance at y=0 [cm]
single track impact parameter resolution 30/√2~21mm
but pt “unknown”  need data with B-on (2009)
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
20
Alignment monitoring


Excluding selected points from
fit
Compute residuals
fit points
residuals
SSD
SPD
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
21
Alignment monitoring

rf residuals in inner pixel layer, with B-off and B-on
data B=0
=26mm
data B=+0.5T data B=-0.5T
=26mm
=26mm
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
simul B=+0.5T
=17mm
Andrea Dainese
22
A second alignment method:
Iterative local approach




Alignment params from minimization of residuals
Local: works on a module-by-module basis
Iterations are used to take into account correlations between
the alignment params of different modules
For the pixels, similar alignment quality as Millepede:
Millepede
 = 49 mm
track-to-track transverse distance at y=0 [cm]
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Iterative
 = 52 mm
track-to-track transverse distance at y=0 [cm]
Andrea Dainese
23
Comparison of Millepede and Iterative

Comparison of alignment parameters is promising for:


estimate of residual misalignment / systematics
investigations of possible problems (outliers)
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
24
Silicon Drift Detectors
calibration & alignment

The two intermediate layers
In SDD, local x determined from drift time:
xloc = (t – t0) × vdrift



two calibration parameters: t0 and vdrift
xloc

Interplay between alignment and calibration
t0 and vdrift (also obtained from injectors) as
additional parameters in Millepede
Geometry only
Geometry + calibration
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
25
Cosmics 2009

Show validation of 2008 alignment
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
26
Summary & Outlook

ALICE Inner Tracking System alignment strategy presented



Strips (SSD) survey validated with cosmics
Most of pixels (SPD) realigned to 10 mm



Hierachical track-based alignment using global fit (Millepede)
need cross-checks with pp, especially on the sides
With SPD+SSD  track impact parameter resolution ~20
mm, but difficult to conclude because momentum is
unknown (B=0)
Next steps:




include SDD (calibration ongoing)
ITS-TPC alignment
cosmics 2009 (data taking in progress)
o data with B=0.5T  performance VS pt!
finalize alignment and monitoring for proton-proton
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
27
EXTRA SLIDES
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
28
TPC-ITS relative alignment






Kalman-filter aligner with 8 model parameters: misalignment
of TPC wrt ITS (6 pars), + TPC calibration (time zero, drift
velocity)
Procedure starts with matching of TPC tracks to ITS standalone tracks
Propagate tracks to reference cylinder at R=80 cm 
residuals
The residuals are fed to the Kalman aligner for fitting
Benchmarked on simulation  OK
First result on cosmic data with B=0 (ITS aligned with
Millepede)
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
29
TPC-ITS relative alignment

TPC vdrift correction from TPC-ITS alignment VS same
from Temp&Pressure sensors  difference <0.5%
(B on)
S.Rosseger

Data re-reconstructed with TPC-ITS alignment  new
realignment  results consistent with 0  OK
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
30
First look at statistics in pp


10k (scaled to 100k) pp events at 0.9 and 14 TeV B=0.5T
Minimum number of points for module in each layer

6 points tracks with pt>250 MeV
SPD SPD SDD SDD SSD SSD
inner outer inner outer inner outer
0.9 TeV
100k
evts
1500
1000
1000
500
100
50
14 TeV
100k
evts
3000
3000
3000
2000
200
200
 First Physics sample(s) look OK (statistics-wise)
to perform First Alignment
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
31
First look at statistics in pp:
checking the alignment with extra clusters


10k (scaled to 100k) pp events at 0.9 and 14 TeV B=0.5T
Number of extra clusters per layer per det type (assuming
the tracking finds all of them)
SPD
(~1%, rf)
SDD
SSD
(~1%, rf, z) (~1%, rf, z)
0.9 TeV
~2000
100k evts
~2000
~2000
14 TeV
~6000
100k evts
~6000
~6000
 First Physics sample(s) look OK (statistics-wise)
to layer-by-layer check of First Alignment with extra cls
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
32
Millepede SPD realignment:
stability in time
Stability test with nine 10k-tracks subsamples
xy at y=0 plots with
same alignment file
From trk
0
10000
20000
30000
40000
50000
60000
70000
80000
Mean (mm)
-13
-4
+6
-3
-2
+1
+5
+8
-1
Sigma (mm)
57
54
52
56
53
53
52
52
54
xy [cm]
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
33
Analysis of Millepede results
XY
mean
no align. -1021
sigma
xy [cm]
-
sectors
174
610
staves
-12
95
HS
1
63
ladders
0
55
Strongest improvements when
aligning staves w.r.t. sectors
VERTEX2009, Veluwe – the Netherlands, 14-18.09.2009
Andrea Dainese
34