CMS Torino meeting,
4th June, 2007
R. Castello on behalf of Torino Tracker’s group
Tracker Alignment with MillePede
CMS Torino meeting R.Castello4th June 2007
Outline
• Why we need alignment?
• Alignment with MillePede Algorithm
• Alignment with TIF trigger configuration on simulated cosmics data
• Preliminary results
• Status of the alignment with TIF real data
• Conclusions and outlook
CMS Torino meeting R.Castello4th June 2007
Why we need alignment?
• The CMS tracker is build in order to optimize the particle momenta resolution.
• It depends on two factors:
21 CpCp
p
C2 depends on MCS
C1 is geometry -dependent
B = magnetic field
L = track length
n = # hit of the track
σx = resolution on the measured point
sistx2
int2
21LBn
C x
~ 10 μm (Si)
Systematic error can be minimized by a correct alignment
CMS Torino meeting R.Castello4th June 2007
The tracker alignment
1. Survey measurements (during assembly) = 100-200 m2. Laser Alignment System (LAS) : alignment of TIB vs TOB etc ...3. Track Based Alignment (cosmics, Z→ etc) = 10 m
The problem16k microstrip modules
6 d.o.f per moduleO (100k) unknowns
• Complex system of equation to solve efficient and fast algorithm
• For CMS tracker alignment 3 algorithms:
HIP MillePede Kalman Filter
(Helsinki, Milano, Perugia) (Hamburg, Torino) (Wien)
CMS Torino meeting R.Castello4th June 2007
MillePede Algorithm
MILLE PEDE ( V. Blobel )
• A measurement can be written as
• Linearised least square combined fit of alignment parameters (global) and track parameters (local)
• MIllePede uses the Method of the Least Squares
residuum, where mk is the
measurement with uncertainty k and dk is the
coefficient vector.
the idea : minimize square of residuum.
fitk
hitkk
Tkk uudamr
kk
kT
k dama
2
2 )(
jj
ji
n
ii daz
11
global
local
CMS Torino meeting R.Castello4th June 2007
Alignment with MillePede
• Interested to the n global parameters.• For a set of N measurements: from (n+N) equations to n
• C’ matrix inversion (Computational time ~ n3 )
• With MillePede you can align @ different levels (Detectors, String, Layer level, etc…)
• 6 degrees of freedom for each alignable structure (6 parameters):
3 shifts (respectively along local x,y, z) and 3 rotations (around x,y,z)
the aim it’s to find a with good uncertainty
),,,,,( wvua
baC '' baC global
CMS Torino meeting R.Castello4th June 2007
MillePede Algorithm in CMSSW(implemented by G. Flucke, Hamburg)
- CMSSW 1_3_1 version - Alignment/MillePedeAlignmentAlgorithm (tag branchV00-07-0X-01) - Cfg file: AlignmentTrackSelectorModule:- selection of Tracking algorithm
- set APE - set cut for tk selection
(pt, #hits,..)
AlignmentProducer: - selection of alignment parameters - geometrical selection in eta, phi,z
- selection of misalignment scenario - set solving method ( inversion, etc.) - set 2 range acceptance PoolSource: - selection of dataset - set number of events
CMS Torino meeting R.Castello4th June 2007
Cosmics simulation @TIF (Tracker Integration Facility)
Old scintillator configuration ( 36k events)
New scintillator configuration ( 20k events)
Since Feb.’07 the 25% of the tracker system is under commissioning
CMS Torino meeting R.Castello4th June 2007
RMS =263 rad
Rod Level (79 alignment
parameters)
RMS =151 m
Results on TIF cosmics simulated sample (old trigger configuration)
with Short Term misalignment scenario (~100 pb-1)
CMS Torino meeting R.Castello4th June 2007
u (Strings level)
Good correlation between u at the start (misaligned) and at the end (after the alignment)
Global correlation : largest correlation of a parameter with any linear combination of all other parameters. A value close to 1 means solution not well determined.
CMS Torino meeting R.Castello4th June 2007
Simulated cosmic sample with old scintillator configuration (x axis)
Sim
ula
ted c
osm
ic s
am
ple
wit
h n
ew
sci
nti
llato
r co
nfig
ura
tion (
y a
xis
)
agreement for u
Old and new MC configuration
To be understood..
CMS Torino meeting R.Castello4th June 2007
u and for real data (10th March run)
RMS =21 m
RMS =79 rad
Rod Level (71 alignment
parameters)
Too precise if compared with the “expected” results
under studying…
CMS Torino meeting R.Castello4th June 2007
Conclusions & outlook
• Preliminary results obtained running MIllePede alignment algorithm on TIF simulated sample for u and parameters at two different hierarchical levels (Strings & Layer )
• No relevant differences between new and old scintillator configuration on simulated data (a first indication of algorithm stability)
• The exercise on real data is going on
• Good impression and relevant hints from recent Hamburg workshop:
http://indico.cern.ch/conferenceDisplay.py?confId=16095