Download ppt - Tracker Alignment with MillePede

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


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


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)


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


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


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


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


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)


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.


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..


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…


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