recent improvements in track reconstructionbmangano/cms_talks/byboris/tracking... · g.petrucciani)...

B.ManganoOpening Session- CMS Week - February 2008

Boris Mangano

(University of California, San Diego)

on behalf of the tracking group (Tracker Project)

Recent Improvements in TrackReconstruction


Outline

• one slide description of track reconstruction in CMS

• improvements in tracking

- description

- effects on tracking performance

- effects on “physics”

• evolution of timing performance over past CMSSW releases


Tracking modules

Trajectory seeding

Trajectory fitting

Trajectory building

Initial estimate of trajectory parametersfrom a small subset of trackermeasurements (hits on seeding layers).

An iterative process which collects all themeasurements associated to the samecharged particle.

Estimate of final track parameters from the fitof the full set of measurements associated tothe same charged particle.

Track collectionfiltering

new

Removal of “ghost” tracks (fakes) + qualityfilter.


glob

al e

ffici

ency

|η|

Porting of code from ORCA

-•- single muon, pt = 1 GeV



ORCA performance

CMSSW_1_3_X

performance

ORCA tracking performance ~ re-obtained with CMSSW_1_3_X release(beginning of 2007)

Actually some new features were already introduced: e.g. mixed seedinggave better tracking efficiency in the high eta region.


New Feature: Tracking with “overlaps”

Original trajectory builder was able to collect only onemeasurement per layer per track.

Nevertheless the tracker has overlapping sensors:

sometimes there is more than one measurementproduced by the same charged particle on the sametracker layer


standard building

building “with overlap”

hit c

olle

ctio

n ef

ficie

ncy

|η|

New Feature: Tracking with “overlaps”

New trajectory builder is able tocollect hits on all the overlapping

sensors of the CMS tracker

hit collection efficiency:

from ~88% to 98%

new trajectory builder

old trajectory builder

fully integrated and used by defaultin CMSSW_1_7_X

Thanks to:

- W.Adam (Wien)

- P.Lenzi (INFN Florence)


New Feature: Handling of Magnetic FieldIn-Homogeneities During Track Fit




|η|

approximate handling ofmagnetic fieldinhomogeneities produces abias in the reconstructedvalue of transversemomentum

Effect is apparent in high etaregion of the tracker

average (pt reco - pt sim)/pt error


-•- single muon, pt = 1 GeV-•- single muon, pt = 10 GeV-•- single muon, pt = 100 GeV

|η|

Thanks to:

- T.Todorov (now in ATLAS)

- M.Mulders (CERN)fully integrated and used by defaultin CMSSW_1_8_X

Track parameterspropagator based onRunge-Kutta methodallows to properly take intoaccount the magnetic fieldinhomogeneities duringthe final fit


average (pt reco - pt sim)/pt error


J/ψ reconstructed mass

mean = 3.096 GeV

plot by M.Cepeda(CIEMAT)


Bias in the reconstructed J/ψ mass iscompletely removed thanks to theRunge-Kutta propagator


CMSSW_1_7_X

CMSSW_1_7_0+ 7 fixes/improvements

|η|

aver

age χ2 / n

dfAdditional New Features: Impact on

Quality of Reconstructed Track

7 independent new features oftrack reconstruction haveexceptionally improved the χ2 ofreconstructed tracks

Individual contributions are shown inthe following slides

QCD 80-120sample


|η|

aver

age χ2 /n

df

default tracking inCMSSW_1_7_X

+ updated materialbudget parameterizationfor track fitting Thanks to:

- A.Bocci (SNS Pisa)

- W.Adam (Wien)

Additional New Features: Impact onQuality of Reconstructed Track

QCD 80-120sample


+ better track anglehypothesis duringclusters re-fitting

|η|

aver

age χ2 / n

df

Thanks to:

-G.Cerati (INFN Milano)

-C.Genta (INFN Florence)



QCD 80-120sample


+ splitting of matchedhits before final fit of thetrack

|η|

aver

age χ2 /n

df

Thanks to:

-G.Cerati (INFN Milano)

-C.Genta (INFN Florence)



QCD 80-120sample


+ Runge-Kuttapropagator during finalfit of the track

|η|

aver

age χ2 /n

df



QCD 80-120sample

Thanks to:

- T.Todorov (now in ATLAS)

- M.Mulders (CERN)


+ new pixel parametersestimator based ontemplates

|η|

aver

age χ2 / n

df

Thanks to:

-M.Swartz, D.Fehling,G.Giurgiu, P.Maksimovic(JHU)

- V.Chiochia (UniversityZurich)



QCD 80-120sample

review of the severalbenefits for trackingdue to new pixel CPEare shown inCMS NOTE 2007-033


+ tool for rejection of

outlier hits

|η|

aver

age χ2 / n

df

Thanks to:

G.Cerati (INFN Milano)



QCD 80-120sample

Impact of outlier rejection ontracking efficiency, fake rateand estimation ofparameters at vertex will bedescribed in a note, currentlyin preparation


+ finer description of

material budget for final fit

of tracks

|η|

aver

age χ2 /n

df

Thanks to:

A.Bocci (SNS Pisa)



QCD 80-120sample


old standard tracking + newtrack collection filter

old standard tracking

QCD 80-120sample

old standard tracking+ new track collectionfilter


CMSSW_16X

new features: track collection filterbased on vertex compatibility

Thanks to:-P.Azzurri (SNS Pisa)-M.Pioppi, P.Janot (CERN)-F.Ambroglini (INFN Perugia)

|η|

|η|Fake rate is almost completely removedwithout spoiling the track reconstructionefficiency

selection is based on d0 and dz(and significances) respect toreconstructed primary vertex

efficiency and fake forpt > 0.9 GeV


New Pixel CPE and New Track Filter:effects on b-tagging

QCD 50-120sample

b-tagging efficiency

mis

-tag

Tests on b-tagging byM.Swartz, D.Fehling,G.Giurgiu, P.Maksimovic(JHU) and T.Boccali (INFNPisa)

track counting, 2nd track (w/o new CPE)

track counting, 2nd track (new pixel CPE)

track counting, 3rd track (w/o pixel CPE)

track counting, 3rd track (new pixel CPE)

Impact on btagging performance are impressive forboth track counting configurations.

The btagging algorithms have not been changed


new features: track collection filterbased on vertex compatibility

studies and plot byM.Lebourgeios (UCSD)

“cut-based” loose electrons:isolation performance

Lowering the tracking fake rate, the electron isolation performanceimproves (without any changes in the isolation algorithm itself!)



new trackingconfiguration

QCD 80-120sample

new features: seeder based on tripletsand vertex-constrained hit pairs

pt spectrum of reconstructedtracks can extended towardlower values:

New seed generator is based ontriplets of hits + hit pairs constrainedby the primary vertex.

Both high efficiency and high seedcollection purity are achieved

up to +65% reconstructed trackincreasing only slightly thealgorithm timing

Thanks to:

-M.Konecki (Warsaw)

-G.Boudoul, T.Legrand (IPNL Lyon)

-G.Petrucciani (SNS Pisa)

normalizeddistributions


Evolution of Timing PerformanceAlmost factor 3 reduction in timing:

SMatrix migration and C++ codeoptimization (V.Innocente,G.Petrucciani)

Almost same timing, despite +65%reconstructed tracks (thanks to newseed generator):

minimum Pt from 0.9 GeV to 0.3GeV

aver

age

time/

even

t (se

cs)

all tests done with:

- BJets_Pt_50-120sample w/o PU

- 2.0 GHz E5335 intelprocessor

- gcc345 compiler


Evolution of Timing Performance

mode

old code (before June2007)

global unpacking

regional unpacking

unpacking on-demand

861

86

7

4

- sample: single tau

- time is per event.Averaged on eventswhich pass L2.5

Thanks to:

- M.Wingham, N.Cripps,R.Bainbridge (Imperial college)

- D.Giordano (INFN Bari)

- J-R. Vlimant (UCSB)

More than a factor 100X inreducing the timing of trackerclusters unpacking.

Clear impact on HLT performance

SiStrip clustersunpacking time (ms)


Conclusion

• Last 12 months have been rich of activities related to track reconstruction

• The current status of the tracking software is well beyond what was availableafter the porting from ORCA:

- almost no fake tracks (from 20% to few percent)

- low pt tracking (from 0.9 GeV to 0.3 GeV)

- faster reconstruction (factor 2 in speed, despite the more sophisticated

algorithms)

• Many additional ideas are under investigation and development.

• Current studies are more “real tracker conditions” oriented.


Conclusion

Higgs -> GammaGamma

sample

performance for 10pb-1 conditions areexpected to be under control:

-resolutions of parameters at vertex arenecessarily degraded

- efficiency and fake rate are almostexactly the same

tracking for ideal conditions

tracking for alignmentconditions after 10pb-1

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