ring recognition and electron identification in the rich detector of the cbm experiment at fair...

Ring Recognition and Electron Identification in the RICH detector of the CBM Experiment at

FAIR

Semeon Lebedev GSI, Darmstadt, Germany and LIT JINR, Dubna, Russia

Claudia HoehneGSI, Darmstadt, Germany

Gennady OsoskovLIT JINR, Dubna, Russia

[email protected]

Electron Identification in CBM RICH CHEP 2009, 20.03.2009 2/18

CBM at FAIR

Facility for Antiproton and Ion Research• accelerator complex serving several experiments at a time (up to 5) from a broad community• SIS100 and SIS300 synchrotrons• highest beam intensities!(e.g. 2x1013/s 90 GeV protons and 109 Au ions/s at 45 AGeV beam energy)• rare isotope beams• first experiments ~2012, fully operational ~2016


CBM physics topics

Physics Topics Observables

In medium modifications of hadrons , , e+e- (μ+μ-)

D0, D±, D±s, c

Deconfinement phase transition,

charm production at threshold

K, , , , D0, D±

J/Ψ,Ψ' e+e- (μ+ μ-)

Critical point Event by event fluctuations

Exploration of the QCD phase diagram in the region of high baryon density and moderate temperature.


CBM experimentAim: hadron and lepton identification in large acceptance, good momentum and

secondary vertex resolution

• tracking, momentum tracking, momentum determination, vertex determination, vertex reconstruction: radiation hard reconstruction: radiation hard silicon pixel/strip detectors silicon pixel/strip detectors (MVD+STS) in a magnetic dipole (MVD+STS) in a magnetic dipole fieldfield

• hadron ID: TOF (& RICH)hadron ID: TOF (& RICH)• photons, photons, 00, , : ECAL: ECAL

• electron ID: RICH & TRD suppression 104

• PSD for event characterizationPSD for event characterization• high speed DAQ and trigger high speed DAQ and trigger → → rare probes!rare probes!

RICHTRD

TOF ECAL

PSDSTS

magnet

About 1000 charged About 1000 charged particles in central Au+Au particles in central Au+Au collision at a beam energy collision at a beam energy

of 25 AGeV in the CBM of 25 AGeV in the CBM acceptance. acceptance.


CBM RICH detector (I)

RICH in CBM will serve for electron identification for momenta up to 10 GeV/c -> study vector mesons and J/Ψ

RICH characteristics:RICH characteristics:• radiatorradiator : : - N2 length 2.5 m (large RICH) - CO2 length 1.5 m (compact RICH)• glass mirrorglass mirror of of 66 mm thickness mm thickness and aluminum and aluminum support structure:support structure: - mirror radius: 4.5m (Large) and 3m (Compact) - size: 22 m2 (Large) and 11.8 m2 (Compact)• photodetector Hamamatsu H8500photodetector Hamamatsu H8500 MAPMTMAPMT:: - 9 m2 -> 200k channels (large RICH) - 2.4 m2 -> 55k channels (compact RICH)• aboutabout 2200 hits/ electron ring hits/ electron ring

Two different options of RICH (Large Two different options of RICH (Large and Compact) are under discussionsand Compact) are under discussions

photodetectors

Large RICH

Compact RICH

mirrors


CBM RICH detector (II)

Part of typical event in the CBM RICH (Large and Compact). RICH hits (blue), found rings

(red), track projections (green).

ElectronsElectrons

PionsPions

Radius versus momentum for Radius versus momentum for reconstructedreconstructed rings in central Au+Au rings in central Au+Au

collisions at 25 AGeV beamcollisions at 25 AGeV beam energy for energy for UrQMD eventsUrQMD events (large RICH) (large RICH). A 3. A 3σσ band band around the mean radiusaround the mean radius is indicated by is indicated by

the solid linesthe solid lines..

Large RICH

Compact RICH

Typical reaction for CBM -> central Au+Au collisions at 25 AGeV beam energySimulation: transport model UrQMD


Reconstruction in the CBM RICH detector

Main problems of ring recognition in CBM RICH:• high ring density (~100 rings per event, many secondary electrons);• many overlapping rings;• distortions and elliptic shape of the rings;• measurement errors (the dimensions of sensitive pad are 0.6x0.6 cm and

mean ring radius is ~6 cm).• ring-track matching (high density of projected tracks)

sketch of RICH hits and sketch of RICH hits and found ringsfound rings

ring-track matchingring-track matchingSketch of the STS and the RICH Sketch of the STS and the RICH detector, track extrapolation and track detector, track extrapolation and track projection onto the photodetector planeprojection onto the photodetector plane


Ring recognition algorithm

Global search. Filter: algorithm compares all ring-candidates and chooses only good rings, rejecting clone and fake rings.

Standalone ring finder.

Local search of ring-candidates, based on local selection of hits and Hough Transform.

Two steps:


Ring recognition algorithm, local searchPreliminary selection of hitsPreliminary selection of hits Histogram of ring centersHistogram of ring centers

HoughHoughTransformTransform

Ellipse fitterEllipse fitter

Ring quality Ring quality calculation calculation

Remove hits Remove hits of found ringof found ring (only best (only best matched hits)matched hits)

Ring arrayRing array


Rejection of fake ring candidates, ring quality calculation

ANN output value for correctly found (solid line) ANN output value for correctly found (solid line) and fake (dashed line) rings and fake (dashed line) rings

Nine ring parameters selected for ring quality calculation:• number of hits in ring; • chi-squared• biggest angle between neighboring hits;• number of hits in a small corridor around the ring; • position of ring on photodetector plane; • major and minor half axes of ellipse; • rotation angle of the ellipse vs. azimuthal angle.

• ANNANN derivesderives ring quality ring quality from from thesethese parameters. parameters. • TThe ANN output he ANN output provides aprovides a ring ring quality quality parameter parameter or probability, or probability, whether ring-candidate was found whether ring-candidate was found correctly or not.correctly or not.


Ring recognition algorithm, global search

Reject candidate with worse quality if it Reject candidate with worse quality if it

shares more than Nshares more than Nmaxmax hits with a better hits with a better

quality ring candidate.quality ring candidate.

NNmaxmax is set to 30% of the total number is set to 30% of the total number

of hits in ringof hits in ring


RICH ring fitting methodsCircle fittingCircle fitting Ellipse fittingEllipse fitting

• NewtonNewton method for nonlinear method for nonlinear equations with one variable equations with one variable is usedis used• 3-4 iterations3-4 iterations• aalgorithmlgorithm is very robust to the initial is very robust to the initial parametersparameters

• pprogram realizationrogram realization of the of the COPCOP (Chernov-Ososkov-Pratt)(Chernov-Ososkov-Pratt), based on , based on the minimization of the functionalthe minimization of the functional 2 2XP Ax Bxy Cy Dx Ey F

• general, as conic section

• Rings in the photodetector plane have a slight elliptic shape

• usage in ring finding algorithm

• Taubin method is usedTaubin method is used• Minimize P(x) by A,B,C,D,E,F, but Minimize P(x) by A,B,C,D,E,F, but measuring deviations measuring deviations along normals to along normals to the curvethe curve. . • non-linearity is avoided by Tailor non-linearity is avoided by Tailor expansionexpansion• non-iterative very fast direct algorithm• no need of starting parameter values

Mean B/A for Mean B/A for CBM RICH ringsCBM RICH rings = 0.9= 0.9Ref: N. Chernov J Math Im Vi, 27 (2007), 231-239.

Thanks to A.Ayriyan (JINR, Dubna) and N. Chernov (USA)Ref: Comp Ph Com Volume 33, Issue 4, 1984, 329-333


Ring parameter correction

Why? How?

B distribution BEFORE BEFORE correction B distribution AFTERAFTER correction

B correction mapB distribution onto photodetector plane

Example for minor half axis of ellipse (B)

Resolution 3.3% Resolution 1.7%

cm cm


Ring finding efficiency

Typical reaction for CBM -> central Au+Au collisions at 25 AGeV beam energy (UrQMD)

Compact RICHCompact RICHLarge RICHLarge RICH

Accepted rings = rings with >= 5 hits

Large Compact

radiator gas and length N2 length 2.5 m CO2 length 1.5 m

photodetector size (No. of channels)

9 m2 (200k) 2.4 m2 (55k)

Efficiency for e+ and e- embedded in central Au+Au collisions at 25 AGeV beam energy


Test for ring finder

120 e+ and 120 e- in 120 e+ and 120 e- in each eventeach event

min #hitsmin #hits

Integrated Integrated

efficiencyefficiency

55 91.0791.07

1010 92.7192.71

1515 95.2895.28Mean number of hits per ring is 22Mean number of hits per ring is 22

> 2 times typical ring density


Electron Identification in RICH

ElectronsElectrons

PionsPions

Radius versus momentum for reconstructedRadius versus momentum for reconstructed rings rings in central Au+Au collisions at 25 AGeV beamin central Au+Au collisions at 25 AGeV beam energy energy

for UrQMD eventsfor UrQMD events (large RICH) (large RICH). .

Pions which were matched to secondary electron RICH rings

Pions – dashed lineElectrons – solid line

BB

AA

A

B

Ellipse fitter

Ring-track distance


Electron Identification in CBM, results

The RICH detector alone yields a pion suppression factor of 500 at an electron identification efficiency of 90% while in combination with TRD a factor 104 is reached at 83% efficiency. Large RICH

Ring finding (95.3%)

RICH el id (90.3%)

RICH+TRD el id (83.3%)

RICH (250)

RICH+TRD (10000)


Summary

• Fast and efficient algorithm for ring recognition in Fast and efficient algorithm for ring recognition in CBM RICHCBM RICH was developed was developed– based on the HT method with local selection of based on the HT method with local selection of hitshits.. – Fast and robust eFast and robust ellipse fitting algorithm has been llipse fitting algorithm has been

implemented for preciseimplemented for precise estimation of ring parameters.estimation of ring parameters.– A global ring search algorithm A global ring search algorithm waswas developed to select developed to select

goodgood rings, while rejecting fake and clone rings.rings, while rejecting fake and clone rings.

• Time of one event reconstruction in RICH (> 100 Time of one event reconstruction in RICH (> 100 rings) varies from 50 ms to 300 ms depending on rings) varies from 50 ms to 300 ms depending on parameters in ring recognition (fast or more efficient parameters in ring recognition (fast or more efficient but slower) on a Pentium4 2GHz.but slower) on a Pentium4 2GHz.

• Ring finding aRing finding algorithm has shown a verylgorithm has shown a very good good performance and robustness to high ring multiplicity performance and robustness to high ring multiplicity environment.environment.

ring recognition and electron identification in the rich detector of the cbm experiment at fair...

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