Status Report particle identification with the RICH detector Claudia Höhne - GSI Darmstadt, Germany...

Status Report

particle identification with the RICH detector

Claudia Höhne - GSI Darmstadt, Germany

• general overview

• focus on ring radius/ Cherenkov angle resolution

• Boris Polichtchouk: results from simulation

Claudia Höhne CBM collaboration meeting 9.-12.03. 2005

particle identification with RICH

• ring finding

ring finder: Hough Transform, Elastic Net to be implemented in framework

→ efficiencies ...

• determination of center and radius of ring/ Cherenkov angle

• matching of rings with tracks

→ tracking (momentum and position resolution), information from other detectors

• pid by combining ring radius and momentum information

detailed knowledge of resolution necessary!


main contribution !

ring radius resolution

N2 radiatorCherenkov angle/ ring radius resolution limited by:

• multiple scattering

• magnetic stray field in RICH

• emission point: particle trajectories do not pass through the center of curvature of the mirror smeared projection in dependence on (,)

• mirror surface: enlargement of focal spot in focal plane due to a deviation of the mirror surface from the ideal spherical curvature

• pixel size: resolution limited due to finite granularity of photodetector

• chromatic dispersion

investigate single rings!

resolution for overlapping rings different!


single photon – ring resolution

NR

σσ =• distinguish between Cherenkov angle resolution for single photons σ

and the resolution for a ring σR consisting of N measured photons

• table 1 from RICH-TDR of LHCb:

• aim at doing a similar study

2% of cmax

σsingle = 2.5mrad


multiple scattering

pp

ms

σσ =momentum dependent error of form

with 0

6.133

2

X

LMeVp =σ

[T. Ypsilantis, J.Seguinot, NIM A343 (1994) 30,

P. Glaessel, NIM A 433 (1999) 17]

typical value for a N2 radiator of 1m and p=1GeV: mradms

9.0=σ

→ mradms

4.1=σ for 2.5 m N2


magnetic stray field

pp

B

σσ =

Bp LBTm

GeV φπ

σ sin123.02

⋅=

momentum dependent error of form

with

o90=Bφ

being the particle angle relative to the magnetic field direction

Bφ

→ ⊥⋅= LBTm

GeVp

12

3.02

πσ


P. Glaessel, NIM A 433 (1999) 17]


magnetic stray field (II)

gaussian shaped field asymmetric field

RICH front wall z=170cm

→ magnetic stray field (By) of order 10mT

length L=2.5m at maximum

z [cm] z [cm]

By

[T]

By

[T]


magnetic stray field (III)

pp

B

σσ =

Bp LBTm

GeV φπ

σ sin123.02

⋅=

momentum dependent error of form

with

o90=Bφ

being the particle angle relative to the magnetic field direction

Bφ

→ ⊥⋅= LBTm

GeVp

12

3.02

πσ

L2.5m, B=10mT GeVp 001.0≤σ→

p=1GeV → mradB1≤σ


P. Glaessel, NIM A 433 (1999) 17]


emission point

rings() - polar angle, azimuth angle

• no diffusion at reflection

• no magnetic field, no multiple scattering

to do:

• quantify and correct for distortions at large

• improve focussing/ position of focal plane

• correct for remaining distortions

= 80o 60o 40o

20o

= 5o

10o 15o

20o 25o

30o 35o

one quarter of mirror/ photodetector:

→ restrict investigation of resolution to "good" area in central region and wait for optimized setup


mirror surface

Be-mirror prototype:

• optical surface roughness σh = 1.6nm (after glass polishing, Al covering and SiO2 coating)→ diffuse reflection of only 12% of total for = 150nm

• image diameter of a point source D0 = 0.4mm (contains 95% of reflected light)

→ angular deviation from nominal curvature σR = 0.03mrad

resulting radius resolution to be determined

Ph.D. thesis of G. Hering (2002), CERES:

ring center resolution of same order as local mirror deviation

→ σ < 0.1mrad


pixel size

• finite granularity of photodetector restricts resolution

• simple estimate can be made from comparing padsize d and ring radius R / Cherenkov angle

→ Boris Polichtchouk

cmd

3.02

==σ

mradL

d

L

R

L

dR

3.12

2tan~

tan ==−+

=−= σ

d=0.6cm padsize

R=5.5cm ring radius

L=225cm radiator length


chromatic dispersion

[nm] [mrad]

600 24.42200 26.15150 28100 36.75

• strong increase of n() in UV region

NR

σσ =

• however, dN/d also increases in UV region andN2:

→ Boris Polichtchouk

[Landolt Boernstein Series, 6th Edition, volume II/8

Ph.D. thesis of Annick Bideau-Mehu (1982)]

N2

4mrad

σ~2mrad

(~0.4cm)


total resolution (I)

• multiple scattering σ ~ 1 mrad (p=1 GeV)

• magnetic stray field σ < 1 mrad (p=1 GeV)

• emission point σ small because of corrections, optimization

• angular deviation of mirror σ < 0.1 mrad

• chromatic dispersion σ > 1 mrad (strongly dependent on min)

• pixel size σ ~ 1-2 mrad

couple of mrad contributions, independent errors

mradmrad

N

padBms

R

7.0...36.030...10

5...4

...2222

==

++++=

σσσσσ

c=24.4 mrad

σ ~2-3% of c


total resolution (II)

gaussian distributed Cherenkov angles/ radii

→ calculate separation power for e and π in terms of σ for different σ

1%

2%

3%

4%

5%


ring – track matching

• matching: combine track and ring with closest distance

• ~ 2/3 of all rings from secondary interactions, very often not reconstructed

• difficulty due to high particle mutiplicities match ring to track (e.g. π) which is nearby

R

additional source for π-misidentification


pid versus R

1% momentum resolution1.3 mrad resolution in azimuth angle0.8 mrad resolution in deep angle200 m position resolution in mirror

• ideal tracking: with of R –distribution due to method for ring center determination

• finite tracking: distribution widened

• cut on R important for efficiency and purity!


efficiency, misidentification

efficiency of e-identification in dependence on a cut on R

R = 0.8cm: > 95%

ideal trackingfinite resolution

ideal trackingfinite resolution

π- misidentification in dependence on a cut in R

35 AGeV: 827 π / event

0.3/827 = 410-4 π-misid. (R = 0.8 cm)

= 610-4 π-misid. (60% acc.)


summary/ outlook

particle identification with the RICH detector

• aim: momentum dependent pid efficiency and purity

• efficiency: ring finders to come

• purity: started with detailed analysis of ring radius resolution

for σ=3% of c we have 3σ separation between e and π at 13.5 GeV/c

impact on detector layout: granularity of photodetector

maximum wavelength range for photodetection

• purity: extend tracking algorithms for extrapolation of tracks to photodetector plane

combine with information from other detectors

Status Report particle identification with the RICH detector Claudia Höhne - GSI Darmstadt, Germany...

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