cbm – much simulation for low-mass vector meson

CBM-Meet, VECC July 21, 2006

1 Premomoy Ghosh

CBM – MUCH Simulationfor

Low-mass Vector Meson

Work done at GSI during

June 2006


2 Premomoy Ghosh

Talk LayoutIntroduction

Effects on track-reconstruction efficiency due to:

• Variation in individual absorber thickness

• Variation in strength of Magnetic Field

Future plan


3 Premomoy Ghosh

The Physics motivation of CBM leads to the requirementof a Muon Chamber (MUCH) for detecting muons from decays of low-mass vector mesons.

Simulation for such a MUCH has been initiated recently.Studies on some the aspects will be presented here.

CBM will be equipped with Silicon Tracking Station (STS)in magnetic field for:•Track reconstruction of all charged particles•Vertex reconstructionTracks reconstructed in STS have to be matched to hits inMUCH


4 Premomoy Ghosh

ChallengesSmall branching ratios

& signal/background

Vector meson

Mass

MeV/c2

MeV

Multiplicity

25 AGeV Au+Au

(central)

Branching Ratio

ρ 770 149.2 22 4.6 * 10-5

ω 782 8.49 0.99 38 9 * 10-5

Central Au+Au @ 25 AGeV Charged particle multiplicity ~ 1600 (UrQMD)


5 Premomoy Ghosh

CBM Much general layout

Carbon absorberDetector layers

STSTarget

Gap between two detector layers = 45 mm

Gap between absorber and adjacent detector layer = 1 mm

Thickness of each detector layer = 10 mm

4 carbon absorbers & 13 detector layersThree detector layers in between 2 absorbers

Sliced absorbers placed in between detector layers – to facilitate efficient track matching for low momentum particles


6 Premomoy Ghosh

Simulation Tools

• CBM analysis framework – cbmroot and cbmroot2

• UrQMD event generator - Au + Au events at 25 GeV/nucleon.

• PLUTO event generator – Muons from light vector meson decay.

• UrQMD events, embedded with PLUTO events or with generated single particle muons, were transported through STS (with magnetic field on) and MUCH.

• Track reconstruction in STS in done with with the option - Ideal Tracking.


7 Premomoy Ghosh

Variation in thicknesses of individual absorbers

• Individual absorber-thicknesses likely to affect track matching due to:- Hit-density- Deviation due to multiple scattering

• Started in cbmroot • Followed previous configuration:

– Studied cases with total thickness of 180, 190 and 200 cm.– Different geometry versions -> varied thicknesses of individual

absorbers. – For different geometry versions run transport and tracking

for 1000 PLUTO events and 100 PLUTO+UrQMD (central) events.

– Studied in terms of # of surviving muons and background tracks.


8 Premomoy Ghosh

cbmrootMuons from PLUTO (rho-old version) and background tracks from UrQMD (central)

Much version STS+CV8 STS+CV20 STS+CV9 STS+CV12 STS+CV18

Absorber thickness

30,40,50,80

=200 cm

80,50,40,30 =200 cm

30,40,50,60 =180 cm

60,50,40,30

=180 cm

45,45,45,45 =180 cm

Reconstructed background tracks per 100 events

513 493 588 652 574

Reconstructed muon tracks per 1000 events

919 908 951 946 951


9 Premomoy Ghosh

Variation of absorber thickness contd.

• Switched over to new framework - cbmroot2• Changes in software structure• MUCH-codes newly installed – not yet thoroughly

tested• PLUTO for rho changed• Magnetic Field changed• Position of MUCH changed• Repeated some parts of the study.


10 Premomoy Ghosh

•Even for same geometry, results in cbmroot2 are very much different from those from cbmroot(!) •Magnetic field or some bugs in codes or something else – What is responsible?•Needs thorough investigation to understand differences in results.

Muons from PLUTO (rho-old version) comparison of cbmroot and cbmroot2

MUCH_CV8 cbmroot cbmroot2

# of reconstructed tracks 919 558

# of tracks on the 1st MUCH-detector

1120 1320


11 Premomoy Ghosh

cbmroot2Muons from PLUTO (rho-new version) + UrQMD Min. Bias

Much version STS+CV8 STS+CV20 STS+CV9 STS+CV12 STS+CV18

Absorber thickness

30,40,50,80

=200 cm

80,50,40,30 =200 cm

30,40,50,60 =180 cm

60,50,40,30

=180 cm

45,45,45,45 =180 cm

Reconstructed background tracks per 1000 events

307 243 415 427 455

Reconstructed Muon tracks per 1000 events

404 363 443 396 402

Reconstructed

Invariant mass (%)

1.4 0.7 1.5 0.9 0.7

No. of STS hits >=6

Variations in results are not much and may be attributed to statistical uncertainties. We choose CV8 for further studies.


12 Premomoy Ghosh

PLUTO + UrQMD (Min. Bias) 1k events Au+Au 25 AGeV

# of muon tracks in STS

# of muon tracks in MUCH

% of reconstructed

signal

rho1390 418 1.8

omega 1352 593 2.8

MUCH_CV8 – 1k embedded eventscomparison between rho and omega

No. of STS hits >=4Each PLUTO event generates 1 dimuon –1k events corresponds to 2k tracks.Why so less reconstructed tracks and signals?

We look into the loss in terms of tracks.


13 Premomoy Ghosh

Muons from PLUTO - rho – MUCH_CV8 – 1k events


14 Premomoy Ghosh


No track below 1 GeV/c in MUCH


15 Premomoy Ghosh

Muons from PLUTO - omega – MUCH_CV8

No track below 1 GeV/c in MUCH


16 Premomoy Ghosh



17 Premomoy Ghosh

Stages where we loose – PLUTO(rho) –1k events – as compared to STS - Full Mag. Field


18 Premomoy Ghosh

After every absorber - loss in y_pt - compared to mu-tracks in STS – Full Mag. Field

Major loss in MUCH acceptance


19 Premomoy Ghosh

How to catch lower-p muons?

– Reduce absorber thickness -> allows more background.– Reduce magnetic field strength that bends low momentum

particles out of acceptance-> affects momentum resolution.– Improvement in track-matching.

We study effects of reducing magnetic field strength

Loss may be due to absorption or bending or due to both


20 Premomoy Ghosh

•To study the effect of reducing magnetic field strength on acceptance at MUCH - selected MUCH_CV8 with PLUTO events for rho (new version) and omega. We present here the case of omega.

•Run with minimal cuts at signal reconstruction (p_min = 0.5 GeV/c, OA = 120, SPd= -0.26 and Spu= 0.04)

•Study with Ideal Tracking


21 Premomoy Ghosh

PLUTO –omega 1k events Au+Au 25 AGeV

# of muon tracks in

STS


% of reconstructed

signal

omega

Full Field

1345 595 3.9

omega

.7*Field

1338 670 5.7

Effect of reducing Mag. Field Strength


22 Premomoy Ghosh

Effect of reducing Mag. Field Strength PLUTO omega 1k events

By reducing mag. field strength, we gain.But, low momentum (<1 GeV/c) muons are still missing – may be due to absorption.


23 Premomoy Ghosh

Effect of reducing Mag. Field Strength – PLUTOomega 1k events


24 Premomoy Ghosh

Effect of reducing Mag. Field Strength PLUTO omega -1k events


25 Premomoy Ghosh

Comparison - momentum resolution – full field and 0.7 Mag. Field –10k events


26 Premomoy Ghosh

Reducing Mag. Field - effect on delta_p/p

PLUTO – omega – 10k events

Full MF 0.9*MF 0.8*MF 0.7*MF 0.6*MF

# of muons detected in STS

13493 13482 13424 13453 13570

# of muons detected in MUCH

6016 6233 6361 6617 6819

Mom. Resolution (muons at MUCH)

sigma (dp/p)*10-2

0.5731 0.6172 0.7148 0.7967 0.9444


27 Premomoy Ghosh

PLUTO (omega) + UrQMD (Min. Bias) 1k events Au+Au 25 AGeV

# of muon tracks in

STS


% of reconstructed

signal

omega

Full Field

1352 593 2.8

omega

.7*Field

1341 583 4.8

Effect of reducing Mag. Field Strength1k embedded events


28 Premomoy Ghosh

Conclusion and Plan

•Difference in results from cbmroot and cbmroot2 needs to be understood.

•More systematic study on absorber thickness and strength of magnetic field is required.

•Present study – singles muons and pions – varying carbon absorber thickness – different momentum (at GSI machine).

•After optimizing absorber thickness and magnetic field strength, depending on the acceptable background and momentum resolution respectively, improvement in track-matching efficiency may be addressed.

Plan:

To install the codes at VECC machine and continue.

cbm – much simulation for low-mass vector meson

Documents