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					            CBM – MUCH Simulation
                      for
             Low-mass Vector Meson


                 Work done at GSI during
                       June 2006


CBM-Meet, VECC           Premomoy Ghosh    1
July 21, 2006
                 Talk Layout
Introduction
Effects on track-reconstruction efficiency
  due to:
• Variation in individual absorber
  thickness
• Variation in strength of Magnetic Field
Future plan

CBM-Meet, VECC      Premomoy Ghosh           2
July 21, 2006
The Physics motivation of CBM leads to the requirement
of 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 reconstruction
Tracks reconstructed in STS have to be matched to hits in
MUCH

CBM-Meet, VECC          Premomoy Ghosh                    3
July 21, 2006
                                      Challenges
                                      Small branching ratios
                                      & signal/background


   Vector   Mass              G           Multiplicity    Branching
   meson    MeV/c2          MeV            25 AGeV          Ratio
                                            Au+Au
                                           (central)
                          149.2 0.7
      ρ          770                          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)



CBM-Meet, VECC                    Premomoy Ghosh                        4
July 21, 2006
          CBM Much general layout
          Gap between two detector layers = 45 mm
          Gap between absorber and adjacent detector layer = 1 mm
          Thickness of each detector layer = 10 mm


Carbon absorber
Detector layers
 Target    STS




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

 Sliced absorbers placed in between detector layers – to facilitate
 efficient track matching for low momentum particles
CBM-Meet, VECC                 Premomoy Ghosh                         5
July 21, 2006
                     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.


  CBM-Meet, VECC                Premomoy Ghosh                          6
  July 21, 2006
     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.

CBM-Meet, VECC              Premomoy Ghosh                         7
July 21, 2006
                                    cbmroot
        Muons from PLUTO (rho-old version) and
        background tracks from UrQMD (central)
Much version        STS+CV8       STS+CV20      STS+CV9       STS+CV12      STS+CV18

Absorber            30,40,50,80   80,50,40,30   30,40,50,60   60,50,40,30   45,45,45,45
thickness            =200 cm       =200 cm       =180 cm       =180 cm       =180 cm



Reconstructed
background
                      513           493            588          652           574
tracks per 100
events
Reconstructed
muon tracks
                      919           908            951          946           951
per 1000
events



   CBM-Meet, VECC                     Premomoy Ghosh                               8
   July 21, 2006
     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.

 CBM-Meet, VECC     Premomoy Ghosh             9
 July 21, 2006
     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          1120         1320
   MUCH-detector

 •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.



CBM-Meet, VECC                 Premomoy Ghosh              10
July 21, 2006
                                   cbmroot2
  Muons from PLUTO (rho-new version) + UrQMD Min. Bias
                                                      No. of STS hits >=6
Much version        STS+CV8    STS+CV20 STS+CV9 STS+CV12 STS+CV18

Absorber            30,40,50,80 80,50,40,30 30,40,50,60 60,50,40,30 45,45,45,45
 Variations
thickness
             in                  not much =180 cm
                    results are =200 cm
                     =200 cm
                                              and may be attributed to
                                                         =180 cm     =180 cm
statistical uncertainties. We choose CV8 for further studies.
Reconstructed
background
                      307         243          415        427          455
tracks per 1000
events
Reconstructed
Muon tracks
                      404         363          443        396          402
per 1000 events
Reconstructed
                      1.4         0.7          1.5         0.9          0.7
Invariant mass
(%)

   CBM-Meet, VECC                  Premomoy Ghosh                             11
   July 21, 2006
                    MUCH_CV8 – 1k embedded events
                    comparison between rho and omega
PLUTO + UrQMD             # of muon        # of muon         % of
(Min. Bias) 1k events
Au+Au 25 AGeV           tracks in STS       tracks in   reconstructed
                                             MUCH           signal
                           1390                   418          1.8
rho
omega                      1352                   593          2.8


Each PLUTO event generates 1 dimuon –                   No. of STS hits >=4
1k events corresponds to 2k tracks.
Why so less reconstructed tracks and signals?
              We look into the loss in terms of tracks.
   CBM-Meet, VECC                Premomoy Ghosh                      12
   July 21, 2006
                 Muons from PLUTO - rho –
                  MUCH_CV8 – 1k events




CBM-Meet, VECC           Premomoy Ghosh     13
July 21, 2006
                 Muons from PLUTO - rho –
                  MUCH_CV8 – 1k events




                        No track below 1 GeV/c in MUCH




CBM-Meet, VECC          Premomoy Ghosh                   14
July 21, 2006
                 Muons from PLUTO - omega –
                         MUCH_CV8




                            No track below 1 GeV/c in MUCH




CBM-Meet, VECC           Premomoy Ghosh                      15
July 21, 2006
                 Muons from PLUTO - rho –
                  MUCH_CV8 – 1k events




CBM-Meet, VECC           Premomoy Ghosh     16
July 21, 2006
       Stages where we loose – PLUTO(rho) –
  1k events – as compared to STS - Full Mag. Field




CBM-Meet, VECC       Premomoy Ghosh                  17
July 21, 2006
After every absorber - loss in y_pt - compared to mu-tracks in STS –
                           Full Mag. Field




                                        Major loss in MUCH acceptance




 CBM-Meet, VECC             Premomoy Ghosh                     18
 July 21, 2006
  How to catch lower-p muons?
Loss may be due to absorption or bending or due to both
   – 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
CBM-Meet, VECC         Premomoy Ghosh                 19
July 21, 2006
•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


CBM-Meet, VECC   Premomoy Ghosh           20
July 21, 2006
       Effect of reducing Mag. Field Strength

PLUTO –
omega 1k
                  # of muon      # of muon         % of
events Au+Au       tracks in      tracks in   reconstructed
25 AGeV
                     STS           MUCH           signal

omega               1345               595         3.9
Full Field

omega               1338               670         5.7
.7*Field

 CBM-Meet, VECC            Premomoy Ghosh                21
 July 21, 2006
     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.




CBM-Meet, VECC        Premomoy Ghosh                            22
July 21, 2006
    Effect of reducing Mag. Field Strength – PLUTO
                    omega 1k events




CBM-Meet, VECC        Premomoy Ghosh                 23
July 21, 2006
    Effect of reducing Mag. Field Strength PLUTO
                   omega -1k events




CBM-Meet, VECC       Premomoy Ghosh                24
July 21, 2006
      Comparison - momentum resolution –
     full field and 0.7 Mag. Field –10k events




CBM-Meet, VECC       Premomoy Ghosh              25
July 21, 2006
  Reducing Mag. Field - effect on delta_p/p
PLUTO – omega – Full MF     0.9*MF 0.8*MF 0.7*MF 0.6*MF
10k events

# of muons      13493       13482          13424     13453   13570
detected in STS
# of muons       6016       6233           6361     6617     6819
detected in
MUCH
Mom. Resolution 0.5731      0.6172         0.7148   0.7967   0.9444
(muons at MUCH)
sigma (dp/p)*10-2

CBM-Meet, VECC            Premomoy Ghosh                              26
July 21, 2006
        Effect of reducing Mag. Field Strength
                 1k embedded events
PLUTO
(omega) +
                   # of muon      # of muon         % of
UrQMD (Min.         tracks in      tracks in   reconstructed
Bias) 1k events
Au+Au 25
                      STS           MUCH           signal
AGeV

omega                1352               593         2.8
Full Field

omega                1341               583         4.8
.7*Field

  CBM-Meet, VECC            Premomoy Ghosh                27
  July 21, 2006
                 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-Meet, VECC           Premomoy Ghosh                      28
July 21, 2006

				
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