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					CBM Calorimeter System

      CBM collaboration meeting, March 2009
               I.Korolko    (ITEP, Moscow)

■ Reconstruction in the CBM ECAL
■ Reconstruction of π0 and η mesons
■ Two photon reconstruction and low mass background
                                 K.Mikhailov (A.Stavinsky)
    Our main efforts during last year

1) Development of reconstruction for the CBM ECAL
    Need it for optimization and physics feasibility studies
    Quality of reconstruction (at high multiplicities)
    “Popularization” of ECAL reconstruction

2) Optimization of the CBM ECAL (reducing price)
                    “Optimized” calorimeter

Main features:

 ~14K channels

 Efficient γ,   π0, η reconstruction
 Electron identification
 Movable design (no central region)

• Requirements- fast and robust at CBM multiplicities

• Cluster finder
     algorithm and performance
• Cluster fitting
• Matching with tracks and photons
     for debugging and efficiencies
           Reconstruction. Cluster finder
      Requirements                     Cluster formation
• Clusters should be large       • Remove maximums near charged tracks
                                     – Use real tracking
   – information for unfolding
                                 • Precluster:
• Clusters should be small
                                     – formed near local maximum
   – hadrons background                   • cut on maximum energy
                                     – find maximum 2x2 matrix near maximum
                                     – add a neighbor to local maximum cell with
                                       minimal energy deposition
                                          • to add information
                                     – check precluster energy
                                          • >0.5GeV
                                 • Cluster: group of preclusters with common
                                 • Clusters with 2 maximums >10%
                                     – Fit procedure is necessarily!
                                     – >3 maximums can be omitted
                 Reconstruction. Fitter

• Isolated photons – most simple case
• Two close photons:
   – Robust reconstruction in case of two separate maxima
   – Recognition of two photons in case of one maximum (χ2 criteria)

• χ2 shape should not depend on photon’s energy
   – same efficiency value for the chosen cut
• separation power of one/two photons in case of one
  maximum is an important criteria
   – example: with 95% efficiency for clusters formed by single
             Reconstruction. Fitter basics
                 Same photons, but different distance between them.

2 separated clusters        cluster with 2 maxima        Cluster with
                                                         1 maximum (2 γ)

• Trivial case              • Shower shape fit           • Should be rejected
                                                         • Shower shape fit
                 Reconstruction. Shower shape
                ( Ecell  Ecell ) 2
                   meas     pred
  2  
        cells          cell


Precise knowledge of shower shape is essential.
If χ2 is used, than what is the error σ2 ?
          Reconstruction. Shower library
                                     Cell 4
• Store mean energy deposition in
  small cells vs. (x, y) for each
   – Energy
   – Theta
   – Phi
• Energy depositions in cluster
  cells are not independent
   – RMS value storing is useless
• Trying analytical formula for σ2
   – Hoping to take into account
     correlations                             Cell 5
          Reconstruction. Errors


– c2 is normalization
   • 95% of photons have χ2<2
– c1 and c0 are determined requiring that χ2
   • does not depend on photon’s energy
   • Does not depend on φ angle

– ci is different for each calorimeter region
Reconstruction. Performance
        Single (isolated) 1 GeV photons


Calibration is perfect (see later…)
         Reconstruction. Performance
             Two close 1 GeV photons (forming 2 maxima)

• Reconstructed energy
  distribution for both
  photons is Ok
• But for each of them…

              Reconstruction. Performance
                     Two close 1 GeV photons (forming 2 maxima)

Reconstruction algorithm tends to increase asymmetry in energy of photons
(also by PHENIX experience)
        Reconstruction. 2 γ recognition
                           Inner calorimeter region
• χ2 criteria allows
  identify ~40% of two
  photon clusters
  – efficiency highly
    depend on distance
    between photons
Reconstruction. Performance
      AuAu 25 GeV UrQMD events

                   • 730 photons in event
                   • 299 photons in calorimeter
                       – 131 with energy > 0.7 GeV
                   • 35% reconstruction
                   • 91% for isolated tracks
                       – rises with increasing
         Reconstruction. Performance
                   AuAu 25 GeV UrQMD events

• 35% reconstruction      Reconstruction efficiency vs. θ and energy
• Boundaries between
  calorimeter regions
  – occupancy
                Reconstruction. Matching 1

• Idea: use shape of
  reconstructed particles
• For each MC/reconstructed
  particle compute:
                    ERe co j
  P  E
   i           i
               MC      j
                          , ESum   ERe co

       cells        ESum

  – only cells with energy
    deposition from current particle
    are in play
  – match with MC particle with      • Clusters with 2 maximums:
                                       99.97% efficiency
             Reconstruction. Matching 2

• Idea: for γ (e±) look at mother e± (γ)             γ
  and grandmother and …
   – for each γ and e± MCTrack:
     P=Pthis+ΣPdaughter                         e-       e+

• Several realizations available
   – Choose γ/e± with maximum P
   – Choose parent e± if daughter γ have
   – …
   – Exact algorithm/constants are defined in
     configuration file                              γReco

      • still under development
         Reconstruction. Conclusions

• Reconstruction algorithms are completed and tested
   – 35% reconstruction efficiency
      • occupancy!
• Calorimeter geometry optimization
   – cost
   – physical observables sensitivity
      • reconstruction efficiency
• Digitization and response nonuniformity impact
   – moving towards realistic geometry
    Reconstruction of π0 and η mesons
• Sergey Kiselev, ITEP Moscow for the ECAL group

• Input info
• Spectra and acceptances
• Ideal reconstruction
• Real reconstruction
   – efficiency
   – true signal, S/B
   – extracting signal by mixing
• Summary
         π0 and η mesons. Input info

• CbmRoot package (trunk JAN09), Geant3
• 2 104 UrQMD Au+Au central events at 25 AGeV
   simulated and reconstructed in ECAL by Misha Prokudin
• the ECAL wall at 12 m from a target
     Size: X x Y = 12 x 9.6 m2 , beam hole 0.8 x 0.8 m2
• pγ cut: pγ > 0.3 GeV/c
• Cluster cut: χ2 < 3
 π0 and η mesons. Vertex γ
spectra           acceptance (%)

                         <accep.> = 50%
π0 and η mesons. Primary π0
spectra           acceptance (%)

                         <accep.> = 12%
 π0 and η mesons. Primary η
spectra           acceptance (%)

                         <accep.> = 9%
          π0 and η mesons. Vertex γ
386 reco γ/event:
    30 not matched with MC tracks
  356 matched with MC tracks
         237 of them Rvtx <0.1 cm
              160 of them are photons
                    98 of them enter ECAL
                    62 “enter” ECAL by decay products

1738 MC tracks/event enter ECAL
     398 of them are photons
         230 of them Rvtx <0.1 cm

      “vertex” γ reco efficiency = 98 / 230 = 43%
π0 and η mesons. Vertex γ


                     peaks at θ=70
                     and 120 because of
                     change in the cell
π0 and η mesons. Primary π0


                     20.1 reco primary π0/ev.:
                          6.3: 2γ enter ECAL
                          7.9: 1γ enter ECAL
                          5.9: 0γ enter ECAL

                     364 primary π0 /ev.:
                         32.7 enter ECAL

                      primary π0
                     reco efficiency = 6.3/32.7
                      = 19 (%)
π0 and η mesons. Primary η


                    1.56 reco primary η:
                         0.43: 2γ enter ECAL
                         0.62: 1γ enter ECAL
                         0.51: 0γ enter ECAL

                    14.3 primary η  2γ :
                        1.6 enter ECAL

                     primary π0
                    reco efficiency = 0.43/1.6
                     = 27 (%)
   true S from
    primary π0
  higher Mγγ
  lower Mγγ

rather Breit-Wigner
than Gauss fit

real reco: ~5 times
broader signal,
37/30/25/23 MeV,
than for ideal reco
  true S from
   primary η

The same remarks as
for π0

real reco: ~5 times
broader signal,
100/102/90/63 MeV,
 than for ideal reco
             ideal vs real S/B

                         S/B2σ(%) (signif.)
    pt (GeV/c)       0.4 – 0.8   0.8 – 1.2   1.2 – 1.6
ideal reconstruction   1.4(51)    3.6(31)    6.0(14)
 real reconstruction   0.2(14)    0.6(12)     1.2(7)
ideal reconstruction 0.05(2.5) 0.10(1.7) 0.15(0.8)
 real reconstruction 0.007(0.8) 0.013(0.6) 0.027(0.3)
π0 and η mesons from another analysis

                            Still some good
                              luck and fine
                              tuning are
5 mixed events
to evaluate Bmix

(S+B) and Bmix
were normalized
at M>0.3 GeV

at high pt
(S+B)-Bmix and true
S are in reasonable

For η higher
statistics needed
2 104 UrQMD Au+Au central events at 25 AGeV with ECAL reco
                          “vertex” γ   primary π0   primary η
      acceptance (%)         50            12           9
    reco efficiency (%)      43            19          27
    converting part (%)      39           69           70

  high pt π0 can be                    primary π0 primary η
  extracted by mixing     σ (MeV)         ~20        ~100
  event technique
                          S/B2σ (%)      ~0.5-1   ~0.01-0.03
  η: ~ 2 order higher       signif.       ~10         0.5
  statistics needed
   Recommendation: test reconstruction at lower system/energy
Two photon reconstruction
 with ECAL and low mass

     Alexei Stavinskiy, Konstantin Mikhaylov

                  ITEP, Russia

2*104 central UrQMD events AuAu@25AGeV (Local analysis)
Full ECAL reconstruction (version of February 2009) with

CBM root January 2009 version

(version with new geometry)


Minimal energy deposited in ECAL 500 MeV.

χ2 (of photon reconstruction) < 3.

Minimal distance between cluster (DBC) > 20 cm.

Particles from target = Vz < 1cm (conventional)
                           WA 98 experiment

A low mass tail on the π0 was observed.
 The tail can result from π0 produced
         downstream from target:
              from decays (K0s, ...)‫‏‬
      from background interaction on
               downstream materials
         (the normalized target out background
        contribution is shown by the open circles
                       In part c))

                      θt             θb
     πtarget                                    πbackground   2b

Differences between target and background pairs:

*mean photon energy higher for target photons

*real emission angle difference for target photons from
 π(η) decay corresponds to measured invariant mass;
 this is not the case for background pairs

*vertex position is fixed for target pairs;
for background pairs vertex position distribution
corresponds to detectors (support) position
Mγγ same mother
       Mγγ vs Vz   π 0





Material (X0) in front of ECAL
Ecut Rcut: Mγγ,same mother
    S/B for 0<Mγγ<120MeV
         DBC cut: Mγγ,same mother
Distance Between Clusters>20cm   Distance Between Clusters>50cm
Signal and Background


           Two-photon correlations


  External conversion:            C2 calculated in EMCAL and
• No close cluster interference   converted+EMCAL agree =>
• No hadron contamination         both effects are under control
Measurement of Direct Photons
via Conversion in CBM
Melanie Klein-Bosing
WWU Munster,Germany
CBM Collaboration Meeting,
Dubna 2008, October
 The feasibility of π and η meson identification with ECAL was shown
 The low mass background in two photon invariant mass was studied

   Two main contribution to the low mass background are:
       Interaction with downstream detector construction
       Decay of long lived particles
   Possible cuts to reduce (slightly) low mass background:
       Cut on gamma energy
       Cut on distance between center of clusters
   Other possible ways:
       Background simulations
       Combined photon pair identification with different detectors

1) Reconstruction in ECAL is 90% ready
    better definition of errors
    development of matching algorithms (converted γ)
    number of users = quality

2) Use fast (25 ns) ECAL for triggering (Alla)

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