ECAL_Monit_April2004

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					CBM Calorimeter System




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


■ Reconstruction in the CBM ECAL
                                 M.Prokudin
■ Reconstruction of π0 and η mesons
                                 S.Kiselev
■ 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)
                    Reconstruction

• Requirements- fast and robust at CBM multiplicities

• Cluster finder
     algorithm and performance
• Cluster fitting
     performance
• 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
                                   cells
                                   -------------------------------------------------
                                 • 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
     photon
             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
                        2
                                                  +




                                                  or


                                                       ?
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

σ2=c2(Emeas(1-Emeas/Ecluster)+c0+c1E2cluster)

– 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




       7.3%


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…




                                                          8.9%
              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
                     acceptance
                       – 131 with energy > 0.7 GeV
                   • 35% reconstruction
                     efficiency
                   • 91% for isolated tracks
                       – rises with increasing
                         isolation
         Reconstruction. Performance
                   AuAu 25 GeV UrQMD events

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

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

       cells        ESum

  – only cells with energy
    deposition from current particle
    are in play
  – match with MC particle with      • Clusters with 2 maximums:
    Pi>0.6
                                       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
     P>const*Pγ
   – …
   – 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

                     reconstruction
                       efficiency




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

                     reconstruction
                       efficiency

                     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

                    reconstruction
                      efficiency

                    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
2γ/ECAL:
  higher Mγγ
1γ+0γ/ECAL:
  lower Mγγ

sum:
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
                              required…
   extracted
  (S+B)-Bmix
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
agreement

For η higher
statistics needed
                          Summary
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
       background

     Alexei Stavinskiy, Konstantin Mikhaylov

                  ITEP, Russia
                        Input

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)

Cuts:

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


       [arXiv:nucl-ex/0006007]
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))
                                                              1t

                                                              1b
                      θt             θb
     πtarget                                    πbackground   2b
                                                              2t

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

ECAL



                   RPC
                   TRD3


                   TRD2



                      TRD1


                         RICH
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
         Signal-Background
VZ<1cm


             S/sqrt(S+B)=20


             S/B=0.25%
           Two-photon correlations
      e+
                       2
 2rec
                            1
1

 e-




  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
                             Conclusions
 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
                      Conclusions

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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