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					Andrea Dell’Acqua       Status of the GEANT4 Physics Evaluation in ATLAS   Slide 1
  CERN EP/SFT




Status of the Geant4 Physics Evaluation in ATLAS

                     Andrea Dell’Acqua
                        CERN EP/SFT
                    dellacqu@mail.cern.ch


On behalf of the ATLAS Geant4 Validation Team
Andrea Dell’Acqua                   Status of the GEANT4 Physics Evaluation in ATLAS                     Slide 2
  CERN EP/SFT




                                                                                         EMB (LAr/Pb,Barrel)
— ATLAS:                  Muon Detectors (μ)     Electromagnetic Calorimeters (μ,e)
                                                                                       & EMEC (LAr/Pb,EndCap)

A Multi-Pur-                                       Solenoid                Forward Calorimeters (e)   FCal (LAr/Cu/W)


pose LHC                                                                                 EndCap Toroid


Detector




                    Barrel Toroid   Inner Detector (e,μ,π)     Hadronic Calorimeters (e,μ,π)          Shielding

                                                                   HEC (LAr/Cu,EndCap) &
                                                              TileCal (Scint/Fe,Barrel/Extended)
Andrea Dell’Acqua   Status of the GEANT4 Physics Evaluation in ATLAS   Slide 3
  CERN EP/SFT




                               This Talk:

 Strategies for G4 physics validation in ATLAS

 Muon energy loss and secondaries production in the
ATLAS calorimeters and muon detectors

 Electromagnetic processes in tracking detectors and
shower simulations in calorimeters

 Hadronic interactions in tracking devices and
calorimeters

 Conclusions
Andrea Dell’Acqua          Status of the GEANT4 Physics Evaluation in ATLAS   Slide 4
  CERN EP/SFT




  Strategies for G4 Physics Validation in ATLAS

 Geant4 physics benchmarking:
              compare features of interaction models with similar features in
             the old Geant3.21 baseline (includes variables not accessible in the
             experiment);
              try to understand differences in applied models, like the effect
             of cuts on simulation parameters in the different variable space
             (range cut vs energy threshold…);

 Validation:
              use available experimental references from testbeams for
             various sub-detectors and particle types to determine prediction
             power of models in Geant4 (and Geant3);
              use different sensitivities of sub-detectors (energy loss, track
             multiplici-ties, shower shapes…) to estimate Geant4 performance;
              tune Geant4 models (“physics lists”) and parameters (range cut)
             for optimal representation of the experimental detector signal with
             ALL relevant respects;
 Andrea Dell’Acqua           Status of the GEANT4 Physics Evaluation in ATLAS     Slide 5
   CERN EP/SFT




   G4 Validation Strategies: Some Requirements…
Geometry            description:
           has to be as close as possible to the testbeam setup (active detectors and
          relevant parts of the environment, like inactive materials in beams);
           identical in Geant3 and Geant4;
           often common (simple) database used (muon detectors, calorimeters) to describe
          (testbeam) detectors in Geant3 and Geant4:

Environment           in the experiment:
           particles in simulations are generated following beam profiles (muon detectors,
          calorimeters) and momentum spectra in testbeam (muon system);
           features of electronic readout which can not be unfolded from experimental
          signal are modeled to best knowledge in simulation (incoherent and coherent
          electronic noise, digitization effect on signal…);

Work        as much as possible in a common simulation framework
  Andrea Dell’Acqua                Status of the GEANT4 Physics Evaluation in ATLAS        Slide 6
    CERN EP/SFT




   Geant4 Setups (1)                                              Muon Detector Testbeam




    Detector plastic
                        Silicon sensor (280 μm thick)
    Cover (3mm thick)
                        FE chip (150 μm thick)
                        PCB (1 mm thick)


Hadronic Interaction in Silicon Pixel Detector
   Andrea Dell’Acqua        Status of the GEANT4 Physics Evaluation in ATLAS        Slide 7
     CERN EP/SFT




   Geant4 Setups (2)                              Electromagnetic Barrel Accordion Calorimeter

      Forward Calorimeter
      (FCal) Testbeam
      Setup


            Excluder
FCal1 Module 0




                                                10 GeV Electron Shower

                        FCal2 Module 0
               Andrea Dell’Acqua                 Status of the GEANT4 Physics Evaluation in ATLAS                                               Slide 8
                 CERN EP/SFT




                         Muon Energy Loss                                                                  Electromagnetic Barrel Calorimeter
                                                                                                             EMB (Liquid Argon/Lead Accordion)
                     Hadronic EndCap Calorimeter (HEC)
                                                                                             10-1
                      (Liquid Argon/Copper Parallel Plate)




                                                                   Fraction events/0.1 GeV
               800                                                                           10-2

               700
                                                                                             10-3
Events/10 nA




               600
               500                              180 GeV μ
               400
                                                                                             10-4            Eμ= 100 GeV, ημ ≈ 0.975
               300
               200                                                                                  0.1    0.2   0.3   0.4   0.5   0.6    0.7    0.8      0.9   1
               100                                                                                                 Reconstructed Energy [GeV]
                 0                                                                             0
                 -100     0     100     200    300     400   500
                                                                   Δ events/0.1 GeV [%]

                                                                                             -0.5
                               Calorimeter Signal [nA]
                                                                                             -1.0
      — G4 simulations (+ electronic noise)
                                                                                             -1.5
      describe testbeam signals well, also in
                                                                                             -2.0
      Tile Calorimeter (iron/scintillator
                                                                                             -2.5
      technology, TileCal);
                                                                                             -3.0
      — some range cut dependence of G4                                                      -3.5
      signal due to contribution from                                                                0.1   0.2   0.3   0.4   0.5   0.6   0.7     0.8      0.9   1
      electromagnetic halo (δ-electrons);                                                                         Reconstructed Energy [GeV]
Andrea Dell’Acqua        Status of the GEANT4 Physics Evaluation in ATLAS        Slide 9
  CERN EP/SFT




                    Secondaries Production by Muons
                                     Muon Detector:
                                     • extra hits produced in dedicated testbeam setup
                                     with Al and Fe targets (10, 20 and 30 cm deep),
                                     about ~37 cm from first chamber or between the
                                     chambers;

                                     • probability for extra hits measured in data at
                                     various muon energies (20-300 GeV);

                                     • Geant4 can reproduce the distance of the extra hit
                                     to the muon track quite well;
Andrea Dell’Acqua   Status of the GEANT4 Physics Evaluation in ATLAS           Slide 10
  CERN EP/SFT




Silicon Detectors – ionisation and PAI model

                                                    Standard ionisation model
                                                    compared to PAI model for 100
                                                    GeV pions crossing a Pixel detector
                                                    module (280 mm thick silicon):
                                                    • distribution around peak identical
                                                    • PAI model does not link properly
                                                    to d-ray production
                                                    • more important is the correct
                                                    spatial distribution of ionisation
                                                    energy loss: range cut should
                                                    match detector resolution (<10 mm
                                                    for Pixels)
Andrea Dell’Acqua           Status of the GEANT4 Physics Evaluation in ATLAS          Slide 11
  CERN EP/SFT




                       Transition Radiation Tracker
    • Very good agreement with data (and G3) for pions and muons
    • Several models tried for describing transition radiation with
      moderate success
               Currently “on-hold” in favour of a home-grown TR model as the G4
               one turns out to be too demanding in terms of geometry and
               tracking
                                                                                   20 GeV electrons




                                        300 GeV muons
        20 GeV pions


                                                                               Deposited energy (keV)
Andrea Dell’Acqua       Status of the GEANT4 Physics Evaluation in ATLAS                            Slide 12
  CERN EP/SFT



   Geant4 Electron Response in ATLAS Calorimetry
Overall signal characteristics:
                                                                              TileCal Electron Energy Resolution
Geant4 reproduces the average electron signal as
function of the incident energy in all ATLAS
calorimeters very well (testbeam setup or analysis
induced non-linearities typically within ±1%)…
                                   EMB Electron Energy Resolution
…but average signal
can be smaller than in G3
and data (1-3% for 20-              GEANT4                GEANT4

700 μm range cut in HEC);
                                           GEANT3        GEANT3
signal fluctuations in EMB
very well simulated;
                                   data                     data

electromagnetic FCal:                                                      TileCal: stochastic term
high energy limit of reso-                                                 41.%GeV1/2 G4, 38.6%GeV1/2
                                                                           data; high energy limit very
lution function ~5% in G4,   0.2     0.3   0.4     0.5   9 9.2 9.4 9.6     comparable;
~ 4% in data and G3;           high energy limit %
                                                         stochastic term
                                                           % × GeV 
                                                                   
 Andrea Dell’Acqua                                 Status of the GEANT4 Physics Evaluation in ATLAS                                             Slide 13
   CERN EP/SFT




          Electron Shower Shapes & Composition (1)
Shower shape analysis:
Geant4 electromagnetic showers in the EMB are more compact longitudinally than in
G3: about 3-13% less signal in the first 4.3X0, but 1.5-2.5% more signal in the
following 16X0, and 5-15% less signal (large fluctuations) in the final 2X0 for 20-245
GeV electrons;

Geant4 electron shower in TileCal starts earlier and is slightly narrower than in G3:
                                        TileCal 100 GeV Electrons                                    TileCal 100 GeV Electrons



                                                                            dE/E per RM
         dE/E per X0




                       0.12
                                                                                          0.6
                        0.1
                                                                                          0.5
                       0.08
                                                                                          0.4
                       0.06                                                               0.3
                       0.04                                                               0.2
                       0.02                                                               0.1
                         0                                                                 0
                              0   2.5    5   7.5   10 12.5 15 17.5 20                           -3   -2     -1      0      1        2      3
                                                   Shower depth [X0 = 2.25cm]                         Distance from shower axis [RM = 2.11cm]
 Andrea Dell’Acqua                    Status of the GEANT4 Physics Evaluation in ATLAS          Slide 14
   CERN EP/SFT




  Geant4 Hadronic Signals in ATLAS Calorimeters
Calorimeter pion response:
                                                                          HEC Pions
Rather difficult start, with inadequate models
(“GHEISHA++”) and “mix-and-match” problems
(transition from low energy to high energy
charged pion models)
fixes suggested by H.P. Wellisch (LHEP, new
energy thresholds in model transition + code
changes) and new models (QGS) improved the
situation dramatically



                                                Quantitative agreements between data and G4 for
                                                most of the observables, with QGS models which
                                                seem to provide the better answer

                           TileCal                          finally going in the right direction!
                     Pion non-linearity
                                                Still a few problems and open questions, that will
                                                require further investigation (in particular shower
                                                shape and pion energy deposition)
 Andrea Dell’Acqua                 Status of the GEANT4 Physics Evaluation in ATLAS                    Slide 15
   CERN EP/SFT




         Geant4 Hadronic Signal Characteristics (1)
Pion energy resolution:
                                                                           TileCal Pion Energy Resolution
good description of experimental pion energy
resolution by QGS in TileCal; LHEP cannot
describe stochastic term, but fits correct high
energy limit;

                 HEC Pion Energy Resolution


                                     stoch. const
                         Data        68.89 5.82

                         QGSP 70.26               6.00

                         G3          64.44        4.70


All recent simulations show definite improvements
as far as QGSP is concerned (and wrt Geant3)
Andrea Dell’Acqua    Status of the GEANT4 Physics Evaluation in ATLAS   Slide 16
  CERN EP/SFT




        Geant4 Hadronic Signal Characteristics (2)
Pion longitudinal shower
profiles:
measured by energy sharing in four
depth segments of HEC; all available
Geant4 models studied;

rather poor description of
experimental energy sharing by QGS;
pion showers start too early;
requires further investigation

LHEP describes longitudinal energy
sharing in the experiment quite well
for pions in the the studied energy
range 20-200 GeV (at the same level
as GCalor in Geant3.21);
Andrea Dell’Acqua   Status of the GEANT4 Physics Evaluation in ATLAS   Slide 17
  CERN EP/SFT




                             Conclusions:
   Geant4 can simulate relevant features of muon, electron and pion
signals in various ATLAS detectors, in most cases better than Geant3;

   remaining discrepancies, especially for hadrons, are addressed and
progress is continuous and measurable;

   ATLAS can has a huge amount of the right testbeam data for the
calorimeters, inner detector modules, and the muon detectors to
evaluate the Geant4 physics models in detail;

   feedback loops to Geant4 team are for most systems established
since quite some time; communication is not a problem;

   Geant4 is definitely becoming a mature and useful product for
larga scale detector response simulation!
 Andrea Dell’Acqua                              Status of the GEANT4 Physics Evaluation in ATLAS                                Slide 18
   CERN EP/SFT




     Geant4 Electron Signal Range Cut Dependence
maximum signal in HEC and FCal found at 20 μm – unexpected signal drop for lower range
cuts;
                                           FCal 60 GeV Electrons                                      HEC 100 GeV Electrons
                                  1.6                                                           4.3
               Frac. [%]
               Sampling




                                                                                   Evis [GeV]
                                  1.5                                    Geant3                                                        Geant3
                                                                                                4.2

                                  61
                                                                                                4.1
                     Edep [GeV]




                                  60
                                  59
                                                                                                2.1
                                   7




                                                                                   σ/E [%]
                     σ/E [%]




                                                                                                 2
                                   6
                                                      20 μm                                     1.9           20 μm
                                   5
                                    10-3       10-2       10-1                                         10-2    10-1   1        10
                                           GEANT4 range cut [mm]                                       GEANT4 range cut [mm]


HEC and FCal have very different readout geometries (parallel plate, tubular gap) and
sampling characteristics, but identical absorber (Cu) and active (LAr) materials;
effect under discussion with Geant4 team (M. Maire et al.), but no solution yet (??);
 Andrea Dell’Acqua         Status of the GEANT4 Physics Evaluation in ATLAS                              Slide 19
   CERN EP/SFT




         Electron Shower Shapes & Composition (2)
Shower composition:
   cell signal significance spectrum is
                                                                            FCal 60 GeV Electrons
   the distribution of the signal-to-noise
   ratio in all individual channels for all                            electronic




                                             Rel. entries
   electrons of a given impact energy;
                                            10-1                          noise                           excess in
                                                            10-2
                                                                        shower signals                   experiment
    to measure this spectrum for simu-                      10-3
    lations requires modeling of noise in                   10-4
    each channel in all simulated events                    10-5
    (here: overlay experimental “empty”                     10-6
    noise events on top of Geant4 events)                          0   50     100     150      200     250      300
                                                                       Cell Signal Significance Γ σnoise 


spectrum shows higher end point for data than for Geant4 and Geant3,
indicating that larger (more significant) cell signals occur more often in the
experiment -> denser showers on average;
 Andrea Dell’Acqua        Status of the GEANT4 Physics Evaluation in ATLAS   Slide 20
   CERN EP/SFT




                     Individual Hadronic Interactions
Inelastic interaction properties:
energy from nuclear break-up in the course of a hadronic inelastic interactions
causes large signals in the silicon pixel detector in ATLAS, if a pixel (small, 50
μm x 400 μm), is directly hit;

this gives access to tests of         Special interaction trigger
single hadronic interaction
modeling, especially concerning
the nuclear part;

testbeam setup of pixel
detectors supports the study                     ~3000 sensitive pixels
of these interactions;

presently two models in Geant4 studied: the parametric “GHEISHA”-type model
(PM) and the quark-gluon string model (QGS, H.P. Wellisch);
 Andrea Dell’Acqua       Status of the GEANT4 Physics Evaluation in ATLAS                               Slide 21
   CERN EP/SFT




 Individual Hadronic Interactions: Energy Release
Interaction cluster:                                      PM                 QGS                 Experiment

differences in shape and average (~5%
too small for PM, ~7% too small for
QGS) of released energy distribution
for 180 GeV pions in interaction
clusters;
                                                                log(energy equivalent # of electrons)
fraction of maximum single pixel
release and total cluster energy
release not very well reproduced by PM
                                                           PM                  QGS                 Experiment
(shape, average ~26% too small);

QGS does better job on average
(identical to data) for this variable, but
still shape not completely reproduced
yet (energy sharing between pixels in
cluster);
 Andrea Dell’Acqua      Status of the GEANT4 Physics Evaluation in ATLAS            Slide 22
   CERN EP/SFT




           More on Individual Hadronic Interactions
Spread of energy:
other variables tested with pixel detector: cluster width, longest distance
between hit pixel and cluster barycenter -> no clear preference for one of the
chosen models at this time (most problems with shapes of distributions);

Charged track multiplicity:
average charged track multiplicity in in-
elastic hadronic interaction described                   PM                QGS   Experiment
well with both models (within 2-3%),
with a slight preference for PM;

				
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