Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 1
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
— ATLAS:
Muon Detectors (μ)
A Multi-Purpose LHC
Detector
Barrel Toroid
Electromagnetic Calorimeters (μ,e)
Solenoid
Slide 2
EMB (LAr/Pb,Barrel)
& EMEC (LAr/Pb,EndCap)
Forward Calorimeters (e)
FCal (LAr/Cu/W)
EndCap Toroid
Inner Detector (e,μ,π)
Hadronic Calorimeters (e,μ,π)
HEC (LAr/Cu,EndCap) &
TileCal (Scint/Fe,Barrel/Extended)
Shielding
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 3
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 4
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 5
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Geant4 Setups (1)
Detector plastic
Cover (3mm thick)
Silicon sensor (280 μm thick)
FE chip (150 μm thick)
PCB (1 mm thick)
Hadronic Interaction in Silicon Pixel Detector
Muon Detector Testbeam
Slide 6
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Geant4 Setups (2)
Electromagnetic Barrel Accordion Calorimeter
Forward Calorimeter
(FCal) Testbeam
Setup
Excluder
FCal1 Module 0
10 GeV Electron Shower
FCal2 Module 0
Slide 7
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Muon Energy Loss
800
Events/10 nA
700
600
180 GeV μ
500
400
300
Electromagnetic Barrel Calorimeter
EMB (Liquid Argon/Lead Accordion)
10-1
Fraction events/0.1 GeV
Hadronic EndCap Calorimeter (HEC)
(Liquid Argon/Copper Parallel Plate)
10-2
10-3
Eμ= 100 GeV, ημ ≈ 0.975
10-4
200
0.1
0.2
100
0.3
0.4
0.5
0.6
0.7
Reconstructed Energy [GeV]
0.8
0.9
1
0.3
0.4
0.5
0.6
0.7
Reconstructed Energy [GeV]
0.8
0.9
1
0
0
400
100
200
300
Calorimeter Signal [nA]
— G4 simulations (+ electronic noise)
500
describe testbeam signals well, also in
Tile Calorimeter (iron/scintillator
technology, TileCal);
— some range cut dependence of G4
signal due to contribution from
electromagnetic halo (δ-electrons);
Δ events/0.1 GeV [%]
0
-100
Slide 8
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
0.1
0.2
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 9
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 10
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 11
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 12
Geant4 Electron Response in ATLAS Calorimetry
Overall signal characteristics:
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 20700 μm range cut in HEC);
GEANT4
GEANT4
GEANT3
signal fluctuations in EMB
very well simulated;
data
electromagnetic FCal:
high energy limit of resolution function ~5% in G4,
~ 4% in data and G3;
TileCal Electron Energy Resolution
0.2
0.3
GEANT3
data
0.4
0.5
high energy limit %
9 9.2 9.4 9.6
stochastic term
% × GeV 
TileCal: stochastic term
41.%GeV1/2 G4, 38.6%GeV1/2
data; high energy limit very
comparable;
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 13
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;
TileCal 100 GeV Electrons
0.12
0.1
dE/E per RM
dE/E per X0
Geant4 electron shower in TileCal starts earlier and is slightly narrower than in G3:
0.08
TileCal 100 GeV Electrons
0.6
0.5
0.4
0.06
0.3
0.04
0.2
0.02
0.1
0
0
2.5
5
7.5
10 12.5 15 17.5 20
Shower depth [X0 = 2.25cm]
0
-3
-2
-1
0
1
2
3
Distance from shower axis [RM = 2.11cm]
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 14
Geant4 Hadronic Signals in ATLAS Calorimeters
Calorimeter pion response:
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
HEC Pions
Quantitative agreements between data and G4 for
most of the observables, with QGS models which
seem to provide the better answer
TileCal
Pion non-linearity
finally going in the right direction!
Still a few problems and open questions, that will
require further investigation (in particular shower
shape and pion energy deposition)
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 15
Geant4 Hadronic Signal Characteristics (1)
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
Data
stoch. const
68.89 5.82
QGSP 70.26
6.00
G3
4.70
64.44
All recent simulations show definite improvements
as far as QGSP is concerned (and wrt Geant3)
TileCal Pion Energy Resolution
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 16
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 17
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 18
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
4.3
1.5
Geant3
Edep [GeV]
61
Geant3
4.2
4.1
60
59
σ/E [%]
7
σ/E [%]
HEC 100 GeV Electrons
Evis [GeV]
Sampling
Frac. [%]
1.6
6
5
10-3
20 μm
10-2
10-1
GEANT4 range cut [mm]
2.1
2
1.9
20 μm
10-2
10-1
1
GEANT4 range cut [mm]
10
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 19
Electron Shower Shapes & Composition (2)
Shower composition:
Rel. entries
cell signal significance spectrum is
the distribution of the signal-to-noise
ratio in all individual channels for all
10-1
electrons of a given impact energy;
to measure this spectrum for simulations requires modeling of noise in
each channel in all simulated events
(here: overlay experimental “empty”
noise events on top of Geant4 events)
FCal 60 GeV Electrons
electronic
noise
10-2
excess in
experiment
shower signals
10-3
10-4
10-5
10-6
0
50
100
150
200
250
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;
300
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 20
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
single hadronic interaction
modeling, especially concerning
the nuclear part;
testbeam setup of pixel
detectors supports the study
of these interactions;
Special interaction trigger
~3000 sensitive pixels
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
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 21
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;
fraction of maximum single pixel
release and total cluster energy
release not very well reproduced by PM
(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);
log(energy equivalent # of electrons)
PM
QGS
Experiment
Andrea Dell’Acqua
CERN EP/SFT
Status of the GEANT4 Physics Evaluation in ATLAS
Slide 22
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 inelastic hadronic interaction described
well with both models (within 2-3%),
with a slight preference for PM;
PM
QGS
Experiment