Atlas & Physics Validations

1. Atlas & Physics Validations Contents Status on physics validations Paper presented to Chep03 by A.Dell’Acqua Testbeam activities in the subdetectors A collection of programs and activities foreseen in the next 12 months in the Atlas subdetectors

2. 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

3. Solenoid EndCap Toroid Shielding • ATLAS: • A Multi-Pur- • pose LHC • Detector EMB (LAr/Pb,Barrel) & EMEC (LAr/Pb,EndCap) Muon Detectors (μ) Electromagnetic Calorimeters (μ,e) Forward Calorimeters (e) FCal (LAr/Cu/W) Barrel Toroid Inner Detector (e,μ,π) Hadronic Calorimeters (e,μ,π) HEC (LAr/Cu,EndCap) & TileCal (Scint/Fe,Barrel/Extended)

4. 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

5. Strategies for G4 Physics Validation in ATLAS • Geant4 physics benchmarking: • comparefeatures of interaction models with similar features in the old Geant3.21 baseline (includes variables not accessible in the experiment); • try to understanddifferences 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); • usedifferent sensitivities of sub-detectors (energy loss, track multiplicities, shower shapes…) to estimate Geant4 performance; • tuneGeant4 models (“physics lists”) and parameters (range cut) for optimal representation of the experimental detector signal with ALL relevant respects;

6. 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

7. Muon Detector Testbeam Detector plastic Cover (3mm thick) Silicon sensor (280 μm thick) FE chip (150 μm thick) PCB (1 mm thick) Geant4 Setups (1) Hadronic Interaction in Silicon Pixel Detector

8. Geant4 Setups (2) Electromagnetic Barrel Accordion Calorimeter Forward Calorimeter (FCal) Testbeam Setup Excluder FCal1 Module 0 10 GeV Electron Shower FCal2 Module 0

9. Geant4 Setups (3) Tile Detector Testbeam Pixel/Tile/Muon Detector Testbeam

10. 10-1 10-2 Fraction events/0.1 GeV 10-3 Eμ= 100 GeV, ημ ≈ 0.975 10-4 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Reconstructed Energy [GeV] 0 -0.5 -1.0 -1.5 Δ events/0.1 GeV [%] -2.0 -2.5 -3.0 -3.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Reconstructed Energy [GeV] Muon Energy Loss Electromagnetic Barrel Calorimeter EMB (Liquid Argon/Lead Accordion) Hadronic EndCap Calorimeter (HEC) (Liquid Argon/Copper Parallel Plate) 800 700 600 180 GeV μ 500 Events/10 nA 400 300 200 100 0 400 -100 0 100 200 300 500 Calorimeter Signal [nA] • G4 simulations (+ electronic noise) 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);

11. agreement at the level of <1% 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;

12. 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)

13. 300 GeV muons 20 GeV pions 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 Deposited energy (keV)

14. GEANT4 GEANT4 GEANT3 GEANT3 data data 0.2 0.3 0.4 0.5 9 9.2 9.4 9.6 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%)… …but average signal can be smaller than in G3 and data (1-3% for 20- 700 μm range cut in HEC); signal fluctuations in EMB very well simulated; electromagnetic FCal: high energy limit of reso- lution function ~5% in G4, ~ 4% in data and G3; TileCal Electron Energy Resolution EMB Electron Energy Resolution TileCal: stochastic term 41.%GeV1/2 G4, 38.6%GeV1/2 data; high energy limit very comparable;

15. TileCal 100 GeV Electrons TileCal 100 GeV Electrons 0.12 0.6 dE/E per X0 dE/E per RM 0.1 0.5 0.08 0.4 0.06 0.3 0.04 0.2 0.02 0.1 0 0 -3 -2 -1 0 1 2 3 0 2.5 5 7.5 10 12.5 15 17.5 20 Shower depth [X0 = 2.25cm] Distance from shower axis [RM = 2.11cm] 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:

16. 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 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) TileCal Pion non-linearity

17. 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; • All recent simulations show definite improvements as far as QGSP is concerned (and wrt Geant3) TileCal Pion Energy Resolution HEC Pion Energy Resolution stoch. const Data 68.895.82 QGSP 70.266.00 G3 64.444.70

18. 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);

19. 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 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 large scale detector response simulation!

20. 4.3 1.6 Geant3 4.2 Geant3 Evis [GeV] Sampling Frac. [%] 1.5 4.1 61 60 2.1 Edep [GeV] 59 σ/E [%] 2 7 1.9 6 σ/E [%] 10-2 10-1 1 10 5 GEANT4 range cut [mm] GEANT4 range cut [mm] 10-3 10-2 10-1 Geant4 Electron Signal Range Cut Dependence • maximum signal in HEC and FCal found at 20 μm – unexpected signal drop for lower range cuts; • 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 (??); FCal 60 GeV Electrons HEC 100 GeV Electrons 20 μm 20 μm

21. 10-1 10-2 10-3 10-4 10-5 10-6 0 50 100 150 200 250 300 Electron Shower Shapes & Composition (2) • Shower composition: • cell signal significance spectrum is • the distribution of the signal-to-noise • ratio in all individual channels for all • electrons of a given impact energy; • to measure this spectrum for simu- • lations requires modeling of noise in • each channel in all simulated events • (here: overlay experimental “empty” • noise events on top of Geant4 events) • 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; FCal 60 GeV Electrons electronic noise excess in experiment shower signals Rel. entries

22. 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; presently two models in Geant4 studied: the parametric “GHEISHA”-type model (PM) and the quark-gluon string model (QGS, H.P. Wellisch); Special interaction trigger ~3000 sensitive pixels

23. Individual Hadronic Interactions: Energy Release • Interaction cluster: • 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); PM QGS Experiment log(energy equivalent # of electrons) PM QGS Experiment

24. 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 well with both models (within 2-3%), with a slight preference for PM; PM QGS Experiment

25. Testbeam activities in the subdetectors A collection of programs and activities foreseen in the next 12 months in the Atlas subdetectors

26. Inner Detector (from Fred Luehring) • TRT testbeam next August. • This testbeam will use 6 production barrel TRT modules • arranged in 3 layers 2 modules wide. • This setup arranged exactly like 1/16 of the barrel TRT assembly. • The setup will have enough channels instrumented to record a track through all three layers of modules. • comments: • The testbeam setup only measures time information and does not directly measure pulse height. • with this testbeam setup it is not possible to remove the radiator from the production modules so that • any study on an electron beam measures the combined effect of ionization and TR

27. Inner Detector (from Fred Luehring) • Resources • Mogens Dam and Jakob Nielson (Niels Bohr Institute) are making a GEANT4 model of this setup based on Andrei and Yura Zalite's work. • Andrei and Yura are at CERN for the next 3 months for consultation. • Fred Luehring: GEANT3 model of the testbeam setup based on the current TRT barrel simulation in GEANT3 • comparisons between models of TR production in the GEANT4 to data and also Pavel's TR model in GEANT3. • As part of this study, Mogens and Jakob will be moving Pavel's TR model into GEANT4 - this will done by rewriting it into C++ or wrapping it. • try comparisons between TR models built into GEANT4 developed by Vladimir Grichine

28. LAr calorimeters (from Mikhail Lelchouk) • The reconstruction and simulation of 2002 EMEC/HEC combined testbeam (EC-1) is under way • Geant3 Based Simulations done : • > muons 120, 150, 180 GeV • > electrons 6-150 GeV • > negative pions 10-200 GeV • > more details: http://wwwatlas.mppmu.mpg.de/HEC_mc/tb_mc_4.0.0/ • 2003 FCal calibration run can be simulated in G4 • combined EMB/Tile (EC-1) testbeam • in 2004 (Guillaume Unal and Vakhtang Tsulaia are ready to work on G4) • EMEC/HEC/FCAL (EC-2) testbeam simulation • Simulations of the Crack Region at Eta=3.2 were done with ATLSIM in the frame of the standard ATLAS software (version of geometry 98_2) • The first version of G4 simulations may be ready for release 8 (December 2003)

29. Tile Calorimeter (from Vakhtang Tsulaia) • Investigation on • energy distribution shapes • calibration constants • shower leakage effects • with the testbeam setup for • different particles (pions, electrons, muons, protons) • different energies (10-200 GeV) • different eta positions of the setup • different physics lists (LHEP, QGSP, FTFP) • work in progress and to be continued in the next year • Start simulations for the low energy range (10 GeV and below) with the existing testbeam configuration • the first step towards the LAr-Tile combined simulations • results of combined simulations before the real testbeam (next year). • Integration of the three-dimensional fast shower parametrisation intoTile G4 simulations. • work in the phase of initial discussions

30. Muon System (from AR) • New testbeam (22 May -> 6 September) • New test of MDT chambers and a first test of combined setup with • trigger chambers + precision chambers as in the experiment • RPC+MDT BOL BML • EIL EML EOL TGC • More systematics with materials in front of muon chambers • to mimic the inert material presence • extra-particle content in muon beams • energy scan 20, 50, 100, 200 GeV • Extensive tests with Al, Fe at different distance from the muon chamber system • Magnetic Field (angular spread upstream) and between EI EM (3 Tm) on • Combined setup studies (Muon/Tile/Pixel detectors) • Resources • D.Rebuzzi & G.Gaudio (Pavia),A.Policicchio(CS), F.Cerutti(CERN) • A.Di Simone (Rome II)