Dario Barberis - DESY · 2001. 11. 28. · LC Workshop, CERN, 15 Nov 2001 Dario Barberis – Genova University/INFN 3 Electromagnetic Physics • G4 Physics studied using separate

LC Workshop, CERN, 15 Nov 2001 Dario Barberis – Genova University/INFN 1

Dario BarberisDario Barberis

Evaluation of GEANT4 Evaluation of GEANT4 Electromagnetic and HadronicElectromagnetic and Hadronic

Physics in ATLASPhysics in ATLAS


The ATLAS detectorThe ATLAS detector


Electromagnetic PhysicsElectromagnetic Physics• G4 Physics studied using separate test beam simulations (simple geometries) for TRT, SCT, Pixel detectors

• G3 and G4 simulations compared “directly” to test beam data

• Detector response simulation very important for overall agreement between simulated and real data

• Physics models tested (or under test):

� Standard energy loss

� PAI (Photon Absorption and Ionisation) energy loss (relevant for the TRT)

� Transition radiation production and absorption


Transition Radiation TrackerTransition Radiation Trackersupport and alignment plate

straws

radiator matrix

carbon fiber shell

zoom of module end plate


TRT: Energy Loss in StrawsTRT: Energy Loss in Straws300 GeV muons

20 GeV pions

20 GeV electrons

Energy loss measured in ATLAS test beam compared to Geant-3 and to Geant-4 simulations (PAI model) including effects of detector and electronics (K.A.Assamagan):

• spectra match reasonably for different particles and energies

• some more checks needed for electrons


TRT: Transition RadiationTRT: Transition RadiationEv

ents

Deposited energy (keV)

Spectrum of energy deposited by 20 GeV electrons in TRT straws, with and without foil radiator in front of the detector (V.Mitsou):• PAI = Geant-4 with PAI model• DATA = 1999 TRT test beam data

• Transition Radiation is produced in foam and foil radiators placed between the straws• The fraction of hits above a given threshold (5 keV) is used to discriminate electrons from hadrons and muons• Test beam data can only measure the convolution of energy loss by ionisation, emission and absorption of TR photons • Geant-4 offers several ways of describing the radiator (as a more or a less regular medium)• So far none of them reproduce the test beam spectra (but V.Grichine produces new models faster than we can test them!)• More work is needed!


Silicon DetectorsSilicon DetectorsStandard ionisation model compared (by M.Klute) to PAI model for 100 GeV pions crossing a Pixel detector module (280 µm thick silicon):

• distribution around peak identical

• PAI model does not link properly to δ-ray production

• PAI model in any case not really applicable for ATLAS silicon detectors

• more important is the correct spatial distribution of ionisation energy loss: range cut should match detector resolution (10 µm for Pixels)


Silicon DetectorsSilicon Detectors

Expected: 78.9 28.2 (from data on Si detectors)

• Variation of Landau width with range cut somewhat disturbing• Need small r-cut as detector resolution ~10 µm• Investigating alternative geometry descriptions of Pixel module (~50k pixels/module, 1750 modules in ATLAS...)


SCT: Detector ResponseSCT: Detector Response• Detector response model very important for comparisons between simulation and test beam

• Strictly not part of Geant-4 (or Geant-3) but cannot be factorised away in real test beam data!

• SCT Geant-4 test beam simulation with correct detector response model reproduces very well available test beam data

SCT efficiency vs threshold for different depletion bias voltages: points are data, lines are Geant-4 simulation plus detector response model (from S.Gadomski)


Hadronic PhysicsHadronic Physics• The Pixel detector can measure the energy released around the interaction point with a very fine granularity

• A first attempt at extracting hadronic interaction events from test beam data did not bring much information as the events were scatteredamongst many different runs under different operating conditions

• In August 2001 we took some dedicated runs in the Pixel test beam with an interaction trigger – data are being analysed right now

• Comparisons will be possible between test beam data and the hadronic interaction models avaliable in Geant-4: “parametric” model (old Gheisha) and “theoretical” (parton-string) model

• Observables are:� local energy deposit and cluster size/shape� multiplicity and angular distribution of (forward) outgoing tracks� their correlations


CalorimetryCalorimetry


Liquid Argon EM Liquid Argon EM CalorimetryCalorimetry


Liquid Argon Liquid Argon EM CalorimetryEM Calorimetry

muons in LAr EM Barrel

electrons in LAr EM Barrel


Liquid Argon Liquid Argon EM CalorimetryEM Calorimetry

• Geant4 describes better than Geant3 energy deposits as measured with muon test beam data – agreement G4-test beam is within 1%

• Geant4 (as well as Geant3) describes well the linearity of electron response

• Geant4 predicts larger energy resolution for electrons that Geant3. Direct comparison with test beam data still in progress


Liquid Argon Liquid Argon Hadronic CalorimetryHadronic Calorimetry

pion shower in LArHadronic End Cap

electron energy resolution in LAr Hadronic End Cap


Liquid Argon Liquid Argon Hadronic CalorimetryHadronic Calorimetrypion energy resolution inLAr Hadronic End Cap

• Electrons:• Geant4 predicts less visible energy in LAr than Geant3 (~3%) and more energy in absorber (~0.1%). Total energy is the same• energy resolution well reproduced by Geant3: Geant4 gives too good resolution

• Pions:• first results of simulation with Geant4 look reasonable• more detailed comparisons with test beam data in progress• open questions being discussed with Geant4 people


TileTile Calorimeter: electronsCalorimeter: electrons

Visible energy vs impact position for 100 GeV and

20 GeV electrons

Electron energy resolution

• Electron energy resolution somewhat too good: sampling term 16% instead of 24% (was the same for Geant-3)

• Visible energy vs impact point has the correct shape but amplitude of variations and energy dependence do not match test beam data


Tile Calorimeter: Tile Calorimeter: muonsmuons

• Energy loss distribution “fatter” than Geant-3 for both Fe and scintillator:⇒ therefore it does not match perfectly test beam data

• but remember: energy loss distribution in silicon narrower than in real data!


Tile Calorimeter: pions Tile Calorimeter: pions GHEISHA in GEANT4 is similar to GHEISHA in GEANT3GHEISHA in GEANT4 is similar to GHEISHA in GEANT3


Tile Calorimeter: Fluka vs G4Tile Calorimeter: Fluka vs G4

Test beam data from 20 to 300 GeV:Test beam data from 20 to 300 GeV:σ/σ/σ/σ/σ/σ/σ/σ/E = E = (43.5 (43.5 ±±±±±±±± 2.5)2.5)%%%%%%%% //////// √√√√√√√√EE + (2.2 + (2.2 ±±±±±±±± 1.2)1.2)%%%%%%%%

e/h = 1.30 e/h = 1.30 ±±±±±±±± 0.10.1


Muon Muon DetectorDetector

EM shower production by muons in absorber: extra hits in Muon Drift Tubes

• Transverse distance of extra hits from muon track in Geant-4 broadly reproduces test beam data

• Detailed agreement better for lighter absorber material


ConclusionsConclusions

� Large progress in the last year in understanding electromagnetic processes, both in tracking and calorimetry.

� Interplay between geometry and physics processes being addressed.

� Work is continuing on both sides (ATLAS and G4) to improve understanding and produce optimised geometries as well as PhysicsLists.

� Possibility to set different cuts and use different PhysicsLists for each detector (by Logical Volume) will help considerably.

� There are still issues that remain to be clarified, but the gradient is positive!

� Collaboration between ATLAS and G4 people on a very good level, there could be faster progress if there was more manpower (especially on our side!).

Documents

Dario Barberis - DESY · 2001. 11. 28. · LC Workshop, CERN, 15 Nov 2001 Dario Barberis – Genova University/INFN 3 Electromagnetic Physics • G4 Physics studied using separate