A review of the strategy and the results for the physics validation of Geant4 and Fluka detector simulation engines for LHC applications is presented. The main focus is on hadronic physics, where most of the efforts are currently concentrated. In particular, comparisons between simulation and test-beam data for the most relevant calorimeter observables, for different experiments and detector technologies, are discussed

Ribon, Alberto

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Physics Validation of LHC SimulationsAlberto Ribon CERN, SwitzerlandE-mail: Alberto.Ribon@cern.chA review of the strategy and the results for the physics validation of Geant4 and Fluka detectorsimulation engines for LHC applications is presented. The main focus is on hadronic physics,where most of the efforts are currently concentrated. In particular, comparisons between simula-tion and test-beam data for the most relevant calorimeter observables, for different experimentsand detector technologies, are discussed.International Europhysics Conference on High Energy PhysicsJuly 21st - 27th 2005Lisboa, PortugalSpeaker.PoS(HEP2005)387cCopyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Physics Validation of LHC Simulations Alberto Ribon1. IntroductionHadronic physics is notoriously a very broad and difficult field, mainly because the underly-ing theory, Quantum Chromodynamics (QCD), cannot produce predictions for observables whosedominant energy range is outside the perturbative high-energy regime. The only current viable ap-proach, in these cases, is to use different simplified models, whose approximated validity is oftenrestricted to particular incident particles, target material types, and interaction energies. By using aproper set of these models it is often possible to cover different regions of interest.In Geant4 [1] a large set of hadronic models are available, and users can choose and combine themaccording to their needs, in terms of application, precision, and computing time. To ease such achoice, a certain number of “educated guess” Physics Lists, each being a complete and consistentcollection of different models, are provided according to use cases.The goal of this paper is to report about the validation of some of these Geant4 Physics Lists(LHEP, QGSC, QGSP, QGSP_BIC, QGSP_BERT), and also of FLUKA simulation engine [2], inthe context of the LHC experiments. We will focus here only on the hadronic physics validation(the electromagnetic physics has been already validated at the percent level).There are two complementary approaches to the physics validation of simulation results: oneis based on thin-target setups with simple geometries, that allow to test, in a clean and simpleenvironment, single interactions or effects; the other one relies on calorimeter test-beam setups, inwhich the observables are the convolution of many complex processes and interactions.We will treat here some of the work that has been carried out in the context of the LCG (LHCComputing Grid Simulation) Physics Validation Project [3], starting first with three thin-targettests, and then with three calorimeter test-beams.2. Pixel test-beamA test-beam with 180GeVc positive pions on silicon sensors of the ATLAS Pixel trackerdetector has been made at CERN in 2001 [4]. It consists of a telescope of four microstrip planes(each double sided), two pixel detector planes (to test two different chips), and a scintillator counter.In a run dedicated to the study of hadronic interactions, the trigger required an energy depositionin the scintillator corresponding to at least three minimum-ionising particles. A total of about800000 interaction events were collected. Only those events in which at least three clusters havebeen reconstructed in each of the three downstream microstrip planes (in both sides) are furtherconsidered in the analysis. We refer to the note [5] for more details. The conclusion is that bothGeant4 and FLUKA describe reasonably well the test-beam data, more or less at the same level,although there are some discrepancies in some observables.3. Neutron production cross-sectionsNeutron production from proton bombardment, and in particular  p  xn  double-differentialcross-section measurements, d2σdΩdEn, i.e. neutron spectrum at fixed angles, are an importantbenchmark for the validation of hadronic models. Here we consider measurements made at LosAlamos Meson Physics Facility (LAMPF) for proton beam energies of 113, 256, 597, and 800MeV ,PoS(HEP2005)387387 / 2Physics Validation of LHC Simulations Alberto Ribonand for angles of 7  5  . 30  , 60  , 120  and 150  [6]. For details, plots and further references weinvite to see the note [7]. The preliminary conclusions from the comparison of the simulationresults with the experimental data are the following. While the Geant4 LHEP Physics List isclearly not suitable for the simulation of (p,xn) double-differential cross-sections, FLUKA andGeant4 QGSP_BERT and QGSP_BIC Physics Lists do all reproduce the double-differential cross-section data measured by Los Alamos at the level of 20% to 50%.4. In-ight pion absorptionData [8] of thin-target experiments on in-flight pi 	 and pi  absorption are compared with theGeant4 and FLUKA simulations. Pion beam energies are between 23MeV and 315MeV , andtargets are made of Al, Cu and Au. The in-flight pion absorption process can affect in particularthe e/h ratio of calorimeters, as well as the energy resolution at low and medium energies, andtherefore needs to be well understood for the simulation of LHC calorimeters. The results reportedin [9] show a generally good agreement between both simulation packages and the experimentaldata. Unfortunately, due to large uncertainties in the data, precise comparisons are not possible.Geant4 and FLUKA agree better with each other for intermediate weight materials like Cu. Thediscrepancies become larger for light materials like Al and for the heavy materials like Au. On theother hand, the multiplicity and type of the outgoing particles are qualitatively similar, indicatingthat the implementation of the processes is correct.5. Calorimeter test-beamsWe discuss here the results of the comparisons between Geant4 simulation, with LHEP andQGSP Physics Lists, with three different calorimeter test-beam data. The first calorimeter test-beam setup is the ATLAS HEC (Hadron End-Cap) [10], which is a sampling calorimeter withcopper as absorber and liquid argon as sensitive part. The second calorimeter test-beam setup isthe ATLAS Tilecal [11], which is the central (barrel and extended barrel) hadronic calorimeter, asampling calorimeter made of iron as absorber and scintillator tiles as active medium. The thirdcalorimeter test-beam setup is the combined CMS ECAL+HCAL [12]. The electromagnetic part isa matrix of PbWO4 crystals, whereas the hadronic part is a sampling calorimeter made of copperas absorber and scintillator tiles as sensitive part. The whole apparatus is embedded in a magneticfield of max 3 Tesla. The results [13] of these comparisons show that the energy resolution and theepi is well reproduced by the Geant4 Physics Lists LHEP and QGSP, whereas showers are shorterand narrower than the real ones, especially for QGSP.6. ConclusionsWe have presented several validation tests for Geant4 Hadronic Physics, some of which alsoshowing the comparison with FLUKA simulation. This work concludes the first round of hadronicphysics validation, with good results. The conclusion is that Geant4 LHEP and QGSP PhysicsLists are in good agreement with experimental data, for most observables, and in particular for thecalorimeter energy resolution and for the epi ratio, essentially matching the physics requirementsPoS(HEP2005)387387 / 3Physics Validation of LHC Simulations Alberto Ribon[14]. These are coming from the demand that the dominant systematic uncertainties for all physicsanalyses should not be due to the imperfect simulation. Longitudinal and transverse shower shapes,however, are not reproduced very well by Geant4, in particular for QGSP Physics List. Work is inprogress to improve them, and more comparisons are expected from the test-beams made in 2004.Another area of interest, in progress, is the one of Geant4 background radiation studies in the LHCcaverns.7. AcknowledgmentsI am pleased to acknowledge the help, assistence, and collaboration of the Geant4 team(J. Apostolakis, G. Folger, M. Kossov, J.P. Wellisch), the FLUKA team (A. Ferrari, P. Sala), andof all the people who have contributed to the LCG Physics Validation Project (F. Gianotti,W. Pokorski. M. Gallas, J. Beringer, C. Alexa, S. Banerjee, T. Carli, S. Constantinescu,G. Daquino, S. Dita, A. Dotti, D. Elvira, A. Kiryunin, A. Lupi, C. Roda, D. Salihagic, P. Schacht,P. Strizenec, D. Barberis, M. Cervetto, B. Osculati).References[1] S. Agostinelli et al, Geant4 Collaboration, Nuclear Instruments and Methods in Physics Research, A506 (2003) 250. See also the Geant4 web page: http://cern.ch/geant4 .[2] A. Fassò, A. Ferrari, J. Ranft, P. Sala, in proceedings of the MonteCarlo 2000 Conference, Lisbon 2000,Springer-Verlag (2001), 159-164, 955-960. See also the FLUKA web page: http://www.fluka.org .[3] Physics Validation Project, http://lcgapp.cern.ch/project/simu/validation/ .[4] T.Lari et al, Test beam results of ATLAS Pixel Sensors, in proceedings of the InternationalWorkshop on Semiconductor Pixel Detectors for Particles and X-Rays, Carmel 2002.[5] A. Ribon, Validation of Geant4 and FLUKA hadronic physics with pixel test-beam data,CERN-LCGAPP-2004-09.[6] M.M. Meier et al., Nuclear Science and Engineering 102 (1989) 310; 110 (1992) 289.W.B. Amian et al., Nuclear Science and Engineering 112 (1992) 78; 115 (1993) 1.[7] J. Beringer, (p,xn) Production Cross Sections: A Benchmark Study for the Validation of HadronicPhysics Simulation at LHC, CERN-LCGAPP-2003-18.[8] D. Ashery et al, Physcal Review C, 23 (1981) 2173.K. Nakai et al, Physical Review Letters, 44 (1980) 1446.[9] W. Pokorski, In-ight Pion Absorption: Second Benchmark Study for the Validation of HadronicPhysics Simulation at the LHC, CERN-LCGAPP-2004-11.[10] ATLAS Collaboration, Liquid Argon Calorimeter Technical Design Report, CERN/LHCC/96-41(1996).[11] ATLAS Collaboration, Tile Calorimeter Technical Design Report, CERN/LHCC 96-42 (1996).[12] CMS Collaboration, Hadron Calorimeter Technical Design Report, CERN/LHCC/97-31 (1997).[13] F. Gianotti et al, Geant4 hadronic physics validation with LHC test-beam data: rst conclusions,CERN-LCGAPP-2004-10.[14] A. De Roeck, F. Gianotti, A. Morsch, W. Pokorski, Simulation physics requirements from the LHCexperiments, CERN-LCGAPP-2004-02.PoS(HEP2005)387387 / 4

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