Validation of Geant4 Hadronic Physics

Transcription

1 Validatin f Geant4 Hadrnic Physics J. Beringer, G. Flger, F. Giantti, A. Ribn, J.P. Wellisch, CERN D. Barberis, M. Cervett, B. Osculati, Gena University and INFN Abstract Recent wrk n the validatin f Geant4 hadrnic physics is presented. The fcus is mainly n simple thin-target measurements, which allw a clean and detailed study f single hadrnic interactins. In particular, tw analyses are discussed: neutrn prductin duble-differential crss-sectins, and pixel tracker test beam. Index Terms Validatin, Geant4, Hadrnic. I. INTRODUCTION Hadrnic physics is ntriusly a very brad and difficult field, mainly because the underlying thery, Quantum Chrmdynamics (QCD), cannt prduce predictins fr bservables whse dminant energy range is utside the perturbative highenergy regime. The nly current viable apprach, in these cases, is t use different simplified mdels, whse apprximated validity is ften restricted t particular incident particles, target material types, and interactin energies. By using a prper set f these mdels it is ften pssible t cver different regins f interest. In Geant4 [] a large set f hadrnic mdels are available, and the user can chse and cmbine them accrding t her/his needs, in terms f applicatin, precisin, and cmputing time. T ease such chice, a certain number f educated guess Physics Lists, each being a cmplete and cnsistent cllectin f different mdels, are prvided accrding t use cases. The gal f this paper is t reprt abut tests f sme f these Physics Lists (LHEP, QGSC, QGSP, QGSP BIC, QGSP BERT: see [] fr mre infrmatin) using thin-target experimental data, which prvides cmplementary infrmatin with respect t the mre traditinal calrimeter test-beam data. In the latter case, in fact, the bservables are the cnvlutin f many cmplex prcesses and interactins, whereas in the frmer case ne can study single hadrnic interactins in a clean and simple envirnment. Bth kind f tests are f curse imprtant, but we fcus here nly n thin-target validatin, and in particular n tw recent analyses. We will treat first the test f Geant4 against pixel tracker test-beam data. Then, we will cmpare Geant4 predictins with experimental neutrn duble-differential crss-sectin measurements at sme fixed angles, fr different incident prtn beam energies and target materials. See fr example [3] fr ther Geant4 validatin tests. II. PIXEL TEST-BEAM DATA A test-beam with 80GeV/c psitive pins n silicn sensrs f the ATLAS Pixel tracker detectr has been made at CERN Fig.. Pixel test-beam setup. Fig.. Pixel detectr: frm left t right, plastic cver (3mm thick), silicn sensr (80µm thick), frnt end read-ut chip (50µm thick), printed circuit bard (mm thick). in 00 [4]. The setup is presented in Fig.. It cnsists f a telescpe f fur micrstrip planes (each duble sided), tw pixel detectr planes (t test tw different chips), and a scintillatr cunter. The pixel detectrs have 50µm pitch in ne dimensin, 400µm in the ther, and thickness f 80µm. Fig. shws a single pixel detectr in detail, and an example f a simulated hadrnic interactin happening in the sensr (as fr the events we wuld like t select in rder t study these interactins). In a run dedicated t the study f hadrnic interactins, the trigger required an energy depsitin in the scintillatr crrespnding t at least three minimum-inising particles. A ttal f abut interactin events were cllected. Only thse events in which at least three clusters have been recnstructed in each f the three dwnstream micrstrip planes (in bth sides) are further cnsidered in the analysis. Befre analysing the real data, a precise alignment f the telescpe planes was perfrmed using tw beam trigger runs, taken just befre and just after the interactin trigger run. Furthermre,

2 the pixel detectrs were individually calibrated, i.e. the energy depsited (in terms f the number f electrn-hle pairs that have been created) was evaluated frm the Time-ver-Threshld read-ut, using pulse injectin and radiactive surces. It is imprtant t nte that this calibratin prcedure invlves energy releases equivalent t minimum-inising particle, whereas in the case f hadrnic interactins happening in the pixel detectrs, the energy depsited can be equivalent t several hundred minimum-inising particles. Therefre, in studying hadrnic interactins we have t extraplate the calibratin int a regin far away frm where it has been measured, and this implies that the abslute energy can be determined nly with sme uncertainty: therefre nly bservables nt directly dependent n the abslute energy shuld be cnsidered when cmparing with simulatin results. The analysis prcedure cnsists f fur steps. First, tracks are recnstructed in the three micrstrip planes dwnstream f the pixel planes. Secnd, the interactin vertex is recnstructed; a better vertex reslutin is bserved in the case that the interactin happens in the secnd pixel plane (because f the larger gemetrical acceptance, limited by the last dwnstream micrstrip plane), therefre nly interactins in this plane are further cnsidered (the reasn is explained in the next step). Third, the cluster clsest in the transverse plane t the interactin is selected, and it is further cnsidered nly if it satisfies tw requirements aimed t select nly thse interactins which happen in the pixel sensr, rather than in the plastic cver r the read-ut chip r the printed circuit bard (see Fig.). A pixel cluster is defined as a cntiguus set f pixels whse signal is abve a given threshld. The first requirement is that the recnstructed vertex is clse enugh t the center f the pixel sensr, taking int accunt the vertex reslutin in the beam directin. The secnd requirement is a large energy depsitin in the pixel detectr, because fr a nuclear interactin in the sensr we expect a large lcal energy depsit frm heavy nuclear fragments f very shrt range. Furth, the cluster prperties f such selected interactin clusters are finally cnsidered. Because f the calibratin uncertainty, the fllwing bservables, which shuld nt be directly affected by it, have been selected. The number f recnstructed tracks is shwn in Fig.3 tgether with simulatin results btained with different Geant4 Physics Lists. The agreement is quite gd fr all cnsidered Physics Lists; this is the mst imprtant bservable fr the purpse f a Tracker simulatin, as these recnstructed tracks are the nly visible effect utside the pixel detectr in which they have been prduced. The rati f the maximum energy in a single pixel f the cluster assciated t the interactin vertex and the ttal energy f that cluster is shwn in Fig.4. In the case f the QGSP Physics Lists the agreement with data is excellent. This bservable is sensitive t the prductin f heavy nuclear fragments and their energies. The cluster size, i.e. the number f pixels in the cluster ass- Fig. 3. Number f recnstructed tracks. Fig. 4. Maximum energy in a single pixel f the cluster divided by the ttal cluster energy. Fig. 5. Number f pixels in the cluster (cluster size).

3 ciated t the interactin vertex is shwn in Fig.5. In this case, QGSP predicts narrwer clusters with respect t the real data. This bservable is sensitive t the verall range distributin f the nuclear fragments prduced in the hadrnic interactins. In the simulatin the prper beam cmpsitin has been taken int accunt: althugh the beam is nminally f π +, the actual beam cmpsitin is 67% p, 9% π + and 4% K +. Mmentum spread arund the nminal 80GeV/c is instead negligible. The beam gemetrical divergency is als taken int accunt in the simulatin. The cnclusins we can draw frm these cmparisns between data and simulatin, under the reasnable assumptin that the calibratin uncertainty des nt affect much the bservables we have cnsidered, are the fllwing. The thery-driven Physics Lists describe the data better than the parametrized ne. The best agreement with data is fr QGSP. Hwever, this Physics List has a prblem in describing the cluster shape. A pssible explanatin f this discrepancy, currently under investigatin, is related t the range f prtns in silicn: 0µm fr 3MeV kinetic energy; mm fr MeV ; cm fr 45MeV. S, if the QGSP prtn spectrum is nly slightly sfter than the real ne, then the cluster shape culd be significantly narrwer, given the size f a pixel, while the energy flw can still match well the data, withut nticeable discrepancies. Sme imprvements in the Binary Cascade mdel, under develpment, culd harden the prtn spectrum and prvide a better agreement with the data. III. NEUTRON PRODUCTION CROSS-SECTIONS Neutrn prductin frm prtn bmbardment, and in particular (p, xn) duble-differential crss-sectin measurements, d σ/dωde n, i.e. neutrn spectrum at fixed angles, are an imprtant benchmark fr the validatin f hadrnic mdels. Here we cnsider measurements made at Ls Alams Mesn Physics Facility (LAMPF), fr prtn beam energies f 3, 56, 597, and 800 MeV, and fr angles f , 60, 0 and 50 [6]. The setup is shwn schematically in Fig.6. The prtn beam is made f macrpulses at rates up t 60Hz, and each macrpulse is made f abut 70 micrpulses, each cntaining 3 8 prtns in a burst < 00ps lng; the spacing between micrpulses is µs, adequate t accmmdate the time f flight fr the slwest neutrns we are interested in (50keV ) ver the lngest path available (60m). The targets were thin (less than 3.5 MeV thick fr the incident prtn energies), giving interactins per incident prtn. Different target materials have been cnsidered: beryllium, carbn, aluminum, irn, tungsten, lead. Fr neutrn detectin plastic scintillatrs were used, and Time-Of-Flight technique was emplyed t measure the neutrn energy, where the start signal (time-zer) was prvided by an inductive beam pickff, and the stp signal was generated by the arrival f the neutrn at the plastic scintillatr. The distances f the neutrn detectrs frm the target range between 3 60 m. 50 Detectr 0 Beam (prtn) Target (Al, Fe, Pb) 60 < Fig. 6. Setup f the neutrn prductin (p, xn) measurements at Ls Alams Mesn Physics Facility (LAMPF). Lw-energy charged particles frm the target were swept ut f the flight paths by magnets, and the remaining charged particles were rejected by vet scintillatrs. The intense gamma flash generated by the beam burst was reduced using uranium filters. The remaining gamma backgrund is estimated using a cmbinatin f different detectr threshlds and massive irn shadw-bars. The latter allw als t crrect fr inscattering neutrns, whereas the backgrund f neutrns prduced in prtn-air cllisins is estimated frm the data withut target and then subtracted. Finally, csmic rays and time-independent surces are estimated frm the average f the TDC channels befre the prmpt phtns. As an example, in Fig.7 the experimentally measured neutrn prductin Fe(p,xn) duble-differential crss-sectin at an angle f 30 fr 56 MeV incident prtns is cmpared t preliminary results btained with Geant4 using different Physics Lists (LHEP, QGSP BERT, QGSP BIC), and t the results btained with Fluka [5] simulatin package. Figures 8, 9, and shw the ratis between the simulatin results and the experimental data fr the different Physics Lists and simulatin packages. Numerical values fr the experimental crss sectins were taken frm the EXFOR database [7]. The errr bars in the figures include bth the experimental statistical and systematic errrs, and a small statistical errr frm the simulatin. Except fr the high end f the neutrn spectrum, all errrs are dminated by the systematic uncertainty frm the neutrn detectr efficiency, which is given as 5 % t 0 % in [6], depending n the neutrn energy. Since the energy dependence f this systematic errr is nt available frm the EXFOR database and culd nt easily be determined therwise fr all cases, a cnservative energyindependent errr f 0 % was used fr the cntributin f the uncertainty in the neutrn detectr efficiency t the systematic errr. In the present wrk we have cmpared simulated neutrn prductin crss-sectins t the measurements [6]. Other, independent measurements are available, fr example [8], [9]. The level f agreement amng these independent measurements is

4 (b/sr/mev) Fe(p,xn) Prductin Crss Sectin at 56MeV, 30deg - PRELIMINARY Rati Simulatin / Data fr. Fe(p,xn) Prductin Crss Sectin Rati at 56MeV, 30deg - PRELIMINARY G4: QGSP_BIC -5-6 G4: QGSP_BERT G4: QGSP_BIC G4: LHEP FLUKA Ls Alams data Neutrn Energy (MeV) Neutrn Energy (MeV) Fig.. Rati QGSP BIC simulatin / data, fr the data shwn in Fig.7 Fig. 7. Simulatin and experimental neutrn prductin duble-differential crss-sectin with 56MeV prtn beam n Irn target at 30. Rati Simulatin / Data fr. Fe(p,xn) Prductin Crss Sectin Rati at 56MeV, 30deg - PRELIMINARY FLUKA Neutrn Energy (MeV) currently being studied. Frm the cmparisn f the simulatin results with the experimental data in [6] we arrive at the fllwing preliminary cnclusins. While the LHEP Physics List is clearly nt suitable fr the simulatin f (p,xn) duble-differential crss-sectins, Fluka and the QGSP BERT and QGSP BIC Physics Lists d all reprduce the duble-differential crss-sectin data measured by Ls Alams at the level f 0 % t 50 %. IV. CONCLUSION We have presented tw validatin tests fr Geant4 Hadrnic Physics. The parametrized Geant4 Physics List, LHEP, is nt able t reprduce sme f the bservables. In general the QGSP Physics List, r at least its variants with Internuclear Cascade mdels, QGSP BERT and QGSP BIC, seems t describe mst bservables reasnably well, even nn-trivial micrscpic nes, like energy-angular crrelatins. Fig. 8. Rati Fluka simulatin / data, fr the data shwn in Fig.7 ACKNOWLEDGMENT The authrs wuld like t thank A. Ferrari and P. Sala fr useful suggestins and help in the use f Fluka. Rati Simulatin / Data fr Fig. 9.. Fe(p,xn) Prductin Crss Sectin Rati at 56MeV, 30deg - PRELIMINARY G4: QGSP_BERT Neutrn Energy (MeV) Rati QGSP BERT simulatin / data, fr the data shwn in Fig.7 REFERENCES [] S. Agstinelli et al, Geant4 Cllabratin, Nuclear Instruments and Methds in Physics Research, A 506 (003) 50. See als the Geant4 web page: [] J. P. Wellisch, Physics f shwer simulatin at LHC, at the example f Geant4, Prceedings f Internatinal Eurphysics Cnference n High Energy Physics 003, Aachen. A. Heikkinen, N. Stepanv and J. P. Wellisch, Bertini intra-nuclear cascade implementatin in Geant4, Prceedings f Cmputing in High energy physics 003, De Jlla. G. Flger and J. P. Wellisch, String partn mdels in Geant4, Prceedings f Cmputing in High energy physics 003, De Jlla. J. P. Wellisch, Hadrnic Shwer Mdels In Geant4 - The Framewrks, Cmput. Phys. Cmmun. 40, 65 (00). V. Lara and J. P. Wellisch, Preequilibrium and equilibrium decays in Geant4, CHEP 000: Cmputing in high energy and nuclear physics, H.P. Wellisch, D. Axen, Phys. Rev. C 54, 39 (996). J. P. Wellisch, Hadrnic Shwer Simulatin With Geant4, Tucsn 997, Calrimetry in high energy physics, 5-59.

5 [3] J. P. Wellisch, V. Ivanchenk, G. Flger, T. Ki and D. H. Wright, The Geant4 hadrnic verificatin suite fr the cascade energy range, Prceedings f Cmputing in High energy physics 003, De Jlla. J. P. Wellisch, Geant4 hadrnic physics status and validatin fr large HEP detectrs, Prceedings f Cmputing in High energy physics 003, De Jlla. J. P. Wellisch, Geant4 Physics Validatin Fr Large HEP Detectrs, Nucl. Instrum. Meth. A 50 (003) 669. [4] T.Lari et al, Test beam results f ATLAS Pixel Sensrs, Prceedings f PIXEL 00, Internatinal Wrkshp n Semicnductr Pixel Detectrs fr Particles and X-Rays, Carmel, Califrnia, 9- Sep 00. e-print Archive: hep-ex/0045 [5] A. Fassò, A. Ferrari, J. Ranft, P. Sala, Prceedings f the MnteCarl 000 Cnference, Lisbn, Octber 000, Springer-Verlag (00), 59-64, See als the Fluka web page: [6] M.M. Meier et al., Nuclear Science and Engineering: (989) 3, M.M. Meier et al., Nuclear Science and Engineering: (99) 89, W.B. Amian et al., Nuclear Science and Engineering: (99) 78, W.B. Amian et al., Nuclear Science and Engineering: 5 (993). [7] EXFOR Experimental Nuclear Reactin Data Retrievals, [8] S. Stamer et al, Physical Review C, 47 (993) 647. [9] X. Ledux, et al, Physical Review Letters, 8 (999) 44.