To improve the understanding of accelerator-induced backgrounds at the SLAC B-Factory, we simulate lost particle backgrounds in the BABAR detector originating from beam-gas interactions and radiative Bhabha scatters. We have extended the GEANT4-based BABAR detector simulation to include beam-line components and magnetic fields up to 8.5 m away from the interaction point. We describe the simulation model and then compare preliminary predicted background levels with measurements from dedicated single- and colliding-beam experiments

/DAPNIA, Saclay

/INFN, Pisa

/SLAC

/Stanford U., Phys. Dept.

/UC, Santa Cruz

Aston, D.

Barlow, N.

Blount, N.L.

Bondioli, M.

Bower, G.

Calderini, G.

Campbell, B.

Cristinziani, M.

Edgar, C.L.

Fieguth, T.

Kozanecki, W.

Lockman, W.S.

Petersen, B.A.

Robertson, S.H.

Strom, D.

U., /Manchester

U., /McGill

U., /Oregon

Wright, D.H.

UNT Digital Library

SLAC-PUB-11241SCIPP05/04May, 2005GEANT4-BASED SIMULATION STUDY OF PEP-II BEAMBACKGROUNDS IN THE BABAR DETECTOR AT THE SLACB-FACTORY W. S.Lockman,SCIPP, Universityof California,SantaCruz,California95064,USAW. Kozanecki,CEA/DSM/DAPNIA, 91991Gif-sur-Yvette,FranceB. Campbell,S.H. Robertson,McGill University, MontrealH3A 2T8,Quebec,CanadaM. Bondioli, G. Calderini,INFN-Pisa,I-56127Pisa,ItalyN. Barlow, C. L. Edgar, ManchesterUniversity, ManchesterM13 9PL,UnitedKingdomD. Aston,G. Bower, M. Cristinziani,T. Fieguth,D. H. Wright, SLAC, MenloPark,CA 94025,USAB. A. Petersen,StanfordUniversity, Stanford,CA 94305,USAN. L. Blount,D. Strom,Universityof Oregon,Eugene,OR 97403,USAAbstractTo improve the understandingof accelerator-inducedbackgroundsat theSLAC B-Factory, we simulatelost par-ticle backgroundsin theBABAR detectororiginatingfrombeam-gasinteractionsand radiative Bhabhascatters.Wehave extendedtheGEANT4-basedBABAR detectorsimu-lationto includebeam-linecomponentsandmagneticfieldsup to 8.5 m away from the interactionpoint. We describethe simulationmodel and then comparepreliminary pre-dicted backgroundlevels with measurementsfrom dedi-catedsingle-andcolliding-beamexperiments.INTRODUCTIONTwo potential sourcesof backgroundsin the BABARdetector[1] include lost particlesfrom beam-gasinterac-tionsandBhabhascatters.Thefirst is single-beamrelatedandscaleswith thebeamcurrentandtheresidualpressure,while thesecondis presentduringcolliding beamrunningonly andscaleswith theluminosity. Two horizontalbeam-separation(B1) dipoles(Fig. 1) sweepoff-energy particlesinto thevacuumchamberwall. Secondariesfrom thesein-teractionsarea potentialsourceof machinerelatedback-groundin the BABAR detector. FurtherimprovementsinPEP-II luminosity will requireraising the beamcurrentsandlowering the  functionat the interactionpoint (IP),possiblyresultingin increasedbackgroundlevels. To un-derstandthe detectorbackgroundsboth in the currentandupgradedmachineconfigurations,the detectorsimulationhasbeenextendedto includethe machinevacuumcham-ber, lattice elementsand their associatedmagneticfieldsup to andincludingthefinal doubletquadrupoles,	  mfrom theIP. We arein thefirst stagesof comparingMonteCarlo predicteddetectoroccupancieswith thoseobtainedfrom single-andcolliding beammeasurements.Work supportedin partby DOEgrantDE-FG02-04ER41286.Now at Universityof California,Irvine, California92697,USADETECTOR MONTE CARLODescriptionChargedparticletracksin theBABAR detectoraremea-suredin a 5-layersilicon vertex tracker (SVT) surroundedby a 40-layer drift chamber(DCH). Both detectorsareembeddedin a 1.5T solenoidalmagneticfield. A ring-imagingCherenkov detector(DIRC) surroundsthe DCH.Electromagneticshowers from photonsand electronsaredetectedin an arrayof CsI crystals(EMC) locatedinsidethesolenoidalcoil of asuperconductingmagnet.The detectorMonte Carlo programconsistsof pack-agesfor event-generation,particle transport,detectorre-sponse,andevent reconstruction.The Monte Carlo eventgeneratorsof relevance for this study include the sin-gle beamLPTURTLE [2] generatorwhich modelsbeam-gas bremsstrahlungand Coulomb scatters,the forwardbeam-beambremsstrahlunggenerator, BBBREM [3] andthe small-angle radiative Bhabha generator, BHLUMI[4]. The particle transportpackage(basedon GEANT4(G4) [5]) is reponsiblefor tracking charged and neutralparticles through the detectorand the low energy ring(LER) andhigh energy ring (HER) beamlines. This pack-age recordsspatial, temporaland kinematic information(“GHits”) in the various subdetectors.The detectorre-sponsepackagedigitizes the GHit information into rawdata(“Digis”) andsimulatesthe level-1 trigger. Thesamereconstructionpackageis usedto processboth real andsimulatedraw data.The physicsprocessesin the G4-basedparticle track-ing packagefall into four categories: decay, electromag-netic, hadronicandelectro-nuclear. The decayprocessisusedfor all unstable,long-lived particles. The followingelectromagneticprocessesare used: Comptonscattering, conversion and photo-electriceffect, multiple scatter-ing,  ionizationandbremsstrahlung, annihilation,ionizationandbremsstrahlung,  -  pair productionbyContributed to Particle Accelerator Conference (PAC05), Knoxville, TN, May 16-20, 20053020100–10–20–30–7.5 –5 –2.5 0 2.5 5 7.5MetersCentimetersPEP-II Interaction Region3.1 GeV3.1 GeV9 GeV9 GeVQF5QD4QF5QD4QF2QF2SK1SK1QD1QD1QD1 QD1B1B1B1 B1Apiary 8.8K1M.SullivanDec. 16, 1996AA AAB BB BDetectorFigure1: Layoutof thePEP-IIinteractionregion,topview.Thebeamseparationdipoles(B1) areshown togetherwiththe quadrupolesQD1, QF2,QD4 andQF5. The 9.0 GeV beamentersfrom the left, the 3.0 GeV   from theupperright.muons,andhadronionization. Thehadronicprocessesin-clude elasticand inelasticscattering,captureand fission.The electro-nuclearprocessesinclude   - and  -nuclearreactions.A specialmodelis usedto treatlow energy neu-tronsfrom thermalenergiesup to 20 MeV; theBertini cas-cademodel[6] is usedfor neutronsabove this energy andalsofor protonsandpions.TheLow Energy Parameterizedmodel[6] is usedfor all otherhadrons.To realisticallymodelbeam-relatedbackgroundsin theBABAR detector, a detailedmodelof thevacuumchambergeometryandmagnetsis required.Unfortunately, theinnersurfacesof thevacuumchamber, especiallyin theregionoftheQF2septum,arenoteasilydescribedby any of thesim-ple constructive solid geometry(CSG)classesavailableintheG4 toolkit. Instead,weapproximatethevacuumcham-berusinga seriesof CSGtrapezoidswhich areassembledto form polytubesandpolyhedra.Theendpointcoordinatesof thesecompositesolids are taken from Solid Edge[7]modelsof thevacuumchamber. Thevacuumchamberandmagnetsasconstructedby theG4 simulationareshown inFig. 2.ValidationThe geometryof the vacuumchamberis first validatedusing virtual reality visualizationtools to compareeachcomponentof the vacuumchamberto its correspondingSolidEdgemodel.Secondly, aperturelocationsandshapesderivedfrom transversesliceviewsat key transitionpointsalong the vacuumchamberare comparedwith the CADdrawingsandwith the aperturesin the LPTURTLE simu-lation. Finally, from a sampleof beam-gasscattered  ’sproducedby LPTURTLE, we correlate(Fig. 3) the loca-tions in  (the horizontalcoordinatein Fig. 1) wherepar-Figure2: G4 descriptionof PEP-II interactiongeometry.The B1 andQD1 magnetsappearin red, QF2, QD4 andQF5magnetsin light blue.Partof theLER vacuumcham-ber(gray)is visibleadjacento QD4andQF5.ticles strike the vacuumchamberaspredictedby G4 andLPTURTLE, respectively. This teststhatboth thevacuumchambergeometryandthebeamline opticsareconsistentbetweenthetwo simulations.Thelargepeaknear cm is dueto a horizontalaperturelimitation in theforwardLER Q2 septumchamber. Most of the entriesin this plotlie alongthediagonal, !"#$&%('*)+%	#, . (cm)LPTURTLEz-5000- 500 (cm)G4z-5000500Figure3:  positionsof LER vacuumchamberhits aspre-dictedby theG4andLPTURTLE simulations,from anen-sembleof bremsstrahlung-andCoulomb-scattered  .NormalizationIn data,an event is definedby all the hits which residein eachsubsystem’s level-1 (L1) latency buffer whena L1triggersignalis received. The time window of this buffer,., is subsystem-dependentandis typically of order1  s.In theMonteCarlo,anevent is definedasthebackgroundhits relatedto a singleBhabha-or beam-gas catter. Thus,the occupancy predictedby the simulationmustbe multi-pliedby thefactor /102"354614 . , where6 is thepredictedBhabhascatteringcrosssection,3 theinstantaneouslumi-nosityand / 0 theexpectednumberof Bhabhascattersinthetimewindow..To normalizesingle beamsimulationsto the data,wemultiply the Monte Carlo occupancy by the factor /1789 4 . 4:/;#$&%	'<)+%	#,>=@?A#$&%	'<)+%	#, , where9 is thesinglebeamcurrent, /B#$&%('<)%(#, the numberof LPTURTLE events,and ?A#$&%	'<)+%	#, the estimatedmachinetime to produce/;#$&%	'<)+%	#, eventsat a beamcurrent of 1A and a NC -equivalentpressureof 1 nTorr.COMPARISON WITH DATAThebeam-gasandluminositybackgroundspredictedbythesimulationdescribedabove,havebeenconfrontedwithsystematicmeasurementsof acceleratorbackgrounds.Indedicatedmachineexperiments,we determinedthebeam-current-and luminosity-dependenceof the occupancy ineachBABAR subdetector, and parameterizedit in termsof beam-gas-,luminosity- andbeam-beam-inducedback-grounds. The beam-gasterms are measuredin single-beammode,filling either the LER or the HER. The lu-minosity term, predictedto be dominatedby radiative-Bhabhadebris, is measuredin collision, and is by def-inition proportionalto the measuredinstantaneouslumi-nosity. The remainingbeam-beamterm is the differencebetweenthe total background-inducedoccupancy, andthesum of the beam-gasand luminosity terms. Touschekandsynchrotron-radiation-inducedbackgroundshavebeenshown to benegligible.Thesubdetectormostsensitiveto beam-gasbackgroundsis the SVT. It consistsof five layersof silicon strip sen-sors, arrangedin overlappinghexagonalpatternsaroundthebeampipe. Eachlayeris double-sided,with azimuthal( D ) strips on one face,and axial (  ) strips on the other.A comparisonof the measuredand predictedoccupan-cies in the SVT D chips is presentedin Fig. 4 for single-beambackgroundsfrom the LER. The angle D is the az-imuthal angledefinedin the coordinatesystemwheretheE -axis is in the PEP-II storageplane pointing outwardsand the F -axis points vertically upward. The peaksnearDGH and DGJIK*L aredominatedby degraded-energypositronsswept into the chamberwall by the B1 mag-nets (Fig. 1). The agreementbetweendata and MonteCarlo, both in shapeandoverall normalizationis remark-ably good,consideringthe simulationassumesa uniformpressurethroughoutthe LER. The other subsystemsarein the processof performingsimilar single-beamcompar-isons.To simulatetheluminositybackgrounds,wehavegener-atedsamplesof forward beambremsstrahlungandsmall-angleradiativeBhabhascattering.However, theresultsaretoo preliminary to be presentedat this time. In the SVT,DCH and DIRC, the predictedMonte Carlo occupanciesfrom the Bhabhascatteringsamplesappearto be at leastoneorderof magnitudelowerthanwhatis observedin data.In theEMC, however, theMonteCarlooccupanciesappearto bein betteragreementwith data,bothin magnitudeandshape.The next stepin this studywill be to model the singlebeamHER backgrounds.Doing so will provide a betterunderstandingof thesystematicuncertaintiesin theMonteCarlosimulation. angleφM0 50 100 150 200 250 300 350Occupancy (%)00.10.20.3 angleφM0 50 100 150 200 250 300 350Occupancy (%)00.050.10.15Figure4: Single-beamSVT D chip occupanciesmeasuredwith a1A  beam(solidcircles)andpredictedby G4(his-tograms)for layer1 (top) andlayer2 (bottom).Thesimu-lationassumesa1 nTorr pressurearoundthering.REFERENCES[1] B. Aubertet al., Nucl. Inst.Meth.A 479, 1 (2002).[2] T. Fieguth et al.,”Modeling Lost-Particle BackgroundsinPEP-IIUsingLPTURTLE”, thisconference,1723-TPPP025.[3] R.KleissandH. Burkhardt,Comput.Phys.Commun.81, 372(1994).[4] S.Jadachet al., Comput.Phys.Commun.102, 229(1997).[5] S.Agostinelliet al. [GEANT4 Collaboration],Nucl. Instrum.Meth.A 506, 250(2003).[6] SeetheGeant4PhysicsReferenceManualathttp://wwwasd.web.cern.ch/wwwasd/geant4/geant4.html(2005).[7] SolidEdgeComputer-AidedDesignsoftware,http://www.solidedge.com(2005).

Geant4-based Simulation Study of PEP-II Beam Backgrounds in the BABAR Detector at the SLAC B-Factory

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Geant4-based Simulation Study of PEP-II Beam Backgrounds in the BABAR Detector at the SLAC B-Factory

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