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The Generator Phase in Gauss

The Generator Phase in Gauss. P. Robbe , LAL Orsay /CERN. 01 1 0 100 11 1 011 01 01 00 01 01 010 10 11 01 00 B 00 l e. Gauss Tutorial CERN, 2 nd June 2010. JobOpts. LHCb Event model. Interface. HepMC MCParticle MCVertex MCHits. Pythia, EvtGen. …. POOL. Event Generation.

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The Generator Phase in Gauss

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  1. The Generator Phase in Gauss P. Robbe, LAL Orsay/CERN 0110100111011010100010101010110100 B00le Gauss Tutorial CERN, 2nd June 2010

  2. Gauss Tutorial JobOpts LHCb Event model Interface HepMC MCParticle MCVertex MCHits Pythia, EvtGen … POOL EventGeneration primary event generator specialized decay package pile-up generation Introduction • Gauss (LHCb simulation software) iscomposed of 2 steps: Generation and Simulation. Monitor JobOpts Init Exchange model GiGa Geant4 Initialize HepMC Cnv Cnv Cnv POOL Geometry Detector Simulation geometry of the detector (LHCb  Geant4)‏ tracking through materials (Geant4)‏ hit creation and MC truth information (Geant4  LHCb)‏

  3. Gauss Tutorial Generation part of Gauss • Takes care of: 1.Beam Parameters 2.Interaction Region Profiles 3.Number of Pile-Up interactions 4.Production of particles (hard-process)‏ 5.Time-evolution of particles (decay, oscillations, CP violation, ...)‏

  4. Gauss Tutorial External Libraries • The most important actions are performed by externallibraries, developpedoutsideLHCb: PYTHIA, EvtGen, ... • Gauss isorganizing the sequence of actions needed to generateevents, callingtheseexternallibrariesat the right moment, though interfaces. • The interfaces to the externalgenerators are generic: generatorscanbeexchangedeasilyonly via configurables, for example to use SHERPAinstead of PYTHIA.

  5. External Generators • In the LHCb simulation software (Gauss), external generator libraries are used for 2 different steps: • Production: to generate the p-p interaction and hadronization up to hadrons (by default Pythia) • Decay: to decay hadrons produced in the first step up to stable particles (by default EvtGen) • The generated events are then given to Geant4 for the simulation of the LHCb detector response. • 3 main types of events are generated: • Minimum Bias • Inclusive B • Signal B

  6. Gauss Tutorial Minimum Bias (MB) Generation • Most simple generation case isgeneration of minimum-bias (all whatisproduced by pp collisions) events. • Sequencelogicis: • 1. Generatepbeammomentum • 2. DeterminenumberN of pile-up interactions • 3. Determinespace positions of the interactions (PV)‏ • 4. GenerateN pp collisions (encapsulated in production tool) • 5. Decay all producedparticles (encapsulated in decaytool) Interface Generators

  7. Minimum Bias • As an illustration, with Pythia it correspond to the processes: • 11:fifjfifj • 12:fififkfk • 13:fifig g • 28:figfig • 53:g gfkfk • 68: g gg g • 91: elastic scatering • 92: single diffraction (A B  X B) • 93: single diffraction (A B  A X) • 94: double diffraction • 95: low-pT production • 421-439:charmonium production • 461-479:bottomonium production

  8. Gauss Tutorial Production Tool • It isused to generate the pp collisions (hard process, hadronization, ...)‏

  9. Gauss Tutorial Pair Creation Flavour Excitation Gluon Splitting Q Q Q Q Q Q Production Tool (Pythia 6)‏ • Default options constitute « LHCb tuning », whichisdone to extrapolateathigherenergieschargedtrackmultiplicitiesseenat the UA5 experiment. • The activatedphysicsprocesses are the dominant ones for LHC energies. TheydefineLHCb « minimum bias » out of which all major samples are generated. Elastic Single diffractive Charmonium production ...

  10. Pythia6 Tuning • In LHCb, tune the chargedparticlemultiplicity, linked to the event structure atlowpT. • This isgoverned by the « multiple interaction model », ieeachhadronic collision is the sum of a varyingnumber of individual parton-parton interactions. • The number of parton-parton interactions per event (then the particlemultiplicity) isadjusted by the parameter: • pTmin, cut-off belowwhich the parton-parton cross-sections are set to 0 CDF pTmin UA5 CDF UA5 UA5 UA5 CTEQ6L ~20 chargedparticles in the LHCb acceptance per interaction.

  11. Gauss Tutorial Otheravailable production tools • Pythia8: C++ Pythia version (Pythia8)‏ • http://home.thep.lu.se/~torbjorn/talks/tutorial81.pdf • Herwig++: • http://hepwww.rl.ac.uk/theory/seymour/herwig/ • Sherpa: • http://projects.hepforge.org/sherpa/dokuwiki/doku.php

  12. Gauss Tutorial Decay Tool (EvtGen)‏ • It isused to decay all particles, and to generate • correct time dependance (CP violation, mixing), • correct angularcorrelations in sequentialdecays (decay of spin 0, 1, 2, … particles) and theirinterferences • Intermediateresonances (withinterferences) • With a detailed description of the B and D decays (Kaon multiplicities, important for the B tagging, etc…) • Availableimplementations: • EvtGen: (default) interface to EvtGen • SeeMichal’spresentation • Documentation:http://lhcb-release-area.web.cern.ch/LHCb-release-area/DOC/gauss/generator/evtgen.php

  13. Gauss Tutorial OtherDecayTool • Sherpa: • http://projects.hepforge.org/sherpa/dokuwiki/doku.php

  14. Gauss Tutorial Inclusive GenerationSequence • Important samples are inclusive samples: bb or cc inclusive samples. • They are obtainedfrom minimum biasgeneration, but requiringthateacheventcontainsat least one particle of a given type (B hadron, D hadron, ...)‏ • To obtain more interestingsamples, a cutisalsoperformedatgeneratorlevel to keeponlyusefulevents: implemented in a « cuttool ».

  15. Gauss Tutorial Cuttool (1)‏ • Accept or reject an eventbased on generatorlevelquantities. • Availableimplementations: • « LHCbAcceptance »: cut on signal direction: 0≤qsignal≤400 mrad. • « DaughtersInLHCb »: cut on direction of decayproducts of signal particle: • 10 mrad≤ qcharged≤ 400 mrad, 5 mrad ≤ qneutral≤ 400 mrad • No cut on L and Ks0daughters, and on neutrinos. • Onlycut on g if they come fromp0 or h.

  16. Gauss Tutorial Cut tool (2)‏ • Variations of DaughtersInLHCb: • DaughtersInLHCbAndFromB: signal particleiscomingfrom a b-hadron decay, • ListOfDaughtersInLHCb: onlyparticles of given types are required to be in the acceptance of LHCb, • SelectedDaughtersInLHCb: onlyparticlescomingfrom the decay of givenparticles are required to be in the acceptance of LHCb,

  17. Excited B states tuning inPythia • Bflavourtagging in hadron collidersisbased on the properties of the otherBdecay in the event, but also on the fragmentation characteristics of the signal B. Measuredat LEP + Spin counting

  18. Gauss Tutorial Signal Generation • To generate signal sample, an extra stepisadded to the generation of inclusive: the presence of a givenparticle (B0, B+, ...) isrequired, and itsdecayisforced to a signal decay mode. • To speed up generationprocess for signal B,SignalRepeatedHadronizationmethodexist: when a b quark isfound, the eventisre-hadronizeduntil the B of interestisfound. (For example, bhadronizes to Bs0with 10% probability) For non B, use SignalPlainmethod.

  19. Gauss Tutorial Signal Generation • The decay of the signal is forced to a given decay mode (if 2 are present in the same event, only one is forced). • To do this, EvtGen aliases are defined. They are copies of particles (have the same properties) but their decay mode can be redefined (in a EvtGen user decay file) without affecting the decay mode of the « normal » particle. • Aliases names are <Name of the particle>sig: • B0sig, anti-B0sig • B+sig, B-sig • D_s0sig, anti-D_s0sig • ...

  20. Gauss Tutorial Requirements for externalgenerators • They must be able to use an externalrandomnumbergenerator: • To be able to use the « Gaudi » randomnumbergenerator, used by the entire Gauss software, to ensureevent-by-eventreproducibility

  21. Gauss Tutorial Requirements for externalgenerators • The must be able to modifytheirinternalparticleproperties to use the LHCb particlepropertydefinitions. • Theseproperties are stored in a database (seebelow) • They are common to the entire LHCb software (reconstruction, analysis, …) PDGId Mass Lifetime EvtGen name LHCb name

  22. Gauss Tutorial HepMC • Format thatwe use to store generatedevents. • In the .sim file, events are stored in HepMC format: • Documentation:http://lcgapp.cern.ch/project/simu/HepMC/HepMC203/html/ • LHCb Specific: • Particles have status code (HepMC::GenParticle::status()) which have specialmeanings: • 1 = stable in Pythia (pfromPrimary Vertex, ...)‏ • 2 = decayed/fragmentated by Pythia (quark, ...)‏ • 3 = Pythia documentation particle (string, ...)‏ • 777, 888 = decayed by EvtGen (all unstableparticles)‏ • 889 = signal particle • 999 = stable in EvtGen (pfrom B decays, ...)‏ • Units are LHCbunits (MeV, mm, and ns). • Wewouldlike to store in HepMC a « universal » process id, common to all generators: under investigation.

  23. Gauss Tutorial Conclusions • The Gauss simulation software allows to use externalgenerators to generate the events of interest for the LHCb physics program. • Flexible interface, thatallows to combine differentgenerators for differenttasks. • C++ generators are particularlywellsuited for ourframework ! • Simple and oldtuningbasedonly on: • Chargedparticlemultiplicities • Excited state fractions • Frompastexperiments (UA5, LEP) in differentenvironments • Weshouldbe able to do betterwith the LHC(b) data !

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