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The Generator Phase in Gauss
P. Robbe, LAL Orsay/CERN Gauss Tutorial CERN, 2nd June 2010 B00le
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Gauss Tutorial Introduction Gauss (LHCb simulation software) is composed of 2 steps: Generation and Simulation. Monitor HepMC MCParticle MCVertex MCHits POOL LHCb Event model Pythia, EvtGen JobOpts Interface … JobOpts Init Exchange model GiGa Geant4 Initialize HepMC Cnv Cnv Cnv POOL Geometry Event Generation primary event generator specialized decay package pile-up generation Detector Simulation geometry of the detector (LHCb Geant4) tracking through materials (Geant4) hit creation and MC truth information (Geant4 LHCb)
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Generation part of Gauss
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, ...)
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Gauss Tutorial External Libraries The most important actions are performed by external libraries, developped outside LHCb: PYTHIA, EvtGen, ... Gauss is organizing the sequence of actions needed to generate events, calling these external libraries at the right moment, though interfaces. The interfaces to the external generators are generic: generators can be exchanged easily only via configurables, for example to use SHERPA instead of PYTHIA.
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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
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Minimum Bias (MB) Generation
Gauss Tutorial Minimum Bias (MB) Generation Most simple generation case is generation of minimum-bias (all what is produced by pp collisions) events. Sequence logic is: 1. Generate p beam momentum 2. Determine number N of pile-up interactions 3. Determine space positions of the interactions (PV) 4. Generate N pp collisions (encapsulated in production tool) 5. Decay all produced particles (encapsulated in decay tool) Interface Generators
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Minimum Bias As an illustration, with Pythia it correspond to the processes: 11: fi fj fi fj 12: fi fi fk fk 13: fi fi g g 28: fi g fi g 53: g g fk fk 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 : charmonium production : bottomonium production
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Gauss Tutorial Production Tool It is used to generate the pp collisions (hard process, hadronization, ...)
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Production Tool (Pythia 6)
Gauss Tutorial Production Tool (Pythia 6) Default options constitute « LHCb tuning », which is done to extrapolate at higher energies charged track multiplicities seen at the UA5 experiment. The activated physics processes are the dominant ones for LHC energies. They define LHCb « minimum bias » out of which all major samples are generated. Q Pair Creation Flavour Excitation Gluon Splitting Elastic Single diffractive Charmonium production ...
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Pythia6 Tuning In LHCb, tune the charged particle multiplicity, linked to the event structure at low pT. This is governed by the « multiple interaction model », ie each hadronic collision is the sum of a varying number of individual parton-parton interactions. The number of parton-parton interactions per event (then the particle multiplicity) is adjusted by the parameter: pTmin , cut-off below which the parton-parton cross-sections are set to 0 CDF pTmin UA5 CDF UA5 UA5 UA5 CTEQ6L ~20 charged particles in the LHCb acceptance per interaction.
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Other available production tools
Gauss Tutorial Other available production tools Pythia8: C++ Pythia version (Pythia8) Herwig++: Sherpa:
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Gauss Tutorial Decay Tool (EvtGen) It is used to decay all particles, and to generate correct time dependance (CP violation, mixing), correct angular correlations in sequential decays (decay of spin 0, 1, 2, … particles) and their interferences Intermediate resonances (with interferences) With a detailed description of the B and D decays (Kaon multiplicities, important for the B tagging, etc…) Available implementations: EvtGen: (default) interface to EvtGen See Michal’s presentation Documentation:
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Other Decay Tool Sherpa:
Gauss Tutorial Other Decay Tool Sherpa:
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Inclusive Generation Sequence
Gauss Tutorial Inclusive Generation Sequence Important samples are inclusive samples: bb or cc inclusive samples. They are obtained from minimum bias generation, but requiring that each event contains at least one particle of a given type (B hadron, D hadron, ...) To obtain more interesting samples, a cut is also performed at generator level to keep only useful events: implemented in a « cut tool ».
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Gauss Tutorial Cut tool (1) Accept or reject an event based on generator level quantities. Available implementations: « LHCbAcceptance »: cut on signal direction: ≤qsignal≤400 mrad. « DaughtersInLHCb »: cut on direction of decay products of signal particle: 10 mrad ≤ qcharged ≤ 400 mrad, 5 mrad ≤ qneutral ≤ 400 mrad No cut on L and Ks0 daughters, and on neutrinos. Only cut on g if they come from p0 or h.
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Cut tool (2) Variations of DaughtersInLHCb:
Gauss Tutorial Cut tool (2) Variations of DaughtersInLHCb: DaughtersInLHCbAndFromB: signal particle is coming from a b-hadron decay, ListOfDaughtersInLHCb: only particles of given types are required to be in the acceptance of LHCb, SelectedDaughtersInLHCb: only particles coming from the decay of given particles are required to be in the acceptance of LHCb,
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Excited B states tuning in Pythia
B flavour tagging in hadron colliders is based on the properties of the other B decay in the event, but also on the fragmentation characteristics of the signal B. Measured at LEP + Spin counting
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Gauss Tutorial Signal Generation To generate signal sample, an extra step is added to the generation of inclusive: the presence of a given particle (B0, B+, ...) is required, and its decay is forced to a signal decay mode. To speed up generation process for signal B, SignalRepeatedHadronization method exist: when a b quark is found, the event is re- hadronized until the B of interest is found. (For example, b hadronizes to Bs0 with 10% probability) For non B, use SignalPlain method.
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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 ...
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Requirements for external generators
Gauss Tutorial Requirements for external generators They must be able to use an external random number generator: To be able to use the « Gaudi » random number generator, used by the entire Gauss software, to ensure event-by-event reproducibility
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Requirements for external generators
Gauss Tutorial Requirements for external generators The must be able to modify their internal particle properties to use the LHCb particle property definitions. These properties are stored in a database (see below) They are common to the entire LHCb software (reconstruction, analysis, …) LHCb name PDGId Mass Lifetime EvtGen name
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HepMC Format that we use to store generated events.
Gauss Tutorial HepMC Format that we use to store generated events. In the .sim file, events are stored in HepMC format: Documentation: LHCb Specific: Particles have status code (HepMC::GenParticle::status()) which have special meanings: 1 = stable in Pythia (p from Primary Vertex, ...) 2 = decayed/fragmentated by Pythia (quark, ...) 3 = Pythia documentation particle (string, ...) 777, 888 = decayed by EvtGen (all unstable particles) 889 = signal particle 999 = stable in EvtGen (p from B decays, ...) Units are LHCb units (MeV, mm, and ns). We would like to store in HepMC a « universal » process id, common to all generators: under investigation.
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Gauss Tutorial Conclusions The Gauss simulation software allows to use external generators to generate the events of interest for the LHCb physics program. Flexible interface, that allows to combine different generators for different tasks. C++ generators are particularly well suited for our framework ! Simple and old tuning based only on: Charged particle multiplicities Excited state fractions From past experiments (UA5, LEP) in different environments We should be able to do better with the LHC(b) data !
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