The Generator Phase in Gauss

The Generator Phase in Gauss

P. Robbe, LAL Orsay/CERN

Gauss TutorialCERN, 2nd June 2010

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Gauss Tutorial

IntroductionGauss (LHCb simulation software) is composed of 2 steps: Generation and Simulation.

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Init

Event Generationprimary event generatorspecialized decay packagepile-up generation

Detector Simulationgeometry of the detector (LHCb Geant4)tracking through materials (Geant4)hit creation and MC truth information (Geant4 LHCb)

Geant4

GiGaC

nvCnv

Cnv

JobOptsExchange model HepMC

MCParticleMCVertex

MCHits

POOL

LHCb Event model

Geometry

HepMC

POOL

Pythia,EvtGen

JobOpts

Interface

…

Monitor

Initialize

Gauss Tutorial

Takes care of:

Generation part of Gauss

1. Beam Parameters

2. Interaction Region Profiles 3. Number of Pile-Up interactions4. Production of particles (hard-process)

5. Time-evolution of particles (decay, oscillations, CP violation, ...)

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Gauss Tutorial

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.

External Libraries

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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

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Most simple generation case is generation of minimum-bias (all what is produced by pp collisions) events. Sequence logic is:

1. Generate p beam momentum2. Determine number N of pile-up interactions3. 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)

Minimum Bias (MB) Generation

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ator

s

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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– 421-439: charmonium production– 461-479: bottomonium production

Gauss Tutorial

Production Tool

It is used to generate the pp collisions (hard process, hadronization, ...)

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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

Q

QPair Creation Flavour Excitation Gluon Splitting

QQQ

Elastic Single diffractive Charmonium production ...

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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

10

PYTHIA6 TUNING

p Tm

in

UA

5 UA

5 UA

5C

DF

CD

F

UA

5

CTEQ6L

~20 charged particles in the LHCbacceptance per interaction.

Gauss Tutorial

Other available production toolsPythia8: 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

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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 interferencesIntermediate 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:

http://lhcb-release-area.web.cern.ch/LHCb-release-area/DOC/gauss/generator/evtgen.php

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Gauss Tutorial

Other Decay Tool

Sherpa: http://projects.hepforge.org/sherpa/dokuwiki/doku.php

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Gauss Tutorial

Inclusive Generation SequenceImportant 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: 0≤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 Ks

0 daughters, and on neutrinos. Only cut on g if they come from p0 or h.

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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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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.

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Excited B states tuning in PYTHIA

Measured at LEP +Spin counting

Gauss Tutorial

Signal GenerationTo 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 Bs

0 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-B0sigB+sig, B-sigD_s0sig, anti-D_s0sig...

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Gauss Tutorial

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

ddd20

Requirements for external generators

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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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Requirements for external generators

Gauss Tutorial

HepMCFormat that we use to store generated events.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 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 multiplicitiesExcited state fractionsFrom past experiments (UA5, LEP) in different environmentsWe should be able to do better with the LHC(b) data !

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