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Use of EvtGen at LHCb. Patrick Robbe, LAL Orsay, 13 May 2004. Outline. Modifications to the B A B AR EvtGen version Interface to Gaudi and Gauss LHCb simulation software EvtGen version used in LHCb: alpha-00-11-07. Adaptations to LHC environment.

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Use of evtgen at lhcb

Use of EvtGen at LHCb

Patrick Robbe, LAL Orsay, 13 May 2004


  • Modifications to the BABAREvtGen version

  • Interface to Gaudi and Gauss LHCb simulation software

  • EvtGen version used in LHCb: alpha-00-11-07

Adaptations to lhc environment
Adaptations to LHC environment

  • Because of incoherent B meson production:

    • Modification of mixing description

    • Modification of CP violation implementations

  • Technical adaptations:

    • Fix some compilation problems,

    • Compilation with Windows C++ compiler,

    • Use of Pythia 2.205 and Photos 2.02 versions.

B 0 b s 0 mixing
B0/Bs0 Mixing

  • In original EvtGen B0 mixing is performed through Y(4S) decay.

  • Following idea by EvtGen authors, implement B0 and Bs0mixing as a decay B0B0, second B0 has 0 lifetime.

  • Then treat last B0 has any other particle.

  • Changes needed in one EvtGen class, EvtParticle.

Cp violation 1
CP Violation (1)

  • For incoherent B production, time integrated CP asymmetries may not be equal to 0.

  • Allow EvtGen to generate randomly the flavour of the B0/B0s when it decays to a CP decay mode. (and then to change from time to time B0 into B0 = “flip” trick)

  • Since the amplitude depends on the B flavour, the correct asymmetry will be generated (normalized at t=0).

Cp violation 2
CP Violation (2)

  • In original EvtGen, the calculations related to CP violation are located in the EvtCPUtil class (Determination of the tag and of Dt)

  • Substitute with a class specific to LHC configuration (EvtIncoherentMixing) which implements the “flip” trick.

  • Change all CP decay models to call EvtIncoherentMixing instead of EvtCPUtil: only one line change.

Lhcb evtgen version
LHCb EvtGen Version

  • All other EvtGen classes (decay models, …) are most often left unchanged, except for:

    • technical changes (Windows compilation),

    • adaptation of decay models to Bs0 decays,

    • updates with new measurement (strong phases in Dalitz plots, …)

    • addition of new decay models that did not exist in EvtGen.

Interface to gauss gaudi 1
Interface to Gauss/Gaudi (1)

  • EvtGen engine and decay models are contained in a GaudiTool (EvtGenTool) which interfaces EvtGen to Gaudi/Gauss.

  • This tool takes as input:

    • The main DECAY.DEC decay table,

    • The user decay table defining the signal decay under study.

  • The particle properties are transferred from the GaudiParticle Property Service to EvtGen (and also Pythia)

Interface to gauss gaudi 2
Interface to Gauss/Gaudi (2)

  • Generation sequence:

    • pp interactions are generated by Pythia.

    • Pythia decays all particles until it finds a particle declared as stable in Pythia.

      • All particles known by EvtGen, ie which have one or more decay modes defined in the generic decay table are declared as stable in Pythia.

    • Pythia Common blocks are converted to HepMC format

    • EvtGenTool reads the HepMC events, calls EvtGen for undecayed particles and updates HepMC events with EvtGen results.

    • HepMC events are given to Geant4.

Signal samples production
Signal samples production

  • For signal production, remove the decay tree of the signal particle (B0 for example) and replace it by the forced decay chain (defined as an alias to B0 in EvtGen).

  • Implementation of CP violation:

    • Ask EvtGen to decay the B0/B0s meson produced by Pythia.

    • If EvtGen requests to change the flavour of the B meson, then reject the full event, otherwise keep it.

    • Not yet used in LHCb (except for B0p+p-p0)

Status conclusions
Status / Conclusions

  • EvtGen used currently in LHCb for massive Monte Carlo productions (DC04)

  • Development foreseen of new decay models which do not exist yet in EvtGen (specific Lb decay models, …)

  • Open issues studied:

    • Polarization issues,

    • Resonances lineshapes (coherence with Pythia).