Use of EvtGen at LHCb

1. Use of EvtGen at LHCb Patrick Robbe, LAL Orsay, 13 May 2004

2. Outline • Modifications to the BABAREvtGen version • Interface to Gaudi and Gauss LHCb simulation software • EvtGen version used in LHCb: alpha-00-11-07

3. 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.

4. 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.

5. 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).

6. 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.

7. 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.

8. 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)

9. 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.

10. 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)

11. 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).