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

2. Outline Modifications to the BABAR EvtGen version
Interface to Gaudi and Gauss LHCb simulation software
EvtGen version used in LHCb: alpha

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 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 Bs0 mixing 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 Gaudi Tool (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 Gaudi Particle 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).