1. EvtGen Miniworkshop, Patrick Robbe LAL Orsay, 21 Jan 2005
Use of EvtGen in LHCb
EvtGen Miniworkshop, Patrick Robbe LAL Orsay, 21 Jan 2005

2. Outline Modifications to the BABAR EvtGen version for LHCb environment
Interface to LHCb simulation software (Gauss)
EvtGen version used in LHCb: alpha (december 2003 release)

3. Introduction
LHCb uses EvtGen to generate Monte Carlo samples, in particular samples containing decays of B hadrons (B0, B+, Bs0, Bc+, Lb)
In this talk, presentation of the interface between EvtGen and the LHCb Simulation software to illustrate what are the important features needed in the LHCb Generation process.

4. LHCb Detector

5. Adaptations to LHC environment
Because of incoherent B meson production:
Modification of mixing description
Modification of CP violation implementations
Technical adaptations:
Compilation with Windows C++ compiler,
Use of Pythia (with HEPEVT common block size of 10000) and Photos 2.09 versions from the GENSER libraries.
Platforms used: Windows, RedHat Linux 7.3 (gcc 3.2, gcc 3.2.3) Scientific Linux (gcc 3.2.3)

6. B0/Bs0 Mixing
In original EvtGen B0 mixing is performed through Y(4S) decay.
In EvtGen Bs0 mixing is implemented as a decay Bs0Bs0. The same procedure was applied for B0 mixing (and second B0/Bs0 has 0 lifetime).
Then treat last B0 has any other particle.
Changes needed in one EvtGen class, EvtParticle.

7. 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/Bs0 when it decays to a CP decay mode. (and then to change from time to time B0 into B0 = “flip”). Use available function in EvtCPUtil to do that.
Since the amplitude depends on the B flavour, the correct asymmetry will be generated (normalized at t=0).

8. 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) in order to contain the changes needed in a single place.
Change all CP decay models to call EvtIncoherentMixing instead of EvtCPUtil: only one line change.

9. 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 measurements (strong phases in D Dalitz plots (D0K0 K+ K-)
addition of new decay models that did not exist in EvtGen (BD K*) .

10. Interface to LHCb Simulation Software

11. Goals of the Generation Phase in LHCb
Generation of 3 types of events:
Minimum Bias (MB) events, where all particles decay generically.
Inclusive B (or D) events, extracted from the MB sample, with at least one B or B in 400 mrad with respect to the z axis. All particles decay generically.
Signal B (or J/y or D) events, with one B or one B forced to decay to the signal decay mode and within 400 mrad of the z axis. All other particles decay generically.
All types of events are generated with pile-up: each event is added to MB events, the number of interactions per event is determined by the beam parameters.

12. Tools
Two Monte Carlo generators: PYTHIA and EvtGen but with a framework which is general enough to be able to use other generators (Herwig for example).
HepMC to store parameters of generated particles and to communicate between all different parts of the Generation software:
Store mother/daughter links, position 4-vector, momentum 4-vector and status code (to recognize between stable/unstable particle or between particles generated by PYTHIA/EvtGen).
Tools available in PYTHIA and HepMC to convert between PYTHIA events and HepMC events (through HEPEVT common block).
“LHCb made” conversion from EvtParticle to HepMC events.
In the future, study how to deal with polarization.

13. Issues (1)
Particle Properties: they must be identical in all parts of the LHCb software.
Solution:
all particles properties are stored in a single place and are available from a “Particle Property Service”.
A temporary evt.pdl file (file used by EvtGen as input for the particle properties) is created at the beginning of the job from the Particle Property Service and is read by EvtGen.
The particle property service gives: PDG Id, Mass, Width, Mass cut-off and Name in EvtGen.
Other informations needed by EvtGen are: Lifetime (deduced from Width), spin and charge (deduced from PDG Id).
Inside EvtGen, particle properties are transmitted to PYTHIA.

14. Issues (2)
Coherence: one particle must always be decayed by the same generator (PYTHIA or EvtGen but not by both).
Solution:
When a particle X is known to EvtGen (ie when there is an entry “DECAY X” in the generic or user decay file of EvtGen), it is declared stable in PYTHIA (MDCY(X,1) = 0).
EvtGen is called then to decay all stable particles in PYTHIA.
Random numbers: events must be reproducible to be able to debug.
All parts of the Simulation software use the same random number generator which is initialized at the beginning of the event with a different seed.

15. Issues (3)
Excited states: the signal particle can be inside a EvtGen decay tree, for example in B**B*0 ( B0 g) p+
EvtGen has then to be called from different places in the Simulation software.
Solution: use a “shared tool” which contains all the interface to EvtGen and whose functions have HepMC objects parameters.
All algorithms will then call the same EvtGen engine.

16. Example: Generation of signal B+ sample
Illustration with generation of a sample of B±J/y K± signal events. (The other B hadron decays generically)
EvtGen is setup with the decay files:
DECAY.DEC for the generic part
USER.DEC for the signal part: (B+sig is an alias for B+)
Decay B+sig
MyJ/psi K SVS;
Enddecay
CDecay B-sig #
Decay MyJ/psi
e+ e PHOTOS VLL;

17. Signal Sample Generation: Step 1
Generate MB events until it contains a B+ or a B- in 400 mrad:
The B± may come from a B** decay: decay with EvtGen all B hadrons with a mass larger than m(B+) before analyzing the event.
The probability to have the signal mode is twice [ More exactly 2-Br(B+J/y K+)] in events with both B+ and B- than in events with only one B±: reject (1-Brsig)/(2-Brsig) of events with only one B±.
The process keeps possible B+/B- production asymmetries.

18. Signal Sample Generation: Step 2
Decay the signal B in the signal mode:
Choose the B± which will be the “signal B” randomly (with 50% probability) if there are one B+ and one B- in 400 mrad.
Substitute this “signal B” with the corresponding alias B+sig or B-sig.
Call EvtGen to decay B+sig or B-sig according to the User decay file.
[For CP Violation: allow EvtGen to flip the flavour of the signal B and keep the event only if the flavour requested by PYTHIA matches the flavour requested by EvtGen, in order to generate integrated asymmetries.]

19. Signal Sample Generation: Step 3
Generate the pile-up MB interactions.
Decay all remaining undecayed particles with EvtGen, according to the generic decay file.
Store the resulting HepMC event for offline analysis (useful for tagging studies for example)
Send the generated event to the Simulation part (Geant4).

20. Conclusions
EvtGen successfully integrated into the LHCb Simulation software and used in Data Challenge 2004 to generate 200 Millions of events.
The interface requires only very small changes to EvtGen itself (mainly activation of incoherent mixing and CP violation).
CP violation generation still experimental and not yet used in massive productions.
Polarization issues not dealt with yet but under study.
Development foreseen of new decay models (specific Lb decay models, …)