Use of Evt Gen in LHCb Evt Gen

Use of Evt. Gen in LHCb Evt. Gen Miniworkshop, Patrick Robbe LAL Orsay, 21 Jan 2005

Outline • Modifications to the BABAR Evt. Gen version for LHCb environment • Interface to LHCb simulation software (Gauss) • Evt. Gen version used in LHCb: alpha-00 -11 -07 (december 2003 release) 2

Introduction • LHCb uses Evt. Gen to generate Monte Carlo samples, in particular samples containing decays of B hadrons (B 0, B+, Bs 0, Bc+, Lb) • In this talk, presentation of the interface between Evt. Gen and the LHCb Simulation software to illustrate what are the important features needed in the LHCb Generation process. 3

LHCb Detector 4

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 2. 226 (with HEPEVT common block size of 10000) and Photos 2. 09 versions from the GENSER libraries. – Platforms used: Windows, Red. Hat Linux 7. 3 (gcc 3. 2, gcc 3. 2. 3) Scientific Linux (gcc 3. 2. 3) 5

B 0/Bs 0 Mixing • In original Evt. Gen B 0 mixing is performed through Y(4 S) decay. • In Evt. Gen Bs 0 mixing is implemented as a decay Bs 0. The same procedure was applied for B 0 mixing (and second B 0/Bs 0 has 0 lifetime). • Then treat last B 0 has any other particle. • Changes needed in one Evt. Gen class, Evt. Particle. 6

CP Violation (1) • For incoherent B production, time integrated CP asymmetries may not be equal to 0. • Allow Evt. Gen to generate randomly the flavour of the B 0/Bs 0 when it decays to a CP decay mode. (and then to change from time to time B 0 into B 0 = “flip”). Use available function in Evt. CPUtil to do that. • Since the amplitude depends on the B flavour, the correct asymmetry will be generated (normalized at t=0). 7

CP Violation (2) • In original Evt. Gen, the calculations related to CP violation are located in the Evt. CPUtil class (Determination of the tag and of Dt). • Substitute with a class specific to LHC configuration (Evt. Incoherent. Mixing) in order to contain the changes needed in a single place. • Change all CP decay models to call Evt. Incoherent. Mixing instead of Evt. CPUtil: only one line change. 8

LHCb Evt. Gen Version • All other Evt. Gen classes (decay models, …) are most often left unchanged, except for: – technical changes (Windows compilation), – adaptation of decay models to Bs 0 decays, – updates with new measurements (strong phases in D Dalitz plots (D 0 K+ K-) – addition of new decay models that did not exist in Evt. Gen (B D K*). 9

Interface to LHCb Simulation Software 10

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. 11

Tools • Two Monte Carlo generators: PYTHIA and Evt. Gen but with a framework which is general enough to be able to use other generators (Herwig for example). • Hep. MC 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/Evt. Gen). – Tools available in PYTHIA and Hep. MC to convert between PYTHIA events and Hep. MC events (through HEPEVT common block). – “LHCb made” conversion from Evt. Particle to Hep. MC events. 12 – In the future, study how to deal with polarization.

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 Evt. Gen as input for the particle properties) is created at the beginning of the job from the Particle Property Service and is read by Evt. Gen. • The particle property service gives: PDG Id, Mass, Width, Mass cut-off and Name in Evt. Gen. • Other informations needed by Evt. Gen are: Lifetime (deduced from Width), spin and charge (deduced from PDG Id). • Inside Evt. Gen, particle properties are transmitted to PYTHIA. 13

Issues (2) • Coherence: one particle must always be decayed by the same generator (PYTHIA or Evt. Gen but not by both). – Solution: • When a particle X is known to Evt. Gen (ie when there is an entry “DECAY X” in the generic or user decay file of Evt. Gen), it is declared stable in PYTHIA (MDCY(X, 1) = 0). • Evt. Gen is called then to decay all stable particles in PYTHIA. • Random numbers: events must be reproducible to be able to debug. – Solution: • 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. 14

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

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) • Evt. Gen 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 1. 000 My. J/psi Enddecay CDecay B-sig # Decay My. J/psi 1. 000 e+ e. Enddecay K+ SVS; PHOTOS VLL; 16

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 Evt. Gen 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 Bthan 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. 17

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 Evt. Gen to decay B+sig or B-sig according to the User decay file. – [For CP Violation: allow Evt. Gen to flip the flavour of the signal B and keep the event only if the flavour requested by PYTHIA matches the flavour requested by Evt. Gen, in order to generate integrated asymmetries. ] 18

Signal Sample Generation: Step 3 • Generate the pile-up MB interactions. • Decay all remaining undecayed particles with Evt. Gen, according to the generic decay file. • Store the resulting Hep. MC event for offline analysis (useful for tagging studies for example) • Send the generated event to the Simulation part (Geant 4). 19

Conclusions • Evt. Gen 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 Evt. Gen 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, …) 20