1. The Generator Phase in Gauss
P. Robbe, LAL Orsay/CERN
Gauss Tutorial
CERN, 2nd June 2010
B00le

2. Gauss Tutorial
Introduction
Gauss (LHCb simulation software) is composed of 2 steps: Generation and Simulation.
Monitor
HepMC
MCParticle
MCVertex
MCHits
POOL
LHCb Event model
Pythia,
EvtGen
JobOpts
Interface …
JobOpts
Init
Exchange model
GiGa
Geant4
Initialize
HepMC
Cnv
Cnv
Cnv
POOL
Geometry
Event Generation
primary event generator
specialized decay package
pile-up generation
Detector Simulation
geometry of the detector (LHCb  Geant4)‏
tracking through materials (Geant4)‏
hit creation and MC truth information (Geant4  LHCb)‏

3. Generation part of Gauss
Gauss Tutorial
Generation part of Gauss
Takes care of:
1. Beam Parameters
2. Interaction Region Profiles
3. Number of Pile-Up interactions
4. Production of particles (hard-process)‏
5. Time-evolution of particles (decay,
oscillations, CP violation, ...)‏

4. Gauss Tutorial
External Libraries
The most important actions are performed by external libraries, developped outside LHCb: PYTHIA, EvtGen, ...
Gauss is organizing the sequence of actions needed to generate events, calling these external libraries at the right moment, though interfaces.
The interfaces to the external generators are generic: generators can be exchanged easily only via configurables, for example to use SHERPA instead of PYTHIA.

5. External Generators
In the LHCb simulation software (Gauss), external generator libraries are used for 2 different steps:
Production: to generate the p-p interaction and hadronization up to hadrons (by default Pythia)
Decay: to decay hadrons produced in the first step up to stable particles (by default EvtGen)
The generated events are then given to Geant4 for the simulation of the LHCb detector response.
3 main types of events are generated:
Minimum Bias
Inclusive B
Signal B

6. Minimum Bias (MB) Generation
Gauss Tutorial
Minimum Bias (MB) Generation
Most simple generation case is generation of minimum-bias (all what is produced by pp collisions) events.
Sequence logic is:
1. Generate p beam momentum
2. Determine number N of pile-up interactions
3. Determine space positions of the interactions (PV)‏
4. Generate N pp collisions (encapsulated in production tool)
5. Decay all produced particles (encapsulated in decay tool)
Interface
Generators

7. Minimum Bias
As an illustration, with Pythia it correspond to the processes:
11: fi fj  fi fj
12: fi fi  fk fk
13: fi fi  g g
28: fi g  fi g
53: g g  fk fk
68: g g  g g
91: elastic scatering
92: single diffraction (A B  X B)
93: single diffraction (A B  A X)
94: double diffraction
95: low-pT production
: charmonium production
: bottomonium production

8. Gauss Tutorial
Production Tool
It is used to generate the pp collisions (hard process, hadronization, ...)‏

9. Production Tool (Pythia 6)‏
Gauss Tutorial
Production Tool (Pythia 6)‏
Default options constitute « LHCb tuning », which is done to extrapolate at higher energies charged track multiplicities seen at the UA5 experiment.
The activated physics processes are the dominant ones for LHC energies. They define LHCb « minimum bias » out of which all major samples are generated. Q
Pair Creation
Flavour Excitation
Gluon Splitting
Elastic
Single diffractive
Charmonium production
...

10. Pythia6 Tuning
In LHCb, tune the charged particle multiplicity, linked to the event structure at low pT.
This is governed by the « multiple interaction model », ie each hadronic collision is the sum of a varying number of individual parton-parton interactions.
The number of parton-parton interactions per event (then the particle multiplicity) is adjusted by the parameter:
pTmin , cut-off below which the parton-parton cross-sections are set to 0
CDF
pTmin
UA5
CDF
UA5
UA5
UA5
CTEQ6L
~20 charged particles in the LHCb
acceptance per interaction.

11. Other available production tools
Gauss Tutorial
Other available production tools
Pythia8: C++ Pythia version (Pythia8)‏
Herwig++:
Sherpa:

12. Gauss Tutorial
Decay Tool (EvtGen)‏
It is used to decay all particles, and to generate
correct time dependance (CP violation, mixing),
correct angular correlations in sequential decays (decay of spin 0, 1, 2, … particles) and their interferences
Intermediate resonances (with interferences)
With a detailed description of the B and D decays (Kaon multiplicities, important for the B tagging, etc…)
Available implementations:
EvtGen: (default) interface to EvtGen
See Michal’s presentation
Documentation:

13. Other Decay Tool Sherpa:
Gauss Tutorial
Other Decay Tool
Sherpa:

14. Inclusive Generation Sequence
Gauss Tutorial
Inclusive Generation Sequence
Important samples are inclusive samples: bb or cc inclusive samples.
They are obtained from minimum bias generation, but requiring that each event contains at least one particle of a given type (B hadron, D hadron, ...)‏
To obtain more interesting samples, a cut is also performed at generator level to keep only useful events: implemented in a « cut tool ».

15. Gauss Tutorial
Cut tool (1)‏
Accept or reject an event based on generator level quantities.
Available implementations:
« LHCbAcceptance »: cut on signal direction: ≤qsignal≤400 mrad.
« DaughtersInLHCb »: cut on direction of decay products of signal particle:
10 mrad ≤ qcharged ≤ 400 mrad, 5 mrad ≤ qneutral ≤ 400 mrad
No cut on L and Ks0 daughters, and on neutrinos.
Only cut on g if they come from p0 or h.

16. Cut tool (2)‏ Variations of DaughtersInLHCb:
Gauss Tutorial
Cut tool (2)‏
Variations of DaughtersInLHCb:
DaughtersInLHCbAndFromB: signal particle is coming from a b-hadron decay,
ListOfDaughtersInLHCb: only particles of given types are required to be in the acceptance of LHCb,
SelectedDaughtersInLHCb: only particles coming from the decay of given particles are required to be in the acceptance of LHCb,

17. Excited B states tuning in Pythia
B flavour tagging in hadron colliders is based on the properties of the other B decay in the event, but also on the fragmentation characteristics of the signal B.
Measured at LEP +
Spin counting

18. Gauss Tutorial
Signal Generation
To generate signal sample, an extra step is added to the generation of inclusive: the presence of a given particle (B0, B+, ...) is required, and its decay is forced to a signal decay mode.
To speed up generation process for signal B, SignalRepeatedHadronization method exist: when a b quark is found, the event is re- hadronized until the B of interest is found. (For example, b hadronizes to Bs0 with 10% probability) For non B, use SignalPlain method.

19. Gauss Tutorial
Signal Generation
The decay of the signal is forced to a given decay mode (if 2 are present in the same event, only one is forced).
To do this, EvtGen aliases are defined. They are copies of particles (have the same properties) but their decay mode can be redefined (in a EvtGen user decay file) without affecting the decay mode of the « normal » particle.
Aliases names are <Name of the particle>sig:
B0sig, anti-B0sig
B+sig, B-sig
D_s0sig, anti-D_s0sig
...

20. Requirements for external generators
Gauss Tutorial
Requirements for external generators
They must be able to use an external random number generator:
To be able to use the « Gaudi » random number generator, used by the entire Gauss software, to ensure event-by-event reproducibility

21. Requirements for external generators
Gauss Tutorial
Requirements for external generators
The must be able to modify their internal particle properties to use the LHCb particle property definitions.
These properties are stored in a database (see below)
They are common to the entire LHCb software (reconstruction, analysis, …)
LHCb name
PDGId
Mass
Lifetime
EvtGen name

22. HepMC Format that we use to store generated events.
Gauss Tutorial
HepMC
Format that we use to store generated events.
In the .sim file, events are stored in HepMC format:
Documentation:
LHCb Specific:
Particles have status code (HepMC::GenParticle::status()) which have special meanings:
1 = stable in Pythia (p from Primary Vertex, ...)‏
2 = decayed/fragmentated by Pythia (quark, ...)‏
3 = Pythia documentation particle (string, ...)‏
777, 888 = decayed by EvtGen (all unstable particles)‏
889 = signal particle
999 = stable in EvtGen (p from B decays, ...)‏
Units are LHCb units (MeV, mm, and ns).
We would like to store in HepMC a « universal » process id, common to all generators: under investigation.

23. Gauss Tutorial
Conclusions
The Gauss simulation software allows to use external generators to generate the events of interest for the LHCb physics program.
Flexible interface, that allows to combine different generators for different tasks.
C++ generators are particularly well suited for our framework !
Simple and old tuning based only on:
Charged particle multiplicities
Excited state fractions
From past experiments (UA5, LEP) in different environments
We should be able to do better with the LHC(b) data !