Event%20Generation

1
Event Generation
LHCb Software Tutorial Updated on 13 July 2007

• Gloria Corti
• CERN

Gauss Tutorial
2
Outline

• Overview of Event generation
• Purpose reminder
• Elements of generation
• structure and various tasks accelerator
  conditions, production of pp collisions, decays
• External libraries
• Event types
• hints for generating a new signal sample and
  controlling decay
• Generator studies and high statistic samples
• random number seeds, asking for a production
• Output Data
• HepMC

3
Purpose a reminder

• For Generator phase of Gauss
• generation of proton-proton collisions
• decay of particles with special attention to
  those of b-hadrons
• Data produced can be studied directly or in
  further processing
• GenCollisions (Global generator event
  information)
• HepMCEvents (Generator event with particles and
  vertices)
• Output (.sim ) can be processed by Gauss
  simulation phase if produced alone, if in a
  complete Gauss job Boole will not use the
  generator information but will propagate it to
  Brunel ? DaVinci

4
Event Generator Phase as stand-alone Gauss job

• Gauss normally run in a single job with all
  phases
• Detector physics simulation is quite time
  consuming
• Require production system resources for
  reasonable statistic
• No more than few hundred (500) events can be
  produced in a single job
• Generator phase can be run by itself to do
  studies at Generator level
• Faster ? reasonable statistics can be produced
  for signal samples by anyone
• Very useful when introducing generator features
  or looking at at production mechanisms or decay
  channels for the first time
• Must do it BEFORE introducing a new decay channel
  in a Gauss release and asking the Production Team
  for events to be produced with Gauss ? Boole ?
  Brunel chain
• You will run generator stand-alone jobs in this
  following practical section

5
Job Options Gauss.opts Generator Stand alone
//------------------------------------------------
----------------- // Phases to be executed
//-----------------------------------------------
------------------ ApplicationMgr.TopAlg
"GaudiSequencer/Generator" //
ApplicationMgr.TopAlg GaudiSequencer/Simulat
ion //----------------------------------------
------------------------- // Generator
Phase //------------------------------------------
----------------------- include
"GAUSSOPTS/Generator.opts Generator.MeasureTime
true // When running Generator stand-alone
uncomment the following // to write on tape only
Generator objects include "GAUSSOPTS/GenStandAlo
ne.opts //--------------------------------------
--------------------------- // Simulation
Phase //------------------------------------------
----------------------- // include
"GAUSSOPTS/Simulation.opts" //
Simulation.MeasureTime true
Commented !
Uncomment for writing out only generator event
Commented !

• A generator stand-alone Gauss job can be run
  with Gauss.opts or
• v200601.opts the geometry is irrelevant !
• ../slc3_ia32_gcc323/Gauss.exe GAUSSOPTS/Gauss.op
  ts

6
Stand alone Event Generation
Monitor
JobOpts
Init
Exchange model
GiGa
Geant4
HepMC
Initialize
Monitor
Cnv
Cnv
Cnv
POOL
Geometry
Detector Simulation
geometry of the detector (LHCb ? Geant4) tracking
through materials (Geant4) hit creation and MC
truth information (Geant4 ? LHCb)
7
Event Generator Phase

• Necessary to generate events for different
  purposes
• feasibilities of some physics analysis in LHCb
• understand detector performance and evaluate
  changes in detector design
• test beam analysis
• LHCb real data physics analysis
• Need different type of events
• Single pp-collisions and bunch crossing at
  different luminosities (pile-up)
• Minimum bias
• Generic beauty (and charm) events
• Signal events b-mesons family in many decay
  modes but also b-hadrons, open charm, heavy
  quarkonia, Higgs, Z0, etc. etc. etc.
• Particle guns (calibration, test beams, single
  beam in IP8)

Physics Events
8
Elements of generation of physics events

• Generation algorithm uses tools, i.e. small
  pieces of code realizing small and specific
  actions of the generation sequence
• PileUpTool Generation of number of pile-up
  events
• SampleGenerationTool generate a given sample
  of events (minimum bias, inclusive, signal, )
• ProductionTool Generation of one p-p
  interaction
• BeamTool Generation of beam
  parameters (3 momentum)
• DecayTool Decay of unstable
  particles,
• CutTool Cut at generator level,
• FullGenEventCutTool Cut on full event
  properties,
• VertexSmearingTool Smearing of primary
  vertex.

Generator.Members "Generation"
Generator.opts
9
Generator Tools

• Each tool has a generic interface and a specific
  implementation this allows to use different
  methods to realize each action. For example,
  generation of p-p interactions can be done with
  Pythia or HERWIG, without changing anything else.
• New in DC06 versions easier to add a new
  generator engine or cuts for specific samples
• A large amount of code is common when using
  Pythia or Herwig and then do not have to be
  rewritten.
• New ideas (for generator level cuts, ) can be
  inserted quickly in Gauss since it is just a
  matter of adding a small piece of code and do not
  require to change the whole software.
• Details on the web
• http//lhcb-comp.web.cern.ch/lhcb-comp/Simulation/
  generators.htm (Gauss ? Generators)

10
ProductionTool
IProductionTool
Abstract Interface
generateEvent( HepMC ) setStable() save/loadParton
Event() turnOn/OffHadronization()
Concrete Implementations
PythiaProduction
Interface to Pythia with possibility to interact
with Pythia common blocks via job options.
HerwigProduction
BcVegPyProduction
SHERPAProduction
11
External libraries

• The functionality in the tools is implemented by
  external libraries available in the physics
  community
• controllable via property of the tools
• Default engines used in Gauss v25r12
• Pythia generates pp interactions up to
  hadronization and some decays (main-stream
  generator)
• LHAPDF generate the parton distributions inside
  the colliding protons
• called by Pythia
• EvtGen generate the decay and evolution of all
  particles
• B hadrons, generic and user/signal tables
• delegate to Pythia when decay not present in
  decay table (called internally)
• Photos implements QED radiative corrections
• used for all kind of decays
• called by EvtGen

Generation.MinimumBias.PythiaProduction.Commands
"pysub msel 1"
12
External libraries used in Gauss v24r7

• Most external libraries for generator phase out
  of the box from GENSER 1.4.1 (LCG application
  area Generator Services)
• Pythia 6.325.2
• PHOTOS 2.15.2
• LHAPDF 4.2 (use CTEQ6L)
• HERWIG 6.510.2 (and JIMMY 4.2.2)
• available for validation and development
• HepMC 1.26
• uses CLHEP,
• two libraries one for Event classes, the other
  interfaces to HepEVT for IO to FORTRAN generator
  libraries
• Specialized libraries maintained by LHCb
• EvtGen
• modified by P.Robbe for hadron machine with
  additional models including for Lb decays
• from a001107, D.Lange A.Rys for Babar
• BcVegPy 2.0 special Bc generator
• for validation and development
• from C.H.Chang, J.X.Wang, X.G.Wu, Comp.Phys.Comm.
  174 (2006) 241
• HiddenValley special Z ? v-quarks generator
• for special productions
• from M. Strassler, hep-ph/0707160

Documentation on the web by authors from Gauss
doxygen but look first on how it is controllable
and interfaced to Gauss
13
Event types and Generator methods

• Minimum Bias
• Keep all events generated by Pythia.
• Inclusive
• Keep events generated by Pythia with at least one
  b-(or c)hadron in 400 mrad w/r to the z axis.
• If all of these hadrons have pz lt 0, flip the
  whole interaction
• Signal
• Keep events generated by Pythia containing one B
  or one B- (or one B0/anti-B0, J/y, Ds/Ds-, ) in
  400 mrad.
• If there are several candidates, choose randomly
  one.
• If it has pzlt0, flip the whole interaction.
• To speed up generation, if the interaction
  contains a b quark, repeat the hadronization
  process of Pythia until the interaction contains
  the B/B-.
• Decay the signal candidate according to a forced
  channel with EvtGen.
• Decay B and B mixing with EvtGen
• Special (Higgs, top, W, Z, )
• Keep all events generated by Pythia with special
  settings and passing specific generator level
  cuts (pT(lepton) gt 4 GeV, )
• Decay Higgs, top, W, Z, with Pythia, all other
  particles with EvtGen
• No pile-up for this mode.

14
Sample Generation
ISampleGenerationTool
Abstract Interface
generate()
ExternalGenerator
Concrete Implementations
Contain useful functions when interfacing with an
external production generator (handling of B,
parity trick, )
MinimumBias
Inclusive
Signal
Other
SignalForcedFragmentation
SignalPlain
SignalRepeatedHadronization
15
Options for event types

• Options for event type chosen at production time,
  included after all other options
• example in Gauss.opts
• Options files have name GSDCTNXU.opts
• GSDCTNXU is LHCb Event type code, used in data
  and in Book-keeping to identify samples
• Each digit has a meaning explained in Note
  LHCb-2005-034

Minimum Bias include DECFILESROOT/options/3000
0000.opts Inclusive b include
DECFILESROOT/options/10000000.opts Bs-gtJ/Psi(mum
)Phi(KK) include DECFILESROOT/options/13144000.
opts
16
DecFile package

• Contains all validated and released EventType
  options
• released independently from Gauss
• allow production of new event types with an
  already released Gauss
• and corresponding EvtGen decay files (dkfiles)
  for signals and to be used in addition to generic
  one containing all decays
• When building the package a tool automatically
  builds job options necessary for a given EVTTYPE
  given specified keywords for production, cuts to
  be applied and special options to include

Generator.opts
ToolSvc.EvtGenDecay.DecayFile
"DECFILESROOT/dkfiles/DECAY-DC06.DEC"
13144000.opts
ToolSvc.EvtGenDecay.UserDecayFile
"DECFILESROOT/dkfiles/Bs_Jpsiphi,mm.dec"
17
Options and dkfile for Minimum Bias
EventType 30000000 Descriptor pp gt ?
NickName minbias Production Pythia
Physics End
minbias.dec
used by Book-keeping
used by EvtTypeSvc and MCDecayFinder
Empty EvtGen directive no special decay
30000000.opts
// Event Type30000000 // // ASCII Decay
Descriptor pp gt ? Generation.EventType
30000000 Generation.SampleGenerationTo
ol "MinimumBias" Generation.MinimumBias.P
roductionTool "PythiaProduction" ToolSvc.EvtGen
Decay.UserDecayFile "DECFILESROOT/dk
files/minbias.dec"
18
Generation algorithm for Mimimum Bias

• Main algorithm Generation
• Initialize all tools
• Generate an event
• Compute the number of interactions per event, N
• Generate an event with N interactions
• Repeat until this event is accepted
• Decay all hadrons in event
• Smear the vertex
• Store in event data store
• Print counters

Fixed luminosity of 2 1032 cm2/s (default)
and 5 1032 cm2/s.
IPileUpTool
MinimumBias Generate interactions (N times)
IProductionTool Obtain the beam parameters for
this event. Generate an event (use the internal
function of the generator used)
IBeamTool
Colliding beams with horizontal crossing angle
285 mrad
IDecayTool
ISmearingVertexTool
Beam spot size sx,y70 mm sz50 mm
19
Options for Signal Event
Bs_Jpsiphi,mm.dec
This is the decay file for the decay BS0 -gt
PSI(-gt MU MU-)PHI(-gt K K-) EventType
13144000 Descriptor B_s0 -gt (J/psi(1S) -gt
mu mu- ,gamma ,gamma) (phi(1020) -gt K
K-)cc NickName Bs_Jpsiphi,mm Physics
Includes radiative mode, No CP violation
Tested Yes By Gerhard Raven Date 29
Mar 2004
used by Book-keeping
In analysis session used
13144000.opts
// Event Type13144000 // // ASCII Decay
Descriptor B_s0 -gt (J/psi(1S) -gt mu mu-
,gamma ,gamma) (phi(1020) -gt K
K-)cc Generation.EventType
13144000 Generation.SampleGenerationTool
"SignalRepeatedHadronization" Generation.SignalRe
peatedHadronization.ProductionTool
"PythiaProduction" Generation.SignalRepeated
Hadronization.SignalPIDList 531,-531 ToolSvc.
EvtGenDecay.UserDecayFile "GAUSSOPTS/Bs_Jpsiphi
,mm.dec"
20
Generation algorithm for Signal Events
IProductionTool Obtain the beam parameters for
this event. Generate an event (use the internal
function of the generator used)
IPileUpTool

• Main algorithm Generation
• Initialize all tools
• Generate an event
• Compute the number of interactions per event, N
• Generate an event with N interactions
• Repeat until this event is accepted
• Decay all hadrons in event
• Smear the vertex
• Apply cut on whole event
• Store in event data store
• Print counters

SignalReapeatedHadronization Generate
interactions (N times). If the interaction
contains a b (or bbar) quark, save the parton
event and re-run only the hadronization. Decay
excited particles heavier than the signal
particle. Check if the interaction contains the
signal Apply generator level cuts to all
particles of interest. If all remaining particles
have pzlt0, invert the interaction. Force the
signal particle to decay to the correct decay
mode.
IBeamTool

• available
• 400 mrad cut on the B direction (default)
• direction of the decay products of the B

IDecayTool
IGenCutTool
IDecayTool
Repeated hadronization used for B0, B, Bs and
Lb. Forced fragmentation for Bc Plain Pythia for
J/y D, Ds,
IDecayTool
ISmearingVertexTool
21
Decays and EvtGen

• Package developed by BABAR (David Lange and
  Anders Ryd) for B decays generation, written in
  C.
• Takes as input
• evt.pdl particles properties (name, mass, width,
  charge, spin, lifetime)
• In LHCb created on the fly from ParticleTable.txt
• DECAY(-DC06).DEC decay table for generic decays
• we use the latest BABAR table with updated
  Branching Ratio
• User .dec file decay table for signal decays
• Dedicated syntax to describe how to perform
  decays in .dec files
• EvtGen also uses PYTHIA to generate some decay
  modes.
• Ensure both use the same particle properties
  (masses, lifetimes,) via Gaudi ParticleProperty
  Service
• EvtGen is contained in a tool, EvtGenDecay, that
  can be called at various stages of the Generation

22
EvtGen Decay Files (1)

• Aliases for signal B hadrons are defined by
  default.
• they can be found in DECAY(-DC06).DEC
• The user decay file sets the decay table for
  these aliases.
• Additional aliases can be defined

B-hadron
Alias
Charge conjugate
B0
B0sig
anti-B0sig
B
Bsig
B-sig
B0s
B_s0sig
anti-B_s0sig
Bc
B_csig
B_c-sig
Lb
Lambda_b0sig
anti-Lambda_b0sig
23
EvtGen Decay Files (2)

• A lot of decay models are available. The most
  common ones are
• PHSP phase space
• SVS (pseudo)scalar to vector (pseudo)scalar
• VSS vector to scalar scalar
• for example for f ? KK
• SVV_HELAMP H argH H0 argH0 H- argH-
  scalar to vector vector with amplitude given by
  helicity amplitude arguments
• for example for Bs ? J/Psi f where the values
  are
• H 0.159 argH 1.563 H0 0.775 argH0
  0.0 H- 0.612 argH- 2.712
• VLL vector to lepton lepton
• PHOTOS_VLL vector to lepton lepton with Photos
  radiative corrections
• for example for J/Psi ? mm
• ISGW2 semi-leptonic,
• Their explanation is available from the EvtGen
  manual
• available from the Gauss web page Gauss ? EvtGen
  ? pdf of manual

24
Example decay file

• Decay file for BS0 -gt PSI(-gt MU MU-) PHI(-gt K
  K-)
• Can define parameters to be used
• Define Hp 0.159
• . . .
• Define aliases for J/psi and phi decays
• Alias MyJ/psi J/psi
• Alias MyPhi phi
• Define charged conjugates
• ChargeConj MyJ/psi MyJ/psi
• ChargeConj MyPhi MyPhi
• Define decay of B0sig alias
• Decay B_s0sig
• BR Decay_products Decay_model (parameters)
• 1.000 MyJ/psi MyPhi SVV_HELAMP Hp pHp Hz
  pHz Hm pHm
• Enddecay
• Define the charge conjugate decay mode for
  anti-particle
• Decay anti-B_s0sig
• 1.000 MyJ/psi MyPhi SVV_HELAMP Hm pHm Hz
  pHz Hp pHp
• Enddecay

25
High statistic samples - requests

• Once an Event type has been chosen .OR. setup up
  and a big production through the whole chain of
  Gauss?Boole?Brunel is necessary
• Send a request to your Physics Working Group and
  the Physics Coordinator, Olivier Schneider, with
  cc to Patrick Robbe and me
• specify Gauss version to use and if decay file is
  new provide it
• It will be ensure all necessary code is released
  (including options!) and request is forwarded to
  Production Team

26
Indipendent samples

• To generate independent samples and to ensure
  reproducibility of the events set random number
  sequence for each event
• Use the same random number generator in whole of
  Gauss (from Gaudi framework)
• The random number sequence is set in
  initialization of each phase
• Three Random numbers are setup via Run and
  FirstEvent Number and their combination change
  either one!
• Event and Run number set in Generator phase and
  retrieved in Simulation
• Random number seeds is also stored in Headers
• Two separate headers GenHeader (event type)
  MCHeader (event time)

Gauss.opts
Gauss.RunNumber 3477 Gauss.FirstEventNumber
1001
27
Type of events Particle Guns

• Used for special studies
• Gaudi Algorihtm that can be controlled given
  particles in ranges of momentum, angle and origin
  vertex

Generator.Members - "Generation",
"GaudiSequencer/GenMonitor" Generator.Members
"ParticleGun", "GaudiSequencer/GenMonitor"
// Particle Gun's options // Note that
internally the ParticleGun expect the units to be
as specified on // the side (this is because of
consistency with the generator format
HepMC) ParticleGun.xVertexMin 0.0 //
mm ParticleGun.xVertexMax 0.0 //
mm ParticleGun.yVertexMin 0.0 //
mm ParticleGun.yVertexMax 0.0 //
mm ParticleGun.zVertexMin 0.0 //
mm ParticleGun.zVertexMax 0.0 //
mm ParticleGun.MomentumMin 1.0 //
GeV ParticleGun.MomentumMax 100.0 //
GeV ParticleGun.ThetMin 0.015 //
rad ParticleGun.ThetMax 0.300 //
rad ParticleGun.PhiMin 0.0 //
rad ParticleGun.PhiMax 6.28 //
rad ParticleGun.PdgCodes -211, 211 // PDG
code of list of particles to be
// generated
Example in Gauss ParticleGun.opts
Momentum
Theta
Phi
28
Generator Event data

• Event data resides in Event Store
• Tree - similar to file system
• Identification by path /Event/Gen/HepMCEvents
• Objects or Containers of objects
• Data access in GaudiAlgorithm
• iteration like any STL-vector
• Dont FORGET the header file of the data class

LHCb convention
const HepMCEvents events getltHepMCEventsgt(Hep
MCEventLocationDefault)
for( HepMCEventsconst_iterator iEv
events-gtbegin() iEv ! events-gtend()
iEv ) for( HepMCGenEventparticle_const
_iterator iP (iEv)-gtpGenEvt()-gtpart
icles_begin() iP ! (iEv)-gtpGenEvt()-gtp
articles_end() iP )
LHCb convention
include Event/HepMCEvent.h
29
Generator Events

• HepMC used for generator event
• External library supported by LCG
• Some C generator provides interface
• Provide access to Pythia, Herwig data (HepEvt
  wrapper provided)
• Generators communicate via HepMC in LHCb
• Pythia?EvtGen
• Very well suited to generator structure

HepMCGenEvent (process) from which access to
HepMCGenVertex(ices)
HepMCGenVertex and HepMCGenParticle holding
incoming particle/end vertex, production
vertex/outgoing particle relationship
30
Generator Events

• HepMCEvent(s)
• LHCb wrapper class to provide general LHCb data
  object functionality
• Access in Transient Event, Persistency
• Link to HepMCGenEvent as a whole from other
  LHCb event data classes (SmartRef)
• One for each pile-up event
• GenCollisions
• Information about each pile-up interaction
• Mandelstam and Bjorken variables
• process type
• reference to HepMCEvent
• GenHeader
• General information of generation
• luminosity, event type, event and run number
• Fast access to pile-ups reference to
  GenCollisions

31
Generator Counters and Monitors

• In the Gauss log files, counters computed during
  the generation are printed to be able to compute
  efficiencies and cross sections
• Pythia cross-sections,
• Number of c and b events,
• Fractions of B, B, B
• Fractions of B0, B, Bs, Lb
• Efficiencies of generator level cuts
• A tool is being put in place to automatically
  extract quantities from log files (F. Ranjard, M.
  Barbera Asin)
• Basic monitoring histograms are filled in
  production
• They can also be run also a posteriori on the
  HepMCEvents and a detailed n-tuple is also
  available (set up in MonitorInDetail.opts)

32
Exercises

• You will get familiar with running Gauss in
  generator stand-alone, how to generate
  independent samples and how the configuration for
  different event types are set
• Guidelines for the exercises on the web
• from the tutorial agenda
• Packages used
• Sim/Gauss v25r12 Mandatory
• Tutorial/Simulation v2r0 Optional
• exercises/README.txt exerciseN.txt
• solutions/exerciseN/
• Advanced exercises also available