/RapidSim

Phase space generation of b hadron decays

Primary LanguageC++MIT LicenseMIT

RapidSim DOI

RapidSim is a fast Monte Carlo generator for simulation of heavy-quark hadron decays. It can be useful for generating background or toy data sets for acceptance studies.

  • It uses TGenPhaseSpace to generate b/c quark hadron decays.
  • FONLL is used to give the b/c the correct production kinematics for the LHC.
  • The daughter particle momenta are smeared correctly using user-defined resolutions.
  • Cuts can be made on daughter particle properties (pT, eta, etc).
  • Particle mass hypotheses can be swapped to investigate effect of mis-identification.

A more detailed description can be found here https://arxiv.org/abs/1612.07489

Setup

RapidSim relies on having a working ROOT installation and gcc4.9 or clang. Optionally, you may also compile against EvtGen as detailed in EvtGen To build do the following, where "" should be the full path, up to and including the "RapidSim" folder, created by the git clone command:

$ # Set environment for gcc, it defines LD_LIBRARY_PATH and FC, CC, CXX. RapidSim also works with clang.
$ # On lxplus do this:
$ source /cvmfs/lhcb.cern.ch/lib/lcg/releases/LCG_87/gcc/4.9.3/x86_64-slc6/setup.sh
$ # Set ROOT environment 
$ source /cvmfs/lhcb.cern.ch/lib/lcg/releases/LCG_87/ROOT/6.08.02/x86_64-slc6-gcc49-opt/bin/thisroot.sh
$ mkdir build
$ cd build 
$ cmake ../ -DCMAKE_INSTALL_PREFIX=<install location>
$ make -j4
$ make -j4 install # This step is optional if you want to install in a specific location

The usage is:

$ # Setup the RAPIDSIM_ROOT environment variable (or add to .bashrc)
$ export RAPIDSIM_ROOT=<install location>
$ # Optionally setup the RAPIDSIM_CONFIG environment variable
$ export RAPIDSIM_CONFIG=/path/to/additional/configuration/files
$ $RAPIDSIM_ROOT/build/src/RapidSim.exe <decay mode> <events to generate> <save tree?> <num times to re-decay each hadron>

To run an example try:

$ $RAPIDSIM_ROOT/build/src/RapidSim.exe $RAPIDSIM_ROOT/validation/Bs2Jpsiphi 10000 1

or

$ $RAPIDSIM_ROOT/bin/RapidSim.exe $RAPIDSIM_ROOT/validation/Bs2Jpsiphi 10000 1

To run the full system validation:

$ source $RAPIDSIM_ROOT/bin/runValidation.sh

Decays

To generate a new decay mode you must write a .decay file using the following syntax:

  • particles are named according to $RAPIDSIM_ROOT/config/particles.dat or $RAPIDSIM_CONFIG/config/particles.dat and separated by spaces
  • a decay is denoted by ->
  • subdecays are demarcated by braces {}, e.g.
Bs0 -> { Jpsi -> mu+ mu- } { phi -> K+ K- }

When RapidSim is first run for a new decay mode it will generate a simple .config file. These files allow properties of the particles and global settings to be configured for each decay. Run RapidSim with 0 events to generate if you just want to produce the .config file. The default values listed below are those obtained when using the automatically generated .config file. Note that when lines are removed from the .config file this behaviour may change.

Re-decay

It is possible to specify a command-line option to re-decay a particular heavy-quark hadron. This means that for a particular hadron, the same parent kinematics are retained but the kinematics of the decay products (and their various detector-level smearings) are recomputed.

Configuration

Global settings should be defined at the start of the file using the syntax:

<setting> : <value>

Particle settings should be defined after the corresponding @# tag using the same syntax.

Global settings

  • seed:

    • Sets the seed for the random number generator
    • Default: 0

  • acceptance:

    • Sets the type of geometric acceptance to apply to the decay
    • Options are Any, ParentIn, AllIn and AllDownstream
    • Default: Any

  • geometry:

    • Sets the detector geometry to apply acceptance for
    • Options are 4pi and LHCb
    • Default: LHCb

  • energy:

    • Sets the pp collision energy (in TeV) used to get the correct parent kinematics
    • Supported: 7, 8, 13 and 14
    • more types may be added in $RAPIDSIM_ROOT/rootfiles/fonll or $RAPIDSIM_CONFIG/rootfiles/fonll

  • parent:

    • Overrides to determined flavour of the parent particle used to get the correct parent kinematics
    • Supported: b and c
    • More types may be added in rootfiles/fonll

  • ptRange:

    • Define the pT range to generate the parent in.
    • Syntax: ptRange : <min> <max>
    • Defaults: 0–300 (for 4pi geometry) or 0–100 (for LHCb geometry)

  • etaRange:

    • Define the pseudorapidity range to generate the parent in.
    • Syntax: etaRange : <min> <max>
    • Defaults: -8–8 for 4pi geometry and 1–6 for LHCb geometry

  • minWidth:

    • Sets the minimum resonance width (in GeV) to be generated
    • Narrower resonances will be generated with a fixed mass
    • Default: 0.001 GeV

  • maxAttempts:

    • Sets the maximum number of attempts allowed to generate each decay
    • Default: 1000

  • paramsStable:

    • Defines the set of parameters to be added to the histograms/tree for each stable particle in the decay.
    • Syntax: paramsStable : <params>, where <params> is a space separated list of the types defined in the Parameters section.
    • Example: paramsStable : P PT

  • paramsDecaying:

    • Defines the set of parameters to be added to the histograms/tree for each decaying particle in the decay.
    • Syntax is paramsDecaying : <params> where <params> is a space separated list of the types defined in the Parameters section.
    • Example: paramsDecaying : M P PT

  • paramsTwoBody:

    • Defines the set of parameters to be added to the histograms/tree for each two-body combination of particles in a 3+-body decay.
    • Syntax is paramsTwoBody : <params> where <params> is a space separated list of the types defined in the Parameters section.
    • Example: paramsTwoBody : M M2

  • paramsThreeBody:

    • Defines the set of parameters to be added to the histograms/tree for each three-body combination of particles in a 4+-body decay.
    • Syntax is paramsThreeBody : <params> where <params> is a space separated list of the types defined in the Parameters section.
    • Example: paramsThreeBody : M M2

  • param:

    • Defines a new parameter to be added to the histograms/tree
    • Syntax is param : <name> <type> <particles> [TRUE], where:
      • <type> must be one of the types defined in the Parameters section,
      • <particles> is a space separated list of particle indices
      • The optional TRUE argument means the parameter will be calculated using the true unsmeared momenta of the particles
    • Example: param : mSq12 M2 1 2 TRUE

  • cut:

    • Applies a cut on a given parameter
    • Syntax is cut : <param> <type> <min/max> [<max>], where
      • <param> is the name of a parameter (must be defined using param)
      • <type> is one of "min", "max", "range" or "veto"
      • <min> and/or <max> define(s) the cut value(s)

  • shape:

    • Sets a 1D or 2D PDF to generate events according to
    • Syntax is shape : <file> <hist> <paramX> [<paramY>], where:
      • <file> is the file containing the histogram (path must be absolute or relative to run directory),
      • <hist> is the name of the histogram,
      • <paramX> is the name of the parameter on the X-axis (must be defined using param)
    • For 2D histograms <paramY> is the name of the parameter on the Y-axis
    • RapidSim supports TH1F, TH1D, TH2F and TH2D histograms,
    • The dimensionality of the histogram will be inferred from the number of parameters given
    • Defaults to phase-space distribution

  • useEvtGen :

    • Perform decays using the external EvtGen generator
    • Syntax is useEvtGen : TRUE
    • Note any value for this parameter will turn EvtGen ON (even FALSE)
    • To turn EvtGen OFF do not define this parameter

  • evtGenUsePHOTOS :

    • If EvtGen is used to generate decays then PHOTOS will be used for final state radiation
    • Syntax is evtGenUsePHOTOS : TRUE
    • Note any value for this parameter will turn PHOTOS ON (even FALSE)
    • To turn PHOTOS OFF do not define this parameter
    • Note also that if EvtGen is not in use then this parameter will have no effect

  • pid :

    • Sets the particle identification scheme to use
    • Syntax is pid : <scheme>, where
      • <scheme> is the name of the file that defines the scheme (default LHCbGenericPID)
    • More types may be defined in $RAPIDSIM_ROOT/config/pid or $RAPIDSIM_CONFIG/config/pid

Particle settings

  • name:

    • A user-friendly name for this particle to be used in variable names
    • Default: Automatically generated unique name of particle

  • smear:

    • The type of momentum or IP smearing to apply to this particle
      • May be used multiple times for the same particle to define different types of smearing
    • Supported momentum types: LHCbGeneric, LHCbElectron, AtlasMuon, or AtlasHadron
    • Supported IP types: LHCbGenericIP
      • More types may be defined in $RAPIDSIM_ROOT/config/smear or $RAPIDSIM_CONFIG/config/smear
    • Default: LHCbElectron (for electrons/positrons), otherwise LHCbGeneric

  • invisible:

    • Whether the particle should be treated as invisible
    • Invisible particles are not included when determining non-truth parameters
    • Invisible particles are also ignored when applying the AllIn or AllDownstream acceptance requirement for the default LHCb geometry
    • A corrected mass parameter will be added automatically if any particles are invisible
    • Default: true (for neutrinos), otherwise false

  • altMass:

    • Adds alternative mass hypotheses for this particle
    • Syntax is altMass : <particles> where <particles> is a space separated list of particle type names as listed in config/particles.dat
    • If alternative hypotheses are defined for multiple particles then parameters will be calculated for all combinations of one or two mis-identifications

  • evtGenModel :

    • Sets the decay model to be used if this particle is decayed using EvtGen
    • Syntax is evtGenModel : <model> [<params>] where <model> is the name of the model as used in DECAY.DEC and <params> is a space separated list of parameters to be passed to the model
    • Default: PHSP

Parameters

  • M: The invariant mass of the combination of the given particles
  • M2: The squared invariant mass of the combination of the given particles
  • MT: The transverse mass of the combination of the given particles
  • E: The energy of the combination of the given particles
  • ET: The transverse energy of the combination of the given particles
  • P: The total momentum of the combination of the given particles
  • PX: The X momentum of the combination of the given particles
  • PY: The Y momentum of the combination of the given particles
  • PZ: The Z momentum of the combination of the given particles
  • PT: The transverse momentum of the combination of the given particles
  • vtxX: The X coordinate of the decay vertex of the given particles
  • vtxY: The Y coordinate of the decay vertex of the given particles
  • vtxZ: The Z coordinate of the decay vertex of the given particles
  • origX: The X coordinate of the origin vertex of the given particles
  • origY: The Y coordinate of the origin vertex of the given particles
  • origZ: The Z coordinate of the origin vertex of the given particles
  • IP : Impact parameter to own primary vertex
  • SIGMAIP : Error on impact parameter to own primary vertex
  • MINIP : Minimum impact parameter to any primary vertex
  • SIGMAMINIP : Error on minimum impact parameter to a primary vertex
  • FD : Flight distance
  • eta: The pseudorapidity of the combination
  • phi: The azimuthal angle of the combination
  • y: The rapidity of the combination
  • gamma: The relativistic gamma factor of the combination
  • beta: The velocity of the combination
  • theta: The angle between the first two particles in the rest frame of the combination of the remaining particles e.g. 1 2 1 3 would give the angle between 1 and 2 in the rest frame of a resonance in m_13 (i.e. theta_13 in a 3-body decay)
  • costheta: The cosine of theta
  • Mcorr: The corrected mass of the combination of the given particles correcting for any invisible particles
  • ProbNNmu : Particle ID feature for the muon hypothesis
  • ProbNNe : Particle ID feature for the electron hypothesis
  • ProbNNpi : Particle ID feature for the pion hypothesis
  • ProbNNk : Particle ID feature for the kaon hypothesis
  • ProbNNp : Particle ID feature for the proton hypothesis

External Generators

EvtGen

EvtGen may be used to generate decays allowing for non-phasespace decay models.

  • EvtGen must be compiled with all external generators included (Pythia, PHOTOS, Tauola)
    • This can be achieved by running the EvtGen setup script
    • Note the manual step to set the required environment variables after installation
  • To compile RapidSim against EvtGen the environment variable EVTGEN_ROOT must be set
  • At present, all identical particles must decay into the same mode

TODO

  • Add more particles to particles.dat
  • Add derived class for particle specific parameters to simplify RapidParam interface
  • Combine .decay and .config files to simplify interface

Authors

  • Greig Cowan 2015
  • Dan Craik 2016
  • Matt Needham 2015