Gauss

Gauss is the official simulation framework at LHCb experiment at CERN. It is used to simulate the LHCb detector and the physics processes that happen. It is basically a pre-defined custom versions of both pythia and geant4. In this post I will describe how to run Gauss on lxplus.

Quick Setup

• Set up Gauss: lb-dev Gauss/v56r3 you can decide which version specifying what is after Gauss so it is like Gauss/<version>

• cd to the Gauss dir cd ./GaussDev_v56r3

• We will need the DecFiles package which is available on CERN GitLab (this is where the pre-defined decay is there) git lb-clone-pkg Gen/DecFiles

• We now build Gauss by running make

This can be done on one command like

lb-dev Gauss/v56r3 \\
cd ./GaussDev_v56r3 \\ 
git lb-clone-pkg Gen/DecFiles \\
cd Gen/DecFiles
make 

Hint: This will install Sim10

Run Gauss

Let's use the example provided in LHCb starterkit. To run the starterkit example which deals with this decay $\left.D^{*+} \rightarrow D^{0}\left(\rightarrow K^{+} K^{-} \mu^{+} \mu^{-}\right) \pi^{+}\right)$. This decay is pre-defined in Gauss and is given the event type # 27175000. This is determined by the Decfiles library. The DecFile controls the decay itself (i.e. what EvtGen does) as well as provide any event-type specific configuration (e.g. generator cuts). They exist in the Gen/DecFile package. The content of the Decfile is the following:

# EventType: 27175000
#
# Descriptor: [D*(2010)+ -> (D0 -> {K+ K- mu+ mu-}) pi+]cc
#
# NickName: Dst_D0pi,KKmumu=DecProdCut
#
# Cuts: DaughtersInLHCb
#
# Documentation: Forces the D* decay in generic b-bbar / c-cbar events + Requires products to be in LHCb acceptance
# EndDocumentation
#
# PhysicsWG: Charm
# Tested: Yes
# Responsible: Luisa Arrabito
# Email: unknown@<nospam>cern.ch
# Date: 20091215
#

Alias MyD0 D0
Alias MyantiD0 anti-D0
ChargeConj MyD0 MyantiD0
                                                                                                                                                                                                                                                           
Decay D*+sig
  1.000 MyD0  pi+    VSS;
Enddecay
CDecay D*-sig
                                                                                                                                                                                                                                                           
Decay MyD0
  1.000 K+ K- mu+ mu- PHSP;
Enddecay
CDecay MyantiD0
#
End  

This Decfile code contains the Information which tells Gauss what to except when calling <event-type>.py as an option. There is a current parser online to help you with event type.

To run Gauss you need to specify the options which tells Gauss about what you need. You can pass the options using the syntax

./run gaudirun.py option Gauss-Job.py

But the best way is to include all the options in Gauss-Job.py which is where you need to call things. For example

Instead of running

./run gaudirun.py 
        '$APPCONFIGOPTS/Gauss/Beam6500GeV-md100-2016-nu1.6.py' \  # Sets beam energy and position
        '$APPCONFIGOPTS/Gauss/EnableSpillover-25ns.py' \  # Enables spillover (only Run2)
        '$APPCONFIGOPTS/Gauss/DataType-2017.py' \ # Sets the data type (organized by year)
        '$APPCONFIGOPTS/Gauss/RICHRandomHits.py' \  # Random hits in RICH for occupancy
        '$DECFILESROOT/options/{eventnumber}.py' \  # Event type containing the signal
        '$LBPYTHIA8ROOT/options/Pythia8.py' \  # Setting Pythia8 as generator
        '$APPCONFIGOPTS/Gauss/G4PL_FTFP_BERT_EmNoCuts.py' \  # Physics simulated by Geant4
        Gauss-Job.py

You can define your own options in Gauss-Job.py As the following:

from Gauss.Configuration import *
importOptions("$APPCONFIGOPTS/Gauss/Beam6500GeV-md100-2016-nu1.6.py")
importOptions("$APPCONFIGOPTS/Gauss/EnableSpillover-25ns.py")
importOptions("$APPCONFIGOPTS/Gauss/DataType-2016.py")
importOptions("$APPCONFIGOPTS/Gauss/RICHRandomHits.py")
importOptions("$DECFILESROOT/options/27175000.py")
importOptions("$LBPYTHIA8ROOT/options/Pythia8.py")
importOptions("$APPCONFIGOPTS/Gauss/G4PL_FTFP_BERT_EmNoCuts.py")


GaussGen = GenInit("GaussGen")
GaussGen.FirstEventNumber = 1
GaussGen.RunNumber = 1082

from Configurables import LHCbApp
LHCbApp().DDDBtag = 'dddb-20170721-3'
LHCbApp().CondDBtag = 'sim-20170721-2-vc-md100'
LHCbApp().EvtMax = 5

To run the following Simulation you cd to Gauss dir and run the following:

./run gaudirun.py ../Gauss-Job.py

Which will be equivalent to

./run gaudirun.py '$APPCONFIGOPTS/Gauss/Beam6500GeV-md100-2016-nu1.6.py'  '$LBPYTHIA8ROOT/options/Pythia8.py' '$DECFILESROOT/options/27175000.py' ../Gauss-Job.py

Which of course will depend on where you have Gauss-Job.py file

Hint: you will need first to run lhcb-proxy-init to obtain the permissions to run Gauss.

Hint: This will take long time ~20 minutes because we asked for a full detector simulation by Geant4, but we can only tell Gauss to run the generator phase of simulation only by adding '$GAUSSOPTS/GenStandAlone.py' as an option.

Now let's give some information about the options which we told gauss to use. The following table summarize that:

Nightly version

This is how we can run the nightly version of Gauss

The general syntax is

lb-run --nighlty lhcb-gauss-dev/SLOTID Gauss/HEAD gaudirun.py Gauss-Job.py

Where SLOTID would be Today or the one that we are interested

The default is Today and could also be used as follows

lb-run --nightly lhcb-gauss-dev Gauss/HEAD  gaudirun.py Gauss-Job.py