Private sample generation for measurement studies	Madgraph/Madevent configuration Control of the generated samples 2_1_x Samples Summary CMSSW configuration Validation 1_6_x samples Summary PYTHIA 6 configuration CMSSW configuration Validation

Madgraph/Madevent configuration

To build the stand-alone code that will take care of generating the matrix element based events for the processes one must register as a MG/ME user here and, after that, upload the process files that describe the events to generate. You can find some examples below. The server takes care of generating stand-alone code which you can download at anytime and install locally. The MG/ME main web page grants access to a personal database with the processes you have generated. You can access it at anytime.

After generating the code download it and compile it following the instructions. To control the generation process a run card is needed. In this card the phase space for the event generation is defined along with other parameters. You can find examples for run cards below.

The output of the generators is a file in the Les Houches format which is used as in input to PYTHIA which will take care of showering the partons (fragmentation+hadronization), adding the initial and final state radiation and other concurrent interactions, decaying the unstable particles, etc. It also takes care of matching the partons generated by the matrix elements with the ones generated with the parton shower algorithm.

Control of the generated samples

The control of the matched samples (after PYTHIA generation) can be done using a tool called MatchChecker provided by the Madgraph team. To download it from CVS to your local Madgraph directory you can do the following:

   export CVSROOT=:pserver:anonymous@cp3wks05.fynu.ucl.ac.be:/usr/local/CVS
   cvs co MatchChecker
   cd MatchChecker
   make

You can run the MatchChecker.sh script giving a steering card as input. An example of such steering card is the following MatchCheckerCard.dat. To run the tool just type

   ./MatchChecker.sh MatchCheckerCard.dat 

This will produce the control plots under MatchChecker/test directory.

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Expand the following sections to see a more detailed description of the samples we have generated and which configurations were chosen, depending on the CMSSW version in use.

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2_1_x Samples

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Summary

Samples are stored at /lustre/lip.pt/data/cmslocal/pedros/Madgraph/ and were generated with CMSSW_2_1_9. The table below summarizes the main characteristics of the files.

SAMPLES SUMMARY
File	Process	Matching	Decays	Cross section (pb)		Obs.
Madgraph-tt3j_inc_run1-200810241009-CMSSW_2_1_9.root	inclusive	at 40 GeV	, ,	314.0	2958	Fastsim, no pile-up
Madgraph-tt3j_inc_run1-200810241049-CMSSW_2_1_9.root	inclusive	at 40 GeV	, ,	312.6	2944	Fastsim, pile-up=5.0
Madgraph-tt3j_inc_run1-200810241642-CMSSW_2_1_9.root	inclusive	at 30 GeV	, ,		6000	Fastsim, no pile-up

CMSSW configuration

Validation

1_6_x samples

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Summary

For our studies we have produced the parton samples for using Madgraph/Madevent. The table below shows the average time spent in generating the matrix elements for 20 kEvents with Madgraph running on a single processor.

PRIVATE SAMPLES USED FOR Vtb STUDIES (PARTON LEVEL)
N(jets)	Sample	N(events)	Total time (h)	Configuration
0	tt0j_mgme.lhe	20000	0.07	tt0j_proc_card.dat
1	ttj_mgme.lhe	20000	0.32	ttj_proc_card.dat
2	ttjj_mgme.lhe	20000	4.35	ttjj_proc_card.dat
3	ttjjj_mgme.lhe	20000	42.39	ttjjj_proc_card.dat

From the parton level samples we generate 3 different scenarios for the top decay, in the context of the standard model:

1. All b :     100%
2. Mix :     90% ,     10% (q=d,s)
3. All q :     100% (q=d,s)

The 3 scenarios are simulated using PYTHIA6 + FastSim ( CMSSW_1_6_9 ) and are described below:

PRIVATE SAMPLES USED FOR Vtb STUDIES (GEN+SIM+DIGI+RECO)
N(jets)	Jet matching	Cross section (pb)	N(events)	Luminosity (1/pb)	Scenario	Sample	Total Size (GB)
0	exclusive	11.00			All b	tt0j_allb_dil_mgme-pythia-CMSSW_1_6_9-FASTSIM-GEN-SIM-DIGI-RECO_{1,...,10}	9.53
					All q	tt0j_allq_dil_mgme-pythia-CMSSW_1_6_9-FASTSIM-GEN-SIM-DIGI-RECO_{1,...,5}	5.07
1	exclusive	7.97			All b	ttj_allb_dil_mgme-pythia-CMSSW_1_6_9-FASTSIM-GEN-SIM-DIGI-RECO_{1,...,6}	7.16
					All q	ttj_allq_dil_mgme-pythia-CMSSW_1_6_9-FASTSIM-GEN-SIM-DIGI-RECO_{1,...,6}	6.83
2	exclusive	3.53			All b	ttjj_allb_dil_mgme-pythia-CMSSW_1_6_9-FASTSIM-GEN-SIM-DIGI-RECO_{1,..,4}	4.38
					All q	ttjj_allq_dil_mgme-pythia-CMSSW_1_6_9-FASTSIM-GEN-SIM-DIGI-RECO_{1,...,4}	4.23
3	inclusive	2.10			All b	ttjjj_allb_dil_mgme-pythia-CMSSW_1_6_9-FASTSIM-GEN-SIM-DIGI-RECO_{1,...,3}	4.95
					All q	ttjjj_allq_dil_mgme-pythia-CMSSW_1_6_9-FASTSIM-GEN-SIM-DIGI-RECO_{1,...,3}	4.81

All these samples (table 1.1 and 1.2) are stored in CASTOR at CERN: /castor/cern.ch/user/p/psilva/ttbar/madgraph. The run card used to generate these samples can be retrieved from here.

PYTHIA 6 configuration

In order to work properly with the matrix elements generated for the hard process the generator needs to activate the algorithm that matches the jets generated by the matrix element with the ones generated by the parton showers. PYTHIA6 provides such a mechanism and we make use of the version provided by the Madgraph group. The version can be downloaded from this link. Note: it has to be installed under the Madgraph directory.

Our samples consist of dilepton channel. As so we disable all the branching ratios for the W except those to electrons, muons or taus. The taus are forced to decay leptonically (either electron or muon plus neutrinos). Taus are decayed with Tauola. A simple solution to force the leptonic decays is to comment all widths except the leptonic ones:

1. open pythia-pgs/libraries/pgs/src/tauola.f
2. go to routine tauola_init (line 230)
3. comment gamprt(3) to gamprt(8) (lines 335 to 342)

There are 2 ways to force the top quark to decay with different braching ratios in PYTHIA 6 (according to T. Sjostrand):

1. "You change the VCKM matrix in commonblock PYDAT2, which then takes effect in the internal calculation"
2. "You set MWID(6)=2 (in commonblock PYINT4) to use the PYUPDA input for top rather than the internal calculation."

The different decay scenarios were simulated using the second option. For that purpose we have created 1 private particle data tables which define the 3 different top decay scenarios in the standard model : PDTable.dat. In order to read this table from within PYTHIA 6 the following piece of code is needed in pythia-pgs/src/pythia.f, just before calling PYINIT.

C...  Open and read Particle data file
         PRINT *,'Opening particle data table'
         OPEN(78,file="PDTable.dat")
         CALL PYUPDA(2,78)
         PRINT *,'Read particle data table from file, closing now'
         CLOSE(78)
C... Force the top quark to decay according to the settings of the particle data table 
         CALL PYGIVE('MWID(6)=2')

After hacking tauola.f and pythia.f you can compile typing make inside the pythia-pgs directory. To steer the simulation one has to take into account if the matrix elements were simulated for an inclusive or exclusive jet multiplicity bin and also if the matching algorithm should use or ordered showers. We use the following card to steer PYTHIA 6: * PythiaCard.dat.

As the ouput of PYTHIA6 is in HEP binary format file we make use of the HEP to HepMC converter provided in this link for HepMC V2.01.08 (more info in here).

To use plain ascci files in HepMC format as input for CMSSW we have followed the instructions given in here. However in order to be compliant with the input expected by CMSSW, somepatches were needed in the IO_Ascii class of the HepMC library... The patches are the following:

1. Change the following code

         HepMC/IO_Ascii.h:124: int m_eventCtr;                                                                    //private event counter
         src/IO_Ascii.cc:16:   : m_mode(mode), m_file(filename, mode), m_finished_first_event_io(0),m_eventCtr(1)   //init also the event counter
         src/IO_Ascii.cc:63:   //  m_file << "\n" << "HepMC::Version " << versionName() << "\n";                    // disable version printing
         src/IO_Ascii.cc:64:   m_file << "HepMC::IO_Ascii-START_EVENT_LISTING\n";                              // remove escape character before
         src/IO_Ascii.cc:71:   if(evt->vertices_size()==0) return;                                                  //ignore null events
        src/IO_Ascii.cc:74:   output( m_eventCtr++ );                                                                 //use private event counter
        src/IO_Ascii.cc:351:  //output( p->momentum().m());                                                      //do not print mass  
   

2. add the following symbolic links under the HepMC installation directory:

         ln -s ./ include
         ln -s src/ lib
   

3. compile the patched version of HepMC
4. compile the converter tool.

You are now ready to convert HEP files to HepMC format and give it as input to CMSSW. To use the converter just do:

   convertStdHep -i input_HEP_file.hep  -o output_HepMC_file.dat

CMSSW configuration

The samples produced with the fast simulation were produced using CMSSW_1_6_9 and the template configuration file included in the TopQuarkAnalysisFramework to produce fast simulation samples from scratch (TopQuarkAnalysis/Examples/test/TtbarRECO_fromScratch_fast.cfg). You can pick our configuration file from here.

In order to generate HLT information from the Madgraph samples we make use of two configuration files that are parte of the HLTrigger/Configuration package. These files take care of generating the RAW data from the Digis and of generating the L1 and HLT information from that:

• to generate the raw: RelVal_Digi_DigiToRaw.cfg
• to generate the L1 and HLT info: RelVal_HLTFromRaw.cfg

For more details look at: HLT tutorial and HLT table.

Validation

• Validation of samples (sub-page: checks for self-consistency of the generated samples)
• Comparison with Alpgen (sub-page: comparison with different generators, fast and full simulation)

-- PedroSilva - 27 Mar 2008