Generating stable KK lepton samples inside CMSSW

Updated Information

Changes in CMSSW311 release

From 31X on, the generator interface is changed. MadGraphInterface and CompHEPInterface will be replaced by LHESource with Pythia6Interface. One need translate the .PEV format to lhe format, using Comphep4.5.1 by ./translator -i input.PEV -o output.lhe, where input.PEV is the PEV mixed by cpythia

Then the output will work with kktau_gen.py

Another way to get LHE format is to regenerate all the events in Comphep4.5.1 with LHE format(new option in addition to old/new format) then use mix in comphep4.5.1 to mix the events to the final lhe format. ./mix events_1.txt events_2.txt -o kkcomphep451.lhe

Introduction

KK leptons are predicted by Universal Extra Dimension(UED) model, a model in which SM fields propagate in extra dimension of size 1/R~TEV. Minimal UED(MUED) is defined in 4+1 dimension, with only 3 parameters R,Lambda and mhiggs KK masses are degenerate at tree level, mass splitting via radiative correction, eg. KK leptons lepton^1->lepton^0+r^1 (LKP). KK leptons will have below collider signatures :Missing energy, Jets, Leptons. So it looks a lot like SUSY.

In MUED, for certain parameter space, mass splitting is less than the SM lepton mass, the KK leptons can only decay via virtual SM W/lepton, thus lepton^1 life time is longer than SM lepton. This make the KK leptons longlived (lifetime is long enough to pass through detector without decaying) or stable(see paper Phys. Lett. B583 309 (2004) for reference). This is just a candidate of Heavy Stable Charged Particle which WG3(SUSY working group 3) is interested in. We would like to investigate the CMS discovery potential on the KK leptons. The first step is to generate such events inside CMSSW.

The default generator interface in CMSSW is Pythia, and there's a version of UED inside pythia, Pythia_UED but it includes only strongly interacting particles. Instead we use the MUED model from KC Kong et al.. This model can be added as an extra model on CompHEP/CalcHEP. And fortunately, our CMSSW has a CompHEP Generator Interface, so we decide use CompHEP with MUED to generate KK leptons. Since CMSSW/GeneratorInterface/ComphepInterface/ only accepts .PEV format events while CompHEP events is in .txt format. We use cpythia to change the CompHEP format to PEV format required by CMSSW ComphepInterface. After this step, we can work in standard CMSSW way to GEN-DIGI-SIMI-RECON the events.

Instruction on Installation of CompHEP, CPythia and MUED

One can install these package on his own Linux PC, but have to take care of all the compiler, libarary issues.

An easy installation works on cmsuaf.fnal.gov for any user, no root privilege is needed to install CompHEP, CPythia etc.. While I also tried lxplus at CERN, but there's isajet libarary problem which prevents installing CompHEP.

Here is the step by step installation instruction (works on CMSUAF for sure) download comphep-4.4p3.tgz(this version generates old Comphep events format.i.e,PEVLIB1.0, which is compatible to ComphepInterface of CMSSW), cpyth-1.2.7.tgz(this is the version to handle the PEVLIB1.0 format CompHEP events) from CompHEP Download webpage. intall them following instruction of the INSTALL file in the tgz files. download MUED model files for CompHEP from MUED model file for CompHEP untar the model files into models/ directories of CompHEP both working directory and source directory, and change the *14.mdl files from to *11.mdl or the largest number +1 in the models folder. e.g: if there're already 10 models in the folder, name the MUED model as the 11th model.

Now you're ready to use these tools.

Study of Parameter Space for MUED with CompHEP.

To study the parmeter space where KK leptons become charged stable particles, we just need calculate the total width of the KK lepton. If the width becomes very small(<10^{-17}GeV, remember width*tau=hbar=6.58*10^-25GeVs, if tau>~100ns, width<~10^-17GeV), it becomes a long-lived particl.

So in CompHEP, we just calculate 2 body decay (dominant) width of KK leptons by input process ~eL(~eR/~mL/~mR/~tL/~tR)->2*x here e, m, t mean e, muon, tau, and L, R are lefthanded or righthanded.

For left-handed KK leptons

~tL becomes(mass ~tL<mass B1(kk photon)+mass tau) stable only when

R-1<95 GeV, LR=20.

R-1<120, LR=10.

R-1<70, LR=60.

~mL becomes(mass ~ML<mass B1+mass u) stable only when

R-1<6.2GeV, LR=20. 95 for ~tL

R-1<7.8, LR=10. 120 for ~tL

R-1<4.5, LR=60. 70 for ~tL

For ~eL, the R^-1 required is even smaller.

And from experimental limits(direct search and EW precise measurment constraints), R^-1>~300GeV. So no lefthanded KK leptons can be HSCP candidates.

LR=20. R-1<=138 for ~mR, 113 for ~eR, 304 for ~tR

LR=10. R-1<=152 for ~mR, 119 for ~eR, 350 for ~tR

LR=30. R-1<=130 for ~mR, 109 for ~eR, 270 for ~tR

LR=60. R-1<=120 for ~mR, 103 for ~eR, 245 for ~tR

LR=90. R-1<=113 for ~mR, 99 for ~eR, 230 for ~tR

So only ~tR is above the experimental lower bound.

Xsection and event topology for KK lepton.

The dominant mode for KK lepton production is direct pair producing, for LHC it's pp->~tR,~TR, ~TR is the antiparticle of ~tR.

In principle, KK Z (Z1) boson can also decay into ~tR +SM tau through the mixing with KK hypercharge gauge boson (B1), but at R^-1=~300GeV, the mixing angle is relatively small, so the cascade decay of ~tR contribution is small compared to direct pair production.

Consider the dominant pp->KG,KG(KK gluon) process with Xsec 800pb at R^-1=300GeV: BR(KK gluon -> left-handed KK quarks) ~ 0.5 BR(left-handed KK quarks -> Z1) ~ 0.33 If we want to get two Z1 from the decays of KK gluon pair, sigma * BRs = 800 pb * (0.5)2 * (0.33)2 = 800 pb * 0.027 ~ 22 pb Now we want each Z1 to decay into right-handed KK tau through mixing. Br(Z1 -> ~tR or ~TR) ~ 1/6 without mixing. I BR(Z1 -> right-handed KK tau) ~ 1/6 * 0.05 = 0.008 (since sin2(theta) ~ 0.05 in hep-ph/0204342) Then sigma * BRs = 22 pb * (0.008)2 = 0.0015 pb = 1.5 fb < 20 fb

There are also KK quarks(Q1) and KK gluon(KG) production associated with KK quarks. They roughly have similar order of magnitude in cross sections in comparison to KK gluon production. So I guess from the indirect production, sigma(pp -> ~tR+~TR+X) might be a few fb. Such low Rinv, the cascade decay may not be negligible( about 20% from naive calculation), but we don't think it is the dominant production mechanism.

Another way to get KK tau is from the decay of level 2 KK taus. However this cross section is about 2.4 fb for Rinv=300 Gev. BR(level 2 KK tau -> level 1 KK tau) ~ 0.5 Therefore, sigma*BRs = 2.4 fb * (0.5)2 = 0.6 fb, It is minor addition.

So we just first study the direct pair production mode. This results in a pair of back to back charged tracks, without missing energy and lots of jets. For the ~tR from Z1 decay, depending on where does Z1 come from, this kind of events will result in several extra high energy jets (KGKG gives 4 jets, KG,Q gives 3 jets, Q1Q1 gives 2 jets), but no missing energy. Unfortunately, current MUED model doesn't include the rare process Z->~tR,tau process (suppressed by Winberg Angle sin(theta_1)). So we have no way to generate cascade ~tR HSCP events, unless we ask theoriest to modify their MUED model and add in the process.

Instruction on Generating KK lepton samples (GEN level in CMSSW)

CompHEP step

cd CompHEP working directory, issue command ./comphep select "enter scattering process", enter process pb, pb -> ~tR, ~TR, and selecting PDF. Then View "Diagrams" will plot all the subproceses. In this case there's 10 of them. Click "Square Diagam"->Symbolic Calculation->write result->C code, then back to click "C-compiler", you'll get a seperate window. select subprocess you like, define PDF, beam energy in INSTATE tab, also you can change the model parameters in this window, I use LR=20, and Rinv=300 since these are the parameters for stable/long-lived KK taus. Click Vega to enter generate events frame. Start integration gives you the Xsection for the subprocess. Click Generate Events->Start Search of Maxima->Number of Events->Generator(Old format) will generate events for this subprocess.

Repeat to get other subprocesses. All the events will be saved in events_?.txt in result folder.

Cpythia step

We need mix all the subprocesses events_*.txt and change the format to .PEV.

copy events_*.txt to cpythia working directory, type

./mixPEV event_*.txt
will mix all the event_*.txt file in the folder into Mix.PEV, scaled to their Xsections.

CMSSW ComphepInterface Step

in your CMSSW working src/ direcotry, check out GeneratorInterface/CommonInterface and GeneratorInterface/ComphepInterface, change line 50 in GeneratorInterface/ComphepInterface/src/comphep.F, ISMEAR=1 to ISMEAR=0(not to smear the KK lepton mass due to it's stable width=0), and change line 1039-1043 to

ELSEIF (pname.eq.'~tR') THEN modified l2 to be ~tR

kfpart= 2000015

ELSEIF (pname.eq.'~TR') THEN modified L2 to be ~TR

kfpart=-2000015

so that cpythia knows ~tR/~TR, Rebuild the package.

put the Mixed.PEV inside ComphepInterface and modify Comphep.cfg accordingly, add

"MDCY(C2000015,1) = 0 !~tR doesn't decay",

in PythiaParameters section to prevent ~tR decay.

cmsRun Comphep.cfg will generate a root file with GEN events. (this is tested and working in CMSSW1_4_5/0)

Instruction on Sim, Digi and Reco KK lepton samples

Currently, we do the KK lepton generation in CMSSW in 4 steps.

GEN+SIM Step

The code relevant to simulation is located in the SimG4Core/CustomPhysics package. The last version of this packege is made for the new plugin scheme for 1_5_X series, therefore running it with 1_4_X(1_4_5 is my working version) needs recompiling with some older tags. Below is a short recipe:

cvs co -r V00-01-08-01 SimG4Core/CustomPhysics

cvs co -r 1.3 SimG4Core/CustomPhysics/interface/CustomPhysics.h

cvs co -r 1.10 SimG4Core/CustomPhysics/src/CustomPhysics.cc

cvs co -r 1.2 SimG4Core/CustomPhysics/src/module.cc

cvs co -r 1.5 SimG4Core/CustomPhysics/BuildFile

cvs co -r V00-02-17 SimG4Core/Generators

One also has to set the proper mass spectrum file in the SimG4Core/CustomPhysics/data/CustomPhysics.cfi, Use data file particles_kktau_300_GeV.txt, put it in SimG4Core/CustomPhysics/data

One can use this kklepton_gen_sim.cfg card to get GEN+SIM sample.

Digi2Raw Step

In 1_6_0_pre8, use HLTrigger/Configuration/test/RelVal_Digi_Digi2Raw.cfg, Input files should be 1_4_X GEN-SIM

To avoid problem with invalid PDG codes during Digi process, execute: cvs co -r V06-11-16-03 PhysicsTools/HepMCCandAlgos/data/genParticleCandidatesFast.cfi and add replace genParticleCandidates.abortOnUnknownPDGCode = false in RelVal_Digi_Digi2Raw.cfg.

You may not need do this if you particle is a using a existing PDG code. It works fine for me without.

HLTFromRaw Step

In 1_6_0_pre8,Use HLTrigger/Configuration/test/RelVal_HLTFromRaw.cfg, Input files should be those output by Digi2Raw Step

RECO Step

In 1_6_0_pre8,Use Configuration/ReleaseValidation/data/reco.cfg, Input files should be those output by HLTFromRaw Step.

Currently available data

on dcache /pnfs/cms/WAX/resilient/jchen/storage/kktau/300/kktau300GeV_GENSIM145_DIGIHLTRECO163_1kevt.root

on castor /castor/cern.ch/user/j/jiechen/storage/kktau/300/kktau300GeV_GENSIM145_DIGIHLTRECO163_1kevt.root

Generating Single Track HSCP samples inside CMSSW

changes since 2X Configuration/StandardSequences/python/Simulation_cff.py is calling SimGeneral/HepPDTESSource/data/pythiaparticle.tbl now, and the mass of PDGID=2000015 is set as 500GeV, one needs change it to proper value.

But this actually doesn't matter, as Geant4 simulation reads in only 3 momentum of the generated track, and use the mass from paticle definition of particles_kktau_300_GeV.txt in SimG4Core/CustomPhysics/data, which is 303.27. So the simulated mass should still be 303.27 GeV. You can check the four momentum in SimTracks_g4SimHits collection.

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The SIM, RECO parts are same as generating KK lepton samples, we only make some modification for the GEN part.

One needs first change source from CompHEPSource to FlatRandomPtGunSource in the GEN cfg file, see example file here singlehscp_gensim.cfg Due to HSCP is not recognized by Pythia, one also needs to

cvs co -r CMSSW_1_4_5 SimGeneral/HepPDTESSource/

and add HSCP particle defination in SimGeneral/HepPDTESSource/data/Pythia6Table.tbl which is called by pythiapdt.cfi, then included by our GEN cfg file singlehscp_gensim.cfg. For KK tau case, I just change the PDG code = 2000015 line with the proper mass, width, lifetime etc.. in the Pythia6Table.tbl. One can easily change any line into a new HSCP particle, be sure to change the FlatRandomPtGunSource PDG id code in GEN cfg file correspondingly.

Available Sample for Single Track HSCP

on dcache /pnfs/cms/WAX/resilient/jchen/storage/singlehscp/300/singleHSCP300GeV_GENSIM145_DIGIHLTRECO163_pt100800GeV_evt1k.root

on castor /castor/cern.ch/user/j/jiechen/storage/singlehscp/300/singleHSCP300GeV_GENSIM145_DIGIHLTRECO163_pt100800GeV_evt1k.root

-- JieChen - 03 Apr 2009