Production Cross Section of Single Top in Association with Z Boson using CMS 2012 data.

Presentations in Single top group TIP

Motivations for tqZ

  • We are Planning to measure tqZ cross-section using 2012 data
  • tqZ is an unmeasured rare standard model process. It is irreducible background for ttbar and ttbar +Z. Measuring will confirm a predicted feature of the standard model and allow other analyses to more accurately account for tbZ as a background.
  • tbZ is three lepton final state
  • We will use DoubleMu, DoubleElectron and MuEG datasets in our analysis.

Analysis Goals

  • Analysis to have a deep look in very supress process tqZ of Standard Model using CMS 2012 data. Any hint of this decay process can lead to Physics Beyond the Standard Model of Particle Physics.
  • Our goal is publish cross section measurment of single top in association of z boson using CMS 2012 data.
  • We will produce a PhD thesis on this cross section measurment in Dec 2014.
  • Our future goal, is to continue this study about rare decay of SM (tqZ) on 13 Tev CMS data with more statistics.


Muhammad Shoaib, Muhammad Waqas, Ashfaq Ahmad and Hafeez Hoorani


The code with instructions has been uploaded on github.

Analysis Abstract

Top quark being the heaviest particle in Standard Model has been discovered long ago but many of its properties have still not been fully investigated. Cross section of top quark in association with a Z boson is an unmeasured rare standard model process and it is also irreducible background for many important searches at the LHC. In this note cross section measurement of single top in association with Z boson in three leptons(electrons, muons) final is presented using integrated luminosity of 19.2 fb-1 at 8 TeV.


Top quark is the heaviest particle discovered so far, with a mass close to the electroweak symmetry breaking scale. Therefore, it can provide an excellent opportunity to probe the new physics beyond the Standard Model (SM). In search for beyond Standard Model, direct evidences for new physics at the TeV-scale may be not easy to find. However, indirect evidences such as modifications of SM interactions which are originated from new physics, are another way of searching for new physics at TeV-scale. Since CMS experimnet has delivered enough statistic during 2012 data taking at 8 TeV, using this statistics, now we can study the vector boson production in association with single top quark. The production of single top in association with Z boson is an irreducible background for FCNC and also its signature can lead us to new physics.

The central feature of the CMS apparatus is a superconducting solenoid of 6m internal diameter, providing a magnetic field of 3.8 T. Within the superconducting solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass/scintillator hadron calorimeter (HCAL). Muons are measured in gas-ionization detectors embedded in the steel return yoke outside the solenoid. Extensive forward calorimetry complements the coverage provided by the barrel and endcap detectors. A more detailed description can be found in ref. [1].

* tqZ possible feynmann diagrams:

  • Theoritical predicted cross section plot of tZ [2]:

  • Theoritical predicted production cross section of top with and without Z [2]:

Analysis Framework

  • Developed AOD based analysis framework
  • Run under CMSSW_5_3_15
  • Have producers for all basic objects
  • Cut flow about Muons and Electron implemented
  • Plus analyzers for various tasks like final cut flow, top reconstruction
  • Adapted some existing tools such as Pz calculation for MET (using W constrain)
  • Recommended MET filter already incorporated
  • Jet Energy Corrections(JEC) Type-I applied.

Datasets and Triggers [6]

For this study of rare decay of SM tqZ, we are using full 2012 dataset of pp collision collected by CMS detector, having integrated luminosity 19.7 fb-1 collected by CMS experiment at 8 TeV. We are interested in all events having three leptons, electrons or muons, in the final state. Therefore for this purpose, we used DoubleMu, DoubleElectron and MuEG Primry Datasets in our analysis, each of which is associated with special HLT path. Golden JSON file [9] with latest reconstruction 22JANRERECO and all datasets are filtered to have good lumi sections only.

Summar12, signal and Background MC is being used for comparison with real data and also for background study. These simulated samples were generated by using Pythis and Powheg and were rescaled with data using integrated luminosity. All used MC datasets have been mentioned below.


* AOD_Datasets.png:

MuEG AOD to PAT Comparison

Size 649.1 GB 4.4TB 6.3TB 7.2TB
No. of Events 2518137 14776453 20791905 21978229
Size Pat 62.6GB 411.4GB 635.8GB 684.8GB
No. of Events Pat 1939498 12662941 19332807 19944520

Double Electron AOD to PAT Comparison

No. ee-A ee-B ee-C ee-D
Size 2.9TB 4.4TB 6.3TB 10.1TB
No. of Events 12964286 14776453 20791905 34526899
Size Pat 171.0GB 564.8GB 894.5GB 964.9GB
No. of Events Pat 7332038 19805884 30570146 31754632

Double Mu AOD to PAT Comparison

No. uu-A uu-B uu-C uu-D
Size 1.5TB 8.7TB 10.7TB 12.1TB
No. of Events 5636274 29308627 36820243 38006513
Size Pat 148.1GB 730.5GB 1.2TB 1.2TB
No. of Events Pat 4721234 20842033 33422729 31322625

JSON Files

  • Cert_190456-208686_8TeV_PromptReco_Collisions12_JSON.txt
  • Cert_190456-208686_8TeV_22Jan2013ReReco_Collisions12_JSON.txt

MC Datasets [7]

Official MC Production is being used for the background study of our analysis. Main background of our analysis are WZ and ttbar process. All background samples are mentioned below in the table.

* MC datasets that used in analysis:


For the data samples, the events are required to fire one of the unprescaled double-electron, double-muon or muon-electron triggers. To avoid double counting of events, trigger path is required exclusively. The logic for trigger path to fire is given belwo.

  • accept all events from the DoubleMu Primary Dataset which have fired the above mentioned dimuon trigger paths;
  • accept all events fromthe DoubleElectron Primary Dataset, if they have fired one of the above dielectron paths and none of the dimuon paths;
  • accept all events from the MuEG Primary Dataset, if they have fired one of the above cross-flavor paths and none of the dimuon or dielectron paths;

* Trigger.png:

Event Topology and Pre-selection

  • Tri-lepton final state.
  • Two leptons should be same flavor and opposite charge.
  • Third lepton should be from top decay with neutrino (MET).
  • One b-tag.
  • Light Jets should be more than 1 .

Physics Object Selection

  • Exactly three !`tight' leptons, two of which must be oppositely charged, same flavour and invariant mass within the z mass window (78-102 GeV) and the remaining lepton is considered for W boson. These selected isolated leptons have transeverse momentum greater than 20 GeV, and with an absolute pseudorapidity less than 2.4 incase of muons he and 2.5 for the selection of electrons.

Primary Vertex selection

  • Events with good vertex cuts:
    • Ndof>5, Z<24.0, rho()<2.0

Muon selection [3] [4]

    • Gobal and PF muons with official tight selection
    • Tight selection as recommended by POG group
    • Pt > 20 GeV and eta < 2.1

Electron Selection [5]

    • For Electron tight selection: EGamma package and EgammaCutBasedEleId used.
    • Isolation between jets and electron > 0.2
    • Delta R between electron and jets > 0.5
    • EGamma recommended tight selection applied.
    • Pt > 20 GeV and eta < 2.1

Jet selection: Official jet ID implemented [8]

    • PFJets with Jet pt> 20 GeV, eta< 3. and CVS disc < 0.679
    • neutralHadronEnergyFraction > 0.99
    • chargedHadronEnergyFraction > 0
    • neutralEmEnergyFraction > 0.99
    • chargedEmEnergyFraction > 0.99
    • JetIso (j, l) < 0.3

bJets Selection Criteria

  • Tag jet: CSV (Combined Secondary Vertex ) disc > 0.679
    • Eta<2.4, pt > 25 GeV, It’s a Medium working point

MET Selection criteria [8]

    • pFMET with no MET Cut.

Trigger Scale Factors

* Trigger_SF_from_CMS AN AN-12-389:

  • Trigger_SF_from_CMS AN AN-12-389:

  • Trigger_SF_from_CMS AN AN-12-389:


Three leptons final state processes that pass the selection criteria of our analysis are being considered as backgrounds. These backgrounds having three leptons in final state, can be three genuine leptons or can be fake. These processes will be estimated by using MC samples which are mentioned above. In these background processes, at least one of the lepton is non-prompt which will be estimated by using fakeable object data driven method which is described in [AN-2010/261 v1 (2010)]. The main background in our analysis scheme are WZ and ttbar events that contains atleast three leptons which are coming from weak bosons decay or from top decays with one mis-reconstructed jet or loose lepton faking as prompt leptons.

An appropriate signal region has been defined and validation of analysis will be performed in this region.

Table : Comparison of the Z+jets background estimated with different sources. The yields have been obtained in the Z+jets enriched region described in the main text. For the data − MC case, the MC samples are basically the tqZ signal and the WZ background.

No. uuu uue uee eee
Z+Jets MC        
data driven from Z+Jets MC        
data driven from Z+Jets data        
Z+Jets delta        
data - MC(WZ), data driven from data        
data driven from data        
WZ delta        

Fake Rate Estimation

Pileup Re-weighting

Analysis Cut-Flow table

* Analysis Cut-Flow table:

All Combinations cut flow table

* All Combinations cut flow table:

Systematic Uncertainties

To calculate the expected significance of our signal, systematic uncertainties which are affecting our process are mentioned below.

  • Luminosity Uncertainity
  • Trigger Efficiency
  • Leptons Eficiency
  • Jet Energy correction and jet energy resulation
  • MET uncertainty
  • Pileup
  • QCD

Table: Summary table of systematic and statistical uncertainies for cross section measurement, numbers are presented in terms of percentage.

Uncertainities uuu uue uee eee
lepton and trigger efficiency        


* Number of Primary vertex after Pileup reweighting:
pileup.png * Number of Primary Vertex after pileup reweight in log scale:
pileup_log.png * Leptons Multiplicity:
Leptons_Multiplicity.png * Pt of Leptons:
Leptons_Pt.png * Missing Transverse Energy in all leptons combinations:
MET_All_Combinations.png * Z Mass After Pre-Selection:
Di-Leptons_Mass.png * W Trans Mass After Preselection:

On-Going Work

02 Feb 2015

  1. Fake Rate Estimation in validation process.
  2. Trying to understand how to implement systematic uncertainties JER, JES, b-tag, lepton uncertainty, pdf, and Trigger.
  3. Background Estimation using data-driven technique, calculated for WZ .
  4. Analysis Note first draft depends on the progress of Analysis.


We have finalized our framework and cut optimization. We have presented our pre-selection results in single top regular meetings. Final reults will be updated very soon in single top group. We are maintaining our twik page also.


Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf AN2014_182_v4.pdf r1 manage 1512.3 K 2015-08-26 - 16:30 MuhammadShoaibcernch Cut_and_Count_Analysis_Note
PNGpng AOD_Datasets.png r2 r1 manage 18.6 K 2014-08-13 - 23:36 MuhammadShoaibcernch datasets
PNGpng CutFlow.png r1 manage 49.2 K 2014-08-29 - 18:38 MuhammadShoaibcernch Analysis Cut-Flow table
PNGpng Di-Leptons_Mass.png r1 manage 47.4 K 2014-08-02 - 18:21 MuhammadShoaibcernch Z Mass After Pre-Selection
PDFpdf Dilepton_trigger_and_lepton_identification_efficiencies.pdf r1 manage 1195.3 K 2015-08-26 - 16:24 MuhammadShoaibcernch Dilepton Trigger Analysis Note
PNGpng Feynmann_diagram.png r1 manage 38.6 K 2014-08-04 - 23:33 MuhammadShoaibcernch tqZ possible diagrams
PNGpng LeptonsCombinations_CutFlow.png r1 manage 14.9 K 2014-08-29 - 18:40 MuhammadShoaibcernch All Combinations cut flow table
PNGpng Leptons_Multiplicity.png r1 manage 30.0 K 2014-08-02 - 18:10 MuhammadShoaibcernch Leptons Multiplicity
PNGpng Leptons_Pt.png r1 manage 47.0 K 2014-08-02 - 18:14 MuhammadShoaibcernch Pt of Leptons
PNGpng MC_datasets.png r1 manage 23.1 K 2014-08-10 - 19:30 MuhammadShoaibcernch MC datasets that used in analysis
PNGpng MET_All_Combinations.png r1 manage 50.6 K 2014-08-02 - 18:17 MuhammadShoaibcernch Missing Transverse Energy in all leptons combinations
PNGpng Trigger.png r1 manage 26.1 K 2014-08-10 - 20:13 MuhammadShoaibcernch  
PNGpng Untitled.png r1 manage 37.3 K 2014-08-02 - 18:06 MuhammadShoaibcernch Leptons Multiplicity
PNGpng W_Trans_Mass.png r1 manage 63.6 K 2014-08-02 - 18:27 MuhammadShoaibcernch W Trans Mass After Preselection
PNGpng cross-section1.png r1 manage 12.6 K 2014-08-04 - 23:38 MuhammadShoaibcernch Theoritical cross section plot of tZ
PNGpng cross-section2.png r1 manage 29.0 K 2014-08-04 - 23:40 MuhammadShoaibcernch cross section of top with and without Z
JPEGjpg ee_trigger_SF.jpg r1 manage 73.1 K 2015-08-25 - 17:38 MuhammadShoaibcernch Trigger_SF_from_CMS AN AN-12-389
PNGpng elec_muon_pt.png r1 manage 36.9 K 2014-08-02 - 17:56 MuhammadShoaibcernch leptons pt
JPEGjpg eu_trigger_SF.jpg r1 manage 69.1 K 2015-08-25 - 17:38 MuhammadShoaibcernch Trigger_SF_from_CMS AN AN-12-389
PNGpng pileup.png r1 manage 25.3 K 2014-08-12 - 14:28 MuhammadShoaibcernch Pileup reweighting
PNGpng pileup_log.png r1 manage 23.3 K 2014-08-12 - 14:32 MuhammadShoaibcernch Number of Primary Vertex after pileup reweight in log scale
JPEGjpg uu_trigger_SF.jpg r1 manage 100.1 K 2015-08-25 - 17:38 MuhammadShoaibcernch Trigger_SF_from_CMS AN AN-12-389
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