Top Production with N-jets and with Jet-Vetoes at the CMS Experiment - PowerPoint PPT Presentation

Top Production with N-jets and with Jet-Vetoes at the CMS Experiment Jet Vetoes and Jet Multiplicity Observables at the LHC - Durham - July 2013 Alexis Descroix on behalf of the CMS Collaboration | 17/07/2013 I NSTITUT F ¨ UR E XPERIMENTELLE K ERNPHYSIK (IEKP) www.kit.edu KIT – University of the State of Baden-Wuerttemberg and National Laboratory of the Helmholtz Association

Introduction At the LHC a large fraction of top quark pair events (tt) are produced with additional jets Investigating these processes is very interesting Test perturbative QCD at top quark energy scale Constrain modeling uncertainties in MC Anomalous tt + jets production can be sign of new physics Background to tt production with Higgs or with other bosons and BSM This presentation gives an overview of measurements of tt events with jets at CMS: Measurement of the jet multiplicity in tt events Investigation of the properties of additional jets: Kinematic properties of additional jets Veto on additional jets Measurement of the additional parton multiplicity Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 2/25 Alexis Descroix

The Compact Muon Solenoid Experiment Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 3/25 Alexis Descroix

Investigated Processes: tt Events tt produced at LHC mostly with gluons in initial state Top quarks decay almost always into a W boson and a bottom quark tt decay signature depends on decays of both W bosons Dilepton channel: ee , e µ , and µµ ✓ : easy two prompt leptons ✓ , two unmeasured neutrinos ✗ ✗ : challenging Lepton+Jets channel: e +jets and µ +jets one prompt lepton ✓ , 4 jets ✗ Dilepton Channel Lepton+Jets Channel Similar signature as Similar signature as Drell-Yan ( Z /γ ∗ ) production W+jets and QCD multijet Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 4/25 Alexis Descroix

Analyzed Samples Datasets from 2011 (7 TeV, 5.0 fb − 1 ) and 2012 (8 TeV, 19.6 fb − 1 ) Simulation of standard tt sample with LO generator: Matrix element from M AD G RAPH (tt + 0,1,2,3 jets) Interfaced via MLM with P YTHIA for parton showering Fact./renorm. scale: Q 2 = m 2 p 2 t + � T Comparison available with NLO tt generators: P OWHEG +P YTHIA MC@NLO+H ERWIG Modeling/correction of dominant backgrounds with data-driven methods: Drell-Yan (dilepton), W +jets and QCD multijet (lepton+jets) Other backgrounds well modeled with MC: M AD G RAPH +P YTHIA ( W +jets, and Drell-Yan) P OWHEG +P YTHIA (single top) P YTHIA (diboson and QCD multijet) Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 5/25 Alexis Descroix

Event Selection -1 CMS Preliminary, 19.6 fb at s = 8 TeV Events 7 10 Dilepton Channel Dilepton Combined jet p > 30 GeV 6 T 10 Data t t Signal 5 10 ≥ 2 leptons with opposite charge ( p T > 20 GeV) t t Other Single Top 4 10 W+Jets γ → µ µ Z / * ee/ 3 10 γ → τ τ Z / * QCD veto, if m ℓℓ < 20 GeV Diboson 2 10 10 ≥ 2 jets ( p T > 30 GeV), one identified as b-jet 1 ee and µµ case: E miss 1.5 > 40 GeV and T data MC N | m ℓℓ − m Z | > 15 GeV N 1 0.5 Kinematic reconstruction of tt system 2 3 4 5 6 7 8 9 Jets -1 CMS Preliminary 5.0 fb , s = 7 TeV Events 5 10 µ ≥ ≥ Data , 3 jets, 2 b-tags Lepton+Jets Channel t t p jet Single Top > 35 GeV 4 10 T W+Jets Z+Jets Only one lepton ( p T > 30 GeV) + veto against QCD Multijet 3 10 Diboson additional leptons (looser cuts) 2 10 ≥ 3 ( 4 ) jets with p T > 35 ( 30 ) GeV 10 1 ≥ 2 selected jets identified as b-jets 1.5 3 4 5 6 7 8 Data/MC 1 0.5 ≥ 3 4 5 6 7 8 Jets Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 6/25 Alexis Descroix

M EASUREMENT OF THE D IFFERENTIAL C ROSS -S ECTION AS A F UNCTION OF THE NUMBER OF J ETS Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 7/25 Alexis Descroix

Correction Back to Particle Level Subtract background from data → N i data − N i bkg Migration from particle level to detector level to be corrected Invert migration effects back to particle level with M AD G RAPH → N i t ¯ t Correction within the visible phase-space Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 8/25 Alexis Descroix

Differential Cross-Section and Theory Comparisons Calculation of the differential cross-section N i d σ t ¯ 1 = 1 t ¯ t t L , with measured cross-section: σ t ¯ t σ t ¯ σ t ¯ dN jets t t Normalization to σ t ¯ t reduces systematic uncertainty Combine results and compare to predictions from: P OWHEG +P YTHIA MC@NLO+H ERWIG M AD G RAPH +P YTHIA with Q 2 scale varied to 4 · Q 2 and 1 / 4 · Q 2 M AD G RAPH +P YTHIA with matrix-element/parton-showering matching threshold varied to 40 and 10 GeV (nominal is 20 GeV) Systematic uncertainties estimated by repeating the measurement with varied assumptions on sources, most important ones are: Jet energy uncertainties Modeling uncertainties ( Q 2 scale, matching threshold, and hadronization uncertainty) Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 9/25 Alexis Descroix

Differential Cross-Section in Lepton+Jets Channel (7 TeV) -1 -1 CMS Preliminary, L=5 fb at s =7 TeV CMS Preliminary, L=5 fb at s =7 TeV jets jets t t t t σ σ dN dN d d p jet p jet > 35 GeV > 35 GeV t t T 1 t T 1 t σ σ -1 -1 10 10 Data (combined) Data (combined) t t MadGraph+Pythia t t MadGraph+Pythia 2 t t MadGraph Q scale up t t MadGraph Q 2 scale down 10 -2 t t MC@NLO+Herwig 10 -2 t t MadGraph match threshold up t t POWHEG+Pythia t t MadGraph match threshold down MC/Data MC/Data 2 0 1 2 3 4 5 0 1 2 3 4 5 1.5 1.5 1 1 0.5 ≥ ≥ 3 4 5 6 7 8 3 4 5 6 7 8 Jet Multiplicity Jet Multiplicity Good agreement of data with predictions from M AD G RAPH +P YTHIA and P OWHEG +P YTHIA MC@NLO+H ERWIG : jet multiplicity lower than data M AD G RAPH +P YTHIA : best description for larger Q 2 /match threshold Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 10/25 Alexis Descroix

Differential Cross-Section in Dilepton Channel (8 TeV) -1 -1 CMS Preliminary, 19.6 fb at s = 8 TeV CMS Preliminary, 19.6 fb at s = 8 TeV dJets dJets σ jet σ jet p > 30 GeV Dilepton Combined Dilepton Combined p > 30 GeV d d T T Data Data 1 σ 1 σ MadGraph+Pythia 10 10 MadGraph+Pythia MC@NLO+Herwig 2 4*Q POWHEG+Pythia 2 Q /4 1 1 Matching up Matching down -1 -1 10 10 -2 -2 10 10 -3 -3 10 10 1.5 1.5 Data/MC Data/MC 1 1 0.5 0.5 ≥ ≥ 2 3 4 5 6 2 3 4 5 6 Jets Jets Consistent with results in dilepton and lepton+jets channels at 7 TeV Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 11/25 Alexis Descroix

Differential Cross-Section in Dilepton Channel (8 TeV) with Higher Jet p T Threshold -1 -1 CMS Preliminary, 19.6 fb at s = 8 TeV CMS Preliminary, 19.6 fb at s = 8 TeV dJets dJets 2 10 σ jet σ jet p > 60 GeV p > 100 GeV Dilepton Combined Dilepton Combined d d T T Data Data 1 σ 1 σ 10 MadGraph+Pythia MadGraph+Pythia 10 MC@NLO+Herwig MC@NLO+Herwig POWHEG+Pythia POWHEG+Pythia 1 1 -1 10 -1 10 -2 10 -2 10 -3 10 -4 10 -3 10 1.5 1.5 Data/MC Data/MC 1 1 0.5 0.5 ≥ ≥ 0 1 2 3 4 5 6 0 1 2 3 4 Jets Jets Different behavior of MC@NLO confirmed at higher jet p T Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 12/25 Alexis Descroix

K INEMATICS OF A DDITIONAL J ETS - D ILEPTON C HANNEL Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 13/25 Alexis Descroix

Definition of Additional Jets in Dilepton Channel The kinematic reconstruction of the tt system assigns the jets from the tt decay Kinematic constraints allow to solve the two-neutrino ambiguity Jet assignment is not straightforward ⇒ choice made with b-jet identification and neutrino energy spectrum Study of kinematic properties of additional jets: Subtract background from data, no correction back to particle level MC distribution scaled with measured tt cross-section Top Production with N-jets and with Jet-Vetoes at CMS 17/07/2013 14/25 Alexis Descroix