t ¯ t production with N-jets and with jet vetoes at CMS Carmen Diez Pardos for the CMS collaboration DESY JetLHC2014: Workshop on Jet Vetoes and Jet Multiplicity Observables at the LHC, 16-18 July 2014 IPPP Durham
Outline Introduction 1 Normalised differential σ t ¯ t as a function of N Jets 2 σ t ¯ t as a function of N additional jets 3 Kinematic distributions for the leading p T additional jets 4 Veto on additional jets 5 Summary 6 CMS Preliminary Feb 2014 [pb] -1 7 TeV CMS measurement (L ≤ 5.0 fb ) 5 -1 10 8 TeV CMS measurement (L ≤ 19.6 fb ) σ 7 TeV Theory prediction Production Cross Section, ≥ n jet(s) 8 TeV Theory prediction 4 10 CMS 95%CL limit ≥ n jet(s) 3 10 2 10 =n jet(s) 10 1 -1 10 γ γ → EW VBF W Z W γ Z γ WW WZ ZZ WV γ tt t tW t tt γ ttZ ggH VH ttH WW qqll qqH t-ch s-ch Th. ∆ σ in exp. ∆ σ H C. Diez Pardos (DESY) Durham, 18 July 2014 2/42
Introduction Motivation At LHC energies, about half of t ¯ t events are produced with additional hard jets (not coming from the t ¯ t decay). Precise understanding of these events is important: Test higher-order QCD predictions Anomalous production of t ¯ t (+jets) could reveal new physics t ¯ t +jets is a background for many searches and for t ¯ tH In general, sizeable uncertainty from QCD radiation for many top quark analysis Theory predictions and models need to be tuned and tested with new measurements Large samples of t ¯ t events provide a great opportunity to study the details of the production mechanisms Potential of constraining QCD radiation at the scale of the top quark mass C. Diez Pardos (DESY) Durham, 18 July 2014 3/42
Introduction Top quark production and decay W decay defines final state: t ¯ t production mainly by gluon fusion at LHC ( ∼ 80% at 7-8 TeV) Semileptonic [e/ µ ]: Dileptonic [e/ µ ]: All-jets: BR ∼ 46% BR ∼ 30% and BR ∼ 5% and small but largest BG (ie. τ +jets: BR ∼ 15% manageable BG (ie. BG (ie. DY+jets) QCD multijet) W+jets) C. Diez Pardos (DESY) Durham, 18 July 2014 4/42
Introduction Introduction Present studies of t ¯ t +jets with 7 TeV data, both in the dilepton and the lepton+jets channels, and 8 TeV data in the dilepton channels. Measurements performed: ⋄ Differential cross-section measurement in the dilepton and l+jets channels as a function of jet multiplicity ⋄ l+jets: t ¯ t production as a function of the additional jet multiplicity. ⋄ dilepton: properties of additional jets, t ¯ t production with a veto on additional jet activity. − → Measurements in visible phase space, corrected to particle level. 7 TeV results: arXiv:1404.3171, submitted to EPJC (lepton+jets, dilepton channels) 8 TeV results: CMS-PAS-TOP-12-041 (dilepton channel) C. Diez Pardos (DESY) Durham, 18 July 2014 5/42
Introduction Generator setups for t ¯ t at CMS Matrix Element + Parton Shower generators Better description of high multiplicities Initial and final state radiation (ISR/FSR) modelling via ME from assumed Q 2 variation Matching procedure to remove double counting between partons produced by ME and PS Next to Leading Order generators More accurate in normalisation Smaller uncertainty on Q 2 Top mass: m t = 172.5 GeV C. Diez Pardos (DESY) Durham, 18 July 2014 6/42
Introduction Radiative corrections The Q 2 scale variations address two aspects: renormalisation and factorisation scale (ME) amount of ISR/FSR For each event, Q 2 is defined as: t + � p 2 MadGraph: Q 2 = m 2 T Powheg/MC@NLO: Q 2 = m 2 t Parton showering: shares Q 2 factor α s with ME implicitely: starting scale changes with ∆ Q 2 MadGraph(+Pythia), the default MC, uses: tree-level diagrams for hard radiation and interferences (up to 3 final-state partons for t ¯ t ) parton showering for soft and collinear region (with Pythia 6.42X) matching with ktMLM (ensures smoothness of N → N+1 jet rates), thresholds varied by a factor 0.5 and 2.0 (nominal = 20 GeV) → Uncertainty on radiation covered by variations of Q 2 and ME-PS matching C. Diez Pardos (DESY) Durham, 18 July 2014 7/42
Introduction Event selection Lepton+jets Dilepton 1 Single muon or 1 Dilepton triggers electron+jets trigger 2 At least two isolated 2 One isolated lepton leptons (p T > 20 GeV, (p T > 30 GeV, e : | η | < 2.4), opposite sign | η | < 2.5, µ : | η | < 2.1) 3 ≥ 2 jets (anti-kT, R < 0.5) 3 ≥ 3 jets (anti-kT, R < 0.5) with p T > 30 GeV, with p T > 35 GeV, | η | < 2.4 | η | < 2.4 4 QCD veto: ( ≥ 4 jets with M ll > 12-20 GeV p T > 30 GeV - additional 5 In ee , µµ : Z veto jet measurement) (76 < M ll < 106 GeV), 4 Loose lepton veto in both E miss > 30-40 GeV channels T 6 At least one b-tagged jet 5 At least two b-tagged jets C. Diez Pardos (DESY) Durham, 18 July 2014 8/42
Introduction Selection and analysis strategy Dilepton channels Event reconstruction: Kinematic reconstruction of the t ¯ t system Background estimation: ⋄ Z /γ ∗ +jets estimated from data ⋄ Other BGs (single top, dibosons, etc) estimated from MC Lepton+jets channels Background estimation ⋄ W+jets estimated from data ⋄ QCD: data driven ⋄ Single top, Z /γ ∗ +jets and diboson are from MC → Signal: t ¯ t MadGraph+Pythia (normalised to NNLO+NNLL) C. Diez Pardos (DESY) Durham, 18 July 2014 9/42
Introduction Control Plots: Reconstructed jet multiplicity 7 TeV, dilepton: p T > 30 GeV, l+jets: p T > 35 GeV Dilepton l+jets -1 -1 CMS, L = 5.0 fb at s = 7 TeV CMS, L = 5.0 fb at s = 7 TeV Events Events 6 6 10 10 Dilepton Combined Lepton+Jets Combined Data Data 5 t t Signal t t Signal 10 5 10 t t Other t t Other Single t Single t 4 10 W+Jets 4 W+Jets γ → µ µ 10 Z / * ee/ γ Z/ *+Jets γ → τ τ Z / * 3 10 Diboson Diboson 3 10 QCD Multijet QCD Multijet 2 10 2 10 10 10 1 -1 1 10 1.5 Data/MC Data/MC 1.5 1 1 0.5 0.5 ≥ ≥ 5 7 5 7 2 3 4 6 8 3 4 6 8 N N jets jets Good description of data within uncertainties. (Similar description with 8 TeV data) C. Diez Pardos (DESY) Durham, 18 July 2014 10/42
Cross Sections: NJets Normalised differential cross sections 1 d σ i 1 x i dN j = σ t ¯ σ t ¯ ∆ i X L t t x i number of events after background subtraction, corrected for detector efficiencies, acceptances and migration to particle level (regularised unfolding). t inclusive t ¯ σ t ¯ t cross section in the same phase space (visible). ∆ X : bin width (=1) Measurement done in the visible phase space: p l T > 20(30) GeV, | η µ | < 2.4 (2.1) dilepton (l+jets), | η e | < 2.5 p jet T > 30(35) GeV dilepton (l+jets), | η jet | < 2.4, jets required ∆ R ( j , l ) > 0.4, b-jets identified by B-hadron Comparing results to predictions from: Different generators (POWHEG+Pythia, MC@NLO+Herwig) MadGraph+Pythia with varied Q 2 scale, matching threshold C. Diez Pardos (DESY) Durham, 18 July 2014 11/42
Cross Sections: NJets Systematic uncertainties Sources considered: Jet energy scale and resolution Background estimate Model uncertainties: Q 2 scale (using samples with 2*Q, 0.5*Q), jet-parton matching threshold (threshold halved/doubled), hadronisation model, the color reconnection modelling and PDF Other sources: luminosity, pileup, b-tagging, lepton identification and trigger efficiencies Uncertainties determined individually for each bin. Normalised cross sections: bin-to-bin correlated uncertainties cancel (luminosity, flat SF, etc.), only shape uncertainties contribute (shape variation for b-tag, BG, scale, hadronisation etc.) Total syst. uncertainty varies from 3-6% in the low multiplicity bins to 20-30% for the highest multiplicities. Most important sources: JES and Q 2 scale and ME/PS Matching C. Diez Pardos (DESY) Durham, 18 July 2014 12/42
Cross Sections: NJets Normalised diff. σ t ¯ t as a function of NJets Results 7 TeV: dilepton and l+jets channels ⋄ MadGraph+Pythia, POWHEG+Pythia provide a reasonable description ⋄ MC@NLO+Herwig doesn’t describe large jet multiplicities -1 -1 CMS, L = 5.0 fb at s = 7 TeV CMS, L = 5.0 fb at s = 7 TeV jets jets 1 σ Dilepton Combined σ Lepton+Jets Combined dN dN d d 1 1 σ 1 σ -1 10 -1 10 Data -2 Data 10 -2 10 MadGraph+Pythia MadGraph+Pythia MC@NLO+Herwig MC@NLO+Herwig -3 10 -3 10 POWHEG+Pythia POWHEG+Pythia Data/MC 1.5 Data/MC 1.5 2 3 4 5 6 0 1 2 3 4 5 1 1 0.5 0.5 ≥ ≥ 5 5 7 2 3 4 6 3 4 6 8 N N jets jets Results are consistent among channels (dilepton, l+jets) C. Diez Pardos (DESY) Durham, 18 July 2014 13/42
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