ZZ→llnn Analysis Thomas Barber University of Cambridge ATLAS UK SM Meeting, September 2007

Tom Barber, University of Cambridge Outline Results of ZZ→llnn analysis with 11.0.4 and 12.0.6 Monte Carlo. Physical Motivation Analysis procedure and description of CSC datasets used. Electron and muon identification in Eventview. Parameters used in cut-based selection. Comparison of full-simulation yields to previous fast sim results. Signal and background yields using tuned cuts. Summary and outlook. 21/02/2019 Tom Barber, University of Cambridge

Neutral Diboson Couplings Standard Model Production Diagrams Forbidden in SM Neutral diboson (ZZ) production in ATLAS By examining cross section at high Z pT, we can put limits on anomalous neutral triple gauge couplings (NTGC). See Pat’s Talk Other groups studying ZZ4 leptons In comparison, the ZZ→llnn (l=e,m) channel has: Larger background (no second lepton pair) But ~6 times larger cross section (0.397 pb) 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Analysis Procedure Athena analysis using EventView 12.0.6 Using Distributed Analysis framework with Ganga for LCG datasets. pAthena for BNL datasets. Event Generators mostly either Pythia or MC@NLO, one AcerMC sample. Event Weights in MC@NLO are handled by subtracting negative events from positive distributions and yields. Cross Sections calculated by running appropriate DC3 joboptions transform to generate ~10,000 events. For MC@NLO, combine production CS with branching ratio from PDG. 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Monte Carlo Datasets Analysis done on both CSC11 and CSC12 signal and background datasets. Two different signal datasets used: 11.0.4 Signal (blue) csc11.005981.Pythiazzllnunu.recon.AOD.v11004213 Pythia generator: leading order 48700 events. 12.0.6 Signal (red) user.HongMa.trig1_misal1_mc12.005932.McAtNlo0310_JIMMY_ZZ_llnunu.recon.AOD.v12000601_pUM02203 MC@NLO generator: next-to-leading order. 118000 events Dataset currently unofficial, official sample in production. In case of 12.0.6.1 datasets, 1mm Bug-fix applied where needed. 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Version 11.0.4 Datasets 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Version 12.0.6 Datasets 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge EventView Inserters EventView used to Create NTuples Remove overlap (R = 0.1). Insertion order: Electrons (+ loose electrons) Photons Muons (+ loose muons) TauJets TauJetCollection pT > 5 GeV, either TauRec or 1p3p Particle Jets ConeTowerParticleJets, R = 0.5, pT > 5 GeV Need robust electron and muon identification 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Electron ID Electrons from egamma (isEM & 0x7FF) == 0, ie no TRT Isolation: Et(R=0.45) < 8 GeV pT > 5 GeV, || < 2.5 11.0.4: Efficiency = 75.0% 12.0.6: Efficiency = 62.6% Much lower in 12.0.6 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Muon ID Muon from MuID algorithm Require a best match, with 2(match)/ndof < 10 2(fit)/ndof < 5 Isolation: Et(R=0.45) < 5 GeV pT > 5 GeV, || < 2.5 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Muon Efficiency 11.0.4: Efficiency = 86.3% 12.0.6: Efficiency = 87.7% 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Lepton Kinematics Electrons and muons show good agreement. Treat them as leptons for rest of analysis. pT shows good agreement between two versions. Z-width not yet implemented in MC@NLO (planned for future) 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Signal Topology ZZ→llnn signal characterised by 2 high-pT leptons and ET(miss). Atlantis event displays Define cuts to separate signal from background channels. Main backgrounds from channels with: Large C.S. (ttbar and Zll) Similar topology (WZ and WW) Following plots are normalised to unit area to compare shapes before any cuts applied Show 12.0.6 distributions (unless specified) 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Lepton Cuts ZZ→llnn signal channel shown in red. Lepton pT > 20 GeV Reduces soft electron background, eg ttbar (magenta) & Z (turquoise) |mll – 91.2 GeV| < 10 GeV Reduces non-Z background, ttbar (magneta), Z (turquoise) & WWll (purple) 20 GeV ±10 GeV 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Third Lepton Veto Require exactly two oppositely charged leptons Define “Loose Leptons” is anything left in Electron/Muon after initial lepton ID, with pT > 5 GeV. N(leptons) = N(quality) + N(loose) Veto events with N(lepton) > 2 to cut down WZ background. 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Missing Energy Using AOD keys: METFinal (11.0.4) METRefFinal_1mmCorrection (12.0.6.1) METRefFinal (>12.0.6.4) Absolute MET cut > 50 GeV removes Zll (blue), Z4l (orange). Also require magnitude and direction to match that of the Z(ll). |MET-Zpt|/Zpt < 0.35 |f(z)-f(met)| < 35 deg Both cuts are set at ~2s from the centre of the signal peak. Helps to remove background from the WZ channel. 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Jet Veto Reduces background from ttbar and Zll (high pT Z) Require events to have no jets with pT > 30 GeV and || < 3.0 Discrepancy between jet multiplicity 11.0.4 and 12.0.6 signal due to Generator, Pythia (LO) vs. MC@NLO (NLO) In the truth, average number of jets per event: 11.0.4: 1.8 12.0.6: 5.3 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Final Cut Cut on pT of the Z. In fast simulation, pT(Z) > 150 GeV For full simulation, only require pT(Z) > 100 GeV Final value can be set to obtain a required S/B ratio. Anomalous coupling enhance cross section at high Z pT, so this cut will not harm anomalous coupling studies. 21/02/2019 Tom Barber, University of Cambridge

Fast Simulation Comparison Run analysis with cuts identical to fast simulation study, by Hassani (ATL-COM-PHYS-2002-012) Top table reproduction of fast sim results, lower table full simulation with 12.0.6 data. (except Wt channel, which is 11.0.4) Now signal is only ~60% of fast simulation prediction. Larger backgrounds from Z+jets and WZ channels. Other channels, no events pass, low statistics. Events expected in 100fb-1 of data 21/02/2019 Tom Barber, University of Cambridge

Tom Barber, University of Cambridge Full Simulation Cuts Now apply MET matching cuts and reduce pT(Z) cut. Plot shows events after cuts, normalised to 100fb-1 Signal efficiency increased, background reduced: 11.0.4 12.0.6  = 3.2%  = 2.6% S/B =2.25 S/B =1.96 Errors on ttbar & Zll large due to low statistics. Events expected in 100fb-1 of data 21/02/2019 Tom Barber, University of Cambridge

Signal Selection Efficiency Plots show number of reco events passing cuts compared to number of true events passing cuts. No ZpT cut is applied. Shows a drop in efficiency at high Z pt. Possibly due to Jet Veto removing high pT(Z) events that also have a high pT jet recoiling. Needs further investigation – possibly tune the jet veto. 21/02/2019 Tom Barber, University of Cambridge

Conclusions and Outlook Good agreement between two CSC datasets except: Lower electron efficiency No Z width in MC@NLO (12.0.6 dataset) Comparison to Fast Simulation shows lower signal efficiency and higher backgrounds. Requiring Missing Et to match the Z pT and  gives much better yields. Still to do: Investigate and optimise the jet veto (to reduce ttbar) and 3rd lepton veto (to reduce WZ). Trigger Aware Analysis still needs to be performed. Investigate Multi-variate analysis techniques, eg ANN, BDT 21/02/2019 Tom Barber, University of Cambridge