Measurement of the ZZ production cross section at 13 TeV with the ATLAS detector Phys. Rev. Lett. 116, 101801 (2016) Stefan Richter (UCL, CERN) � Jonatan Rosten (Cambridge) IoP HEPP & APP conference 21-23 March 2016 University of Sussex
In short Measure fiducial inclusive cross section for ZZ at √ s = 13 TeV in the four-lepton channel, using 3 . 2 fb − 1 of data “ Z ” = Z / γ ∗ with mass between 66–116 GeV (CMS uses 60–120 GeV) ℓ = e , µ Also extrapolate to ‘total’ phase space and all Z boson decays Paper: Phys. Rev. Let. 116, 101801 (2016) 2 2 Stefan Richter
Introduction Cambridge Motivations: � • Good test of the electroweak sector of the Standard Model at unprecedented energy � • Important background to searches for rare multilepton final states (like H → ZZ) � • First step towards di ff erential cross sections, aTGCS, etc. 3 3 Jonatan Rosten
Introduction Cambridge q Z g Z g q Z Z Two examples of important Feynman diagrams Three leptonic channels: 4e, 2e2 µ , 4 µ Clean channel, small backgrounds Small cross section: statistically limited 4 4 Jonatan Rosten
Fiducial lepton definition Generator-level Prompt final-state muons and electrons ‘Dressing’ to account for Bremsstrahlung: add four-momenta (∆ η ) 2 + (∆ ϕ ) 2 = 0 . 1 � of prompt photons within ∆ R = p ⊥ > 20 GeV | η | < 2 . 7 5 5 Stefan Richter
Lepton selection Reconstructed Lepton identification Electrons: electromagnetic calorimeter deposits + tracking info Muons: tracking and/or muon spectrometer info, calorimeter signature consistent with muon p ⊥ > 20 GeV | η | < 2 . 47 (electrons) or 2 . 7 (muons) Associated with primary vertex Transverse impact parameter significance | d 0 / σ ( d 0 ) | < 5 (electrons) or 3 (muons) Longitudinal impact parameter w.r.t. primary vertex | z 0 sin θ | < 0 . 5 mm Isolated from other tracks/energy deposits 6 6 Stefan Richter
Event selection Same for fiducial and reconstructed except for some reconstruction quality requirements Exactly 4 leptons in 2 same-flavour opposite-charge pairs ∆ R ℓℓ > 0 . 2 If 4 same-flavour leptons, form pairs such that | m 12 − m Z | + | m 34 − m Z | is minimised Z candidate selection: 66 GeV < m 12 , m 34 < 116 GeV In reconstructed: single-muon or dielectron trigger matched by selected leptons, hard-scatering vertex, and at most 1 muon without inner-detector or muon-system track ( standalone , calorimeter-tagged ) 7 7 Stefan Richter
Candidate event (dilepton masses 95 and 88 GeV)
Backgrounds Cambridge Two types of backgrounds, Irreducible and Fake leptons Irreducible backgrounds have four genuine leptons Triboson processes (ZZZ, WZZ, etc) ZZ → [4 τ , 2 τ 2l] tu Z � Well modelled in MC 9 9 Jonatan Rosten
Fake lepton backgrounds Cambridge Fake lepton backgrounds: jets can be misidentified as leptons One or two identified leptons might be jets � • Not modelled well in MC, use data driven “fake factor” method � • Equivalent to the matrix method, except no leptons faking jets 10 10 Jonatan Rosten
Fake factor Cambridge Control region of leptons with inverted definition cuts Lepton-like Jet-like Electrons Pass ID and ISO cut Fail ID xor ISO cut Muons Pass d0 and ISO cut Fail d0 or ISO cut Assumption: Three lepton events are from Z+fake leptons (except ZZ, WZ) � Go through data, find Z+lepton events, save info on jet-like and lepton-like leptons 11 11 Jonatan Rosten
Data driven background Cambridge F mis-ID = L J N misid. leptons � N ``` j − N ``` j � � N `` jj − N `` j j � × F 2 = × F mis-ID − mis-ID bkg ZZ ZZ Assume fake rate is the same for second fake Done in p and η bins, for each channel T 12 12 Jonatan Rosten
Background yields Cambridge √ s = 13 TeV, 3.2 fb -1 Channel Total 4 ` ZZ ! 2 ` 2 τ , 4 τ 0 . 07 ± 0 . 02 ZZZ , WZZ , WWZ 0 . 17 ± 0 . 05 t¯ t Z 0 . 30 ± 0 . 09 0 . 09 + 1 . 08 Data driven Background with 1–2 − 0 . 04 0 . 62 + 1 . 08 Total − 0 . 11 13 13 Jonatan Rosten
Yields Cambridge In 2015, LHC delivered 3.2 ± 0.2 fb -1 of useful √ s = 13 TeV, 25 ns data Channel 4 e 2 e 2 µ 4 µ Total 4 ` Observed 15 ± 29 18 ± 62 ± − 2 . 2 2 . 2 0 . 25 + 0 . 40 0 . 17 + 1 . 00 0 . 62 + 1 . 08 Expected background 0 . 20 ± 0 . 05 − 0 . 05 − 0 . 04 − 0 . 11 14 14 Jonatan Rosten
Dilepton masses (before on-shell requirement) 180 Z candidate mass [GeV] ATLAS -1 160 s = 13 TeV, 3.2 fb Data → 140 ZZ 4l +1.08 Expected background: 0.62 120 -0.11 100 80 T,ll 60 p Leading- 40 Phys. Rev. Let. 116, 101801 (2016) 20 20 40 60 80 100 120 140 160 180 Subleading- Z candidate mass [GeV] p T,ll 15 15 Stefan Richter
Four-lepton mass 18 Events / 20 GeV ATLAS 16 -1 s = 13 TeV, 3.2 fb 14 Data → → q q ZZ 4l 12 → → gg ZZ 4l 10 Prediction uncertainty +1.08 Expected background: 0.62 8 -0.11 Phys. Rev. Let. 116, 101801 (2016) 6 4 2 0 200 300 400 500 600 700 Mass of four-lepton system [GeV] m 4l 16 16 Stefan Richter
Four-lepton p ⊥ 25 Events / 10 GeV ATLAS -1 s = 13 TeV, 3.2 fb 20 Data → → q q ZZ 4l → → gg ZZ 4l 15 Prediction uncertainty +1.08 Expected background: 0.62 -0.11 10 Phys. Rev. Let. 116, 101801 (2016) 5 0 0 50 100 150 200 250 Transverse momentum of four-lepton system [GeV] p T,4l 17 17 Stefan Richter
Four-lepton rapidity 14 Events / 0.2 ATLAS Data → → -1 12 s = 13 TeV, 3.2 fb q q ZZ 4l → → gg ZZ 4l 10 Prediction uncertainty +1.08 Expected background: 0.62 -0.11 8 Phys. Rev. Let. 116, 101801 (2016) 6 4 2 0 − − − 3 2 1 0 1 2 3 Rapidity of four-lepton system y 4l 18 18 Stefan Richter
Correction factor C ZZ Corrects measured cross section for detector effects C ZZ ≡ selected reconstructed events fiducial events Determined using simulated signal samples 4 e 2 e 2µ 4µ C ZZ 0 . 55 ± 0 . 02 0 . 63 ± 0 . 02 0 . 81 ± 0 . 03 Relative uncertainties in %: Source 4 e 2 e 2µ 4µ Statistical 0 . 7 0 . 5 0 . 5 Theory (generator, PDFs) 2 . 5 2 . 5 2 . 5 Experimental efficiencies 2 . 3 2 . 2 2 . 0 Momentum scales and resolutions 0 . 4 0 . 2 0 . 1 Total 3 . 5 3 . 3 3 . 2 19 19 Stefan Richter
Extrapolation factor A ZZ Extrapolates fiducial cross section to total phase space A ZZ ≡ fiducial events on-shell events = 0 . 39 ± 0 . 02 Determined using simulated signal samples Relative uncertainties in %: Source Uncertainty Statistical 0.9 Generator 3.4 Parton shower 0.8 PDFs 0.8 QCD scales 0.3 Total 3.7 20 20 Stefan Richter
Cross section extraction Cambridge N chan exp = σ fid chan L C chan + N chan DD + N chan ZZ Irr ZZ BR chan + N chan N chan exp = σ tot ZZ L C chan ZZ A chan DD + N chan Irr Likelihood model, poisson distributions model for statistical part � Y Pois ( N chan obs , N chan L stat = exp ) � chan Multiplied by Gaussians for systematic uncertainties 21 21 Jonatan Rosten
Cross section results Cambridge O ( α 2 Measurement s ) prediction σ fid 8.4 + 2 . 4 − 2 . 0 (stat.) + 0 . 4 − 0 . 2 (syst.) + 0 . 5 6 . 9 + 0 . 2 − 0 . 3 (lumi.) fb − 0 . 2 fb ZZ ! e + e − e + e − σ fid 14.7 + 2 . 9 − 2 . 5 (stat.) + 0 . 6 − 0 . 4 (syst.) + 0 . 9 13 . 6 + 0 . 4 − 0 . 6 (lumi.) fb − 0 . 4 fb ZZ ! e + e − µ + µ − σ fid 6.8 + 1 . 8 − 1 . 5 (stat.) + 0 . 3 − 0 . 3 (syst.) + 0 . 4 6 . 9 + 0 . 2 − 0 . 3 (lumi.) fb − 0 . 2 fb ZZ ! µ + µ − µ + µ − σ fid 29.7 + 3 . 9 − 3 . 6 (stat.) + 1 . 0 − 0 . 8 (syst.) + 1 . 7 27 . 4 + 0 . 9 − 1 . 3 (lumi.) fb − 0 . 8 fb ZZ ! ` + ` − ` 0 + ` 0− σ tot 16.7 + 2 . 2 − 2 . 0 (stat.) + 0 . 9 − 0 . 7 (syst.) + 1 . 0 15 . 6 + 0 . 4 − 0 . 7 (lumi.) pb − 0 . 4 pb ZZ pp pp ZZ ZZ 4l 4l → → → → ATLAS ATLAS Preliminary Fiducial Fiducial 4e 4e -1 -1 Comparison with s s = 13 TeV, 3.2 fb = 13 TeV, 3.2 fb NNLO Measurement Measurement 2 2 e2 e2 µ µ Tot. uncertainty Tot. uncertainty Stat. uncertainty Stat. uncertainty 2 α α 2 prediction prediction s s 4 µ 4 µ ± ± 1 1 σ σ ± ± 2 2 σ σ Combined Combined Theory: PLB 750 (2015) 407 Theory: PLB 750 (2015) 407 CT10 NNLO CT10 NNLO 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 σ / / σ σ σ 22 data data theory theory 22 Jonatan Rosten
Cross section results Cambridge Total cross section, comparison with NLO 24 [pb] LHC Data 2015 ( s =13 TeV) 22 -1 ATLAS ZZ → llll (m 66-116 GeV) 3.2 fb ATLAS ll 20 LHC Data 2012 ( s =8 TeV) ZZ tot work in progress -1 ATLAS ZZ → ll(ll/ ν ν ) (m 66-116 GeV) 20.3 fb σ 18 ll -1 CMS ZZ → llll (m 66-116 GeV) 19.6 fb ll 16 LHC Data 2011 ( s =7 TeV) -1 ATLAS ZZ → ll(ll/ ν ν ) (m 66-116 GeV) 4.6 fb ll 14 -1 CMS ZZ llll (m 60-120 GeV) 5.0 fb → ll Tevatron Data ( s =1.96 TeV) 12 -1 CDF ZZ → ll(ll/ ν ν ) (on-shell) 9.7 fb -1 10 D0 ZZ → ll(ll/ ν ν ) (m 60-120 GeV) 8.6 fb ll 8 6 MCFM, CT14 NLO ZZ (p p ) 4 ZZ (pp) 2 0 0 2 4 6 8 10 12 14 s [TeV] 23 23 Jonatan Rosten
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