Measurement of the ZZ production cross section in pp collisions at s - PowerPoint PPT Presentation

Measurement of the ZZ production cross section in pp collisions at √ s = 13 TeV with the ATLAS detector Stefan Richter (University College London, CERN) on behalf of the ATLAS Collaboration XXII Epiphany Conference · Cracow, Poland · 7–9 January 2016

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 ℓ = e , µ Also extrapolate to ‘total’ phase space and all Z boson decays Leptonically decaying Z is not strictly separable from γ ∗ → “Z” ≡ Z / γ ∗ with mass between 66–116 GeV (CMS uses 60–120 GeV) Paper: [1512.05314] (submitted to PRL) 2 Stefan Richter for ATLAS: ZZ @ 13 TeV

ZZ production at the LHC q g Z Z g q Z Z Predicted four-lepton mass spectrum at 8 TeV [1509.07844]: 3 Stefan Richter for ATLAS: ZZ @ 13 TeV

Why measure ZZ? Standard Model test at 13 TeV Appears in Higgs and new-physics analysis background and/or sidebands Limits on anomalous gauge couplings: Z / γ ∗ red blob vertex forbidden in SM W ± Towards vector boson scattering: W W ∓ 4 Stefan Richter for ATLAS: ZZ @ 13 TeV

ZZ → 4 ℓ channel Advantage: extremely clean • tiny background • excellent Z mass resolution Disadvantage: small cross section • 0 . 45% × 15 . 6 pb (NNLO) ≈ 70 fb 5 Stefan Richter for ATLAS: ZZ @ 13 TeV

Number of events Predicted number of passing signal selection background events σ pp → ZZ = N data − N background L BR ZZ → 4 ` A ZZ C ZZ Integrated luminosity Leptonic Correction for Extrapolation from branching detector effects measurement to ratio full phase space

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 7 Stefan Richter for ATLAS: ZZ @ 13 TeV

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 ) | < 3 Longitudinal impact parameter | z 0 sin θ | < 0 . 5 mm ( z 0 w.r.t. primary vertex) Isolated from other tracks/energy deposits 8 Stefan Richter for ATLAS: ZZ @ 13 TeV

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-scattering vertex, and at most 1 muon without inner-detector or muon-system track ( standalone , calorimeter-tagged ) 9 Stefan Richter for ATLAS: ZZ @ 13 TeV

Considered backgrounds • Four genuine prompt leptons • ZZ → [4 τ , 2 τ 2 ℓ ] → 4 ℓ + neutrinos • ZZZ , WZZ , WWZ • t¯ tZ • — from simulation • 1–2 nonprompt or misidentified leptons • {Z, WZ, WW} + jets • t ¯ t • ... • — data-driven estimate 10 Stefan Richter for ATLAS: ZZ @ 13 TeV

Background composition (yields) Background process Expected events ZZ → 2 ℓ 2 τ , 4 τ 0 . 07 ± 0 . 02 ZZZ, WZZ, WWZ 0 . 17 ± 0 . 05 t ¯ tZ 0 . 30 ± 0 . 09 0 . 09 + 1 . 08 1–2 misidentified leptons* − 0 . 04 0 . 62 + 1 . 08 Total − 0 . 11 * Derived using data-driven method 11 Stefan Richter for ATLAS: ZZ @ 13 TeV

Yields Channel 4e 2e2µ 4µ Total 4 ℓ Observed 15 30 18 63 0 . 25 +0 . 40 0 . 17 +1 . 00 0 . 62 +1 . 08 Expected background 0 . 20 ± 0 . 05 − 0 . 05 − 0 . 04 − 0 . 11 12 Stefan Richter for ATLAS: ZZ @ 13 TeV

Correction factor C ZZ Corrects measured cross section for detector effects C ZZ ≡ selected reconstructed events fiducial events Determined using simulated signal samples 4e 2e2µ 4µ C ZZ 0 . 55 ± 0 . 02 0 . 63 ± 0 . 02 0 . 81 ± 0 . 03 Relative uncertainties in %: Source 4e 2e2µ 4µ Statistical 0 . 7 0 . 5 0 . 5 Theoretical 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 13 Stefan Richter for ATLAS: ZZ @ 13 TeV

Extrapolation factor A ZZ Extrapolates fiducial cross section to total phase space A ZZ ≡ fiducial events on-shell events ≈ 0 . 39 ± 0 . 2 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 14 Stefan Richter for ATLAS: ZZ @ 13 TeV

Cross section extraction Maximum-likelihood fits: • Fiducial per-channel cross sections • Fiducial combined cross section • Total combined cross section Signal and background yields treated as Poisson variables Systematic uncertainties treated as Gaussian nuissance parameters 15 Stefan Richter for ATLAS: ZZ @ 13 TeV

Signal kinematics

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 -0.11 120 100 80 T,ll 60 p Leading- 40 20 20 40 60 80 100 120 140 160 180 Subleading- p Z candidate mass [GeV] T,ll 17 Stefan Richter for ATLAS: ZZ @ 13 TeV

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 6 4 2 0 200 300 400 500 600 700 Mass of four-lepton system m [GeV] 4l 18 Stefan Richter for ATLAS: ZZ @ 13 TeV

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 5 0 0 50 100 150 200 250 Transverse momentum of four-lepton system p [GeV] T,4l 19 Stefan Richter for ATLAS: ZZ @ 13 TeV

Four-lepton rapidity 14 Events / 0.2 ATLAS Data → → 12 -1 q q ZZ 4l s = 13 TeV, 3.2 fb → → gg ZZ 4l 10 Prediction uncertainty +1.08 Expected background: 0.62 -0.11 8 6 4 2 0 − − − 3 2 1 0 1 2 3 Rapidity of four-lepton system y 4l 20 Stefan Richter for ATLAS: ZZ @ 13 TeV

Results

Fiducial and total cross sections Measurement NNLO 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 → ℓ + ℓ − ℓ ′ + ℓ ′− 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 Theory prediction: [1507.06257] Fiducial cross-section predictions include fiducial acceptance correction for final-state photon radiation (lowers by ∼ 4%) 22 Stefan Richter for ATLAS: ZZ @ 13 TeV

Theory comparison (fiducial) → → pp ZZ 4l ATLAS Fiducial 4e -1 s = 13 TeV, 3.2 fb Measurement µ 2 e2 Tot. uncertainty Stat. uncertainty α 2 prediction s 4 µ ± σ 1 ± σ 2 Combined Theory: PLB 750 (2015) 407 CT10 NNLO 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 σ σ / data theory 23 Stefan Richter for ATLAS: ZZ @ 13 TeV

Total cross section vs. √ s 24 [pb] MCFM, CT14 NLO ATLAS 22 ZZ (p p ) LHC Data 2015 ( s =13 TeV) 20 ZZ (pp) ZZ tot → -1 ATLAS ZZ llll (m 66-116 GeV) 3.2 fb σ ll 18 LHC Data 2012 ( s =8 TeV) → -1 CMS ZZ llll (m 66-116 GeV) 19.6 fb 16 ll LHC Data 2011 ( s =7 TeV) → ν ν -1 ATLAS ZZ ll(ll/ ) (m 66-116 GeV) 4.6 fb 14 ll → -1 CMS ZZ llll (m 60-120 GeV) 5.0 fb ll 12 Tevatron Data ( s =1.96 TeV) → ν ν -1 CDF ZZ ll(ll/ ) (on-shell) 9.7 fb → ν ν -1 D0 ZZ ll(ll/ ) (m 60-120 GeV) 8.6 fb 10 ll 8 6 Prediction is NLO 4 NNLO not yet ready 2 0 0 2 4 6 8 10 12 14 s [TeV] 24 Stefan Richter for ATLAS: ZZ @ 13 TeV

Conclusions ZZ production cross section measured at √ s = 13 TeV Total uncertainty ca. 15%, statistically dominated Agreement with NNLO Standard Model prediction Starting to be sensitive to gg-initiated loop-induced production! Future goals with more data: • differential cross sections • limits on anomalous gauge couplings • ... Thank you! Questions? 25 Stefan Richter for ATLAS: ZZ @ 13 TeV

Backup

Candidate event (dilepton masses 95 and 88 GeV)

Theory considerations Double parton scattering ( ∼ 1%) included in measurement but not in prediction NLO corrections to loop-induced process could increase prediction by ∼ 4–5% [1509.06734] NLO- α electroweak corrections could decrease prediction by ∼ 7–8% [1305.5402], [Biedermann, Denner, Dittmaier, Hofer, Jäger; to be submitted] 28 Stefan Richter for ATLAS: ZZ @ 13 TeV