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

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Measurement of the ZZ production cross section in pp collisions at √s = 13 TeV with the ATLAS detector

Stefan Richter (University College London, CERN)

n behalf of the ATLAS Collaboration

XXII Epiphany Conference · Cracow, Poland · 7–9 January 2016

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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)

Stefan Richter for ATLAS: ZZ @ 13 TeV

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ZZ production at the LHC

q q Z Z g g Z Z Predicted four-lepton mass spectrum at 8 TeV [1509.07844]:

Stefan Richter for ATLAS: ZZ @ 13 TeV

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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

Towards vector boson scattering:

W± W∓ W

Stefan Richter for ATLAS: ZZ @ 13 TeV

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ZZ → 4ℓ channel

Advantage: extremely clean

tiny background
excellent Z mass resolution

Disadvantage: small cross section

0.45% × 15.6 pb (NNLO) ≈ 70 fb

Stefan Richter for ATLAS: ZZ @ 13 TeV

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σpp→ZZ = Ndata − Nbackground L BRZZ→4` AZZ CZZ

Number of events passing signal selection Predicted number of background events Correction for detector effects Extrapolation from measurement to full phase space Leptonic branching ratio Integrated luminosity

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Fiducial lepton definition

Generator-level

Prompt final-state muons and electrons ‘Dressing’ to account for Bremsstrahlung: add four-momenta

f prompt photons within ∆R =
(∆η)2 + (∆ϕ)2 = 0.1

p⊥ > 20 GeV |η| < 2.7

Stefan Richter for ATLAS: ZZ @ 13 TeV

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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 |d0/σ (d0)| < 3 Longitudinal impact parameter |z0 sin θ | < 0.5 mm (z0 w.r.t. primary vertex)

Isolated from other tracks/energy deposits

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Event selection

Same for fiducial and reconstructed except for some reconstruction quality requirements Exactly 4 leptons in 2 same-flavour, opposite-charge pairs ∆Rℓℓ > 0.2 If 4 same-flavour leptons, form pairs such that |m12 − mZ| + |m34 − mZ| is minimised Z candidate selection: 66 GeV < m12,m34 < 116 GeV

In reconstructed: single-muon or dielectron trigger matched by selected leptons, hard-scattering vertex, and at most 1 muon without inner-detector

r muon-system track (standalone, calorimeter-tagged )

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Considered backgrounds

Four genuine prompt leptons
ZZ → [4τ, 2τ2ℓ] → 4ℓ + neutrinos
ZZZ,WZZ,WWZ
t¯

tZ

— from simulation
1–2 nonprompt or misidentified leptons
{Z, WZ, WW} + jets
t¯

t

...
— data-driven estimate

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Background composition (yields)

Background process Expected events ZZ → 2ℓ2τ , 4τ 0.07 ± 0.02 ZZZ, WZZ, WWZ 0.17 ± 0.05 t¯ tZ 0.30 ± 0.09 1–2 misidentified leptons* 0.09+1.08

−0.04

Total 0.62+1.08

−0.11

* Derived using data-driven method Stefan Richter for ATLAS: ZZ @ 13 TeV

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Yields

Channel 4e 2e2µ 4µ Total 4ℓ Observed 15 30 18 63 Expected background 0.20 ± 0.05 0.25+0.40

−0.05

0.17+1.00

−0.04

0.62+1.08

−0.11

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Correction factor CZZ

Corrects measured cross section for detector effects CZZ ≡ selected reconstructed events fiducial events Determined using simulated signal samples

4e 2e2µ 4µ CZZ 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 Stefan Richter for ATLAS: ZZ @ 13 TeV

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Extrapolation factor AZZ

Extrapolates fiducial cross section to total phase space AZZ ≡ fiducial events

n-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 Stefan Richter for ATLAS: ZZ @ 13 TeV

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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

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Signal kinematics

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Dilepton masses (before on-shell requirement)

Z candidate mass [GeV]

T,ll

p Subleading- 20 40 60 80 100 120 140 160 180 Z candidate mass [GeV]

T,ll

p Leading- 20 40 60 80 100 120 140 160 180

Data 4l → ZZ

ATLAS

1

= 13 TeV, 3.2 fb s

0.11

+1.08

Expected background: 0.62

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Four-lepton mass

[GeV]

4l

m Mass of four-lepton system 200 300 400 500 600 700 Events / 20 GeV 2 4 6 8 10 12 14 16 18

Data 4l → ZZ → q q 4l → ZZ → gg Prediction uncertainty

ATLAS

1

= 13 TeV, 3.2 fb s

0.11

+1.08

Expected background: 0.62 Stefan Richter for ATLAS: ZZ @ 13 TeV

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Four-lepton p⊥

[GeV]

T,4l

p Transverse momentum of four-lepton system 50 100 150 200 250 Events / 10 GeV 5 10 15 20 25

Data 4l → ZZ → q q 4l → ZZ → gg Prediction uncertainty

ATLAS

1

= 13 TeV, 3.2 fb s

0.11

+1.08

Expected background: 0.62 Stefan Richter for ATLAS: ZZ @ 13 TeV

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Four-lepton rapidity

4l

y Rapidity of four-lepton system 3 − 2 − 1 − 1 2 3 Events / 0.2 2 4 6 8 10 12 14

Data 4l → ZZ → q q 4l → ZZ → gg Prediction uncertainty

ATLAS

1

= 13 TeV, 3.2 fb s

0.11

+1.08

Expected background: 0.62 Stefan Richter for ATLAS: ZZ @ 13 TeV

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Results

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Fiducial and total cross sections

Measurement NNLO prediction σ fid

ZZ→e+e−e+e−

8.4 +2.4

−2.0(stat.) +0.4 −0.2(syst.) +0.5 −0.3(lumi.) fb

6.9+0.2

−0.2 fb

σ fid

ZZ→e+e−µ+ µ−

14.7 +2.9

−2.5(stat.) +0.6 −0.4(syst.) +0.9 −0.6(lumi.) fb

13.6+0.4

−0.4 fb

σ fid

ZZ→µ+ µ−µ+ µ−

6.8 +1.8

−1.5(stat.) +0.3 −0.3(syst.) +0.4 −0.3(lumi.) fb

6.9+0.2

−0.2 fb

σ fid

ZZ→ℓ+ ℓ− ℓ′+ ℓ′−

29.7 +3.9

−3.6(stat.) +1.0 −0.8(syst.) +1.7 −1.3(lumi.) fb

27.4+0.9

−0.8 fb

σ tot

ZZ

16.7 +2.2

−2.0(stat.) +0.9 −0.7(syst.) +1.0 −0.7(lumi.) pb

15.6+0.4

−0.4 pb

Theory prediction: [1507.06257] Fiducial cross-section predictions include fiducial acceptance correction for final-state photon radiation (lowers by ∼4%)

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Theory comparison (fiducial)

theory

σ /

data

σ 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

Measurement

Tot. uncertainty
Stat. uncertainty

prediction

2 s

α σ 1 ± σ 2 ± Theory: PLB 750 (2015) 407 CT10 NNLO

Combined 4µ 2 µ e2 4e ATLAS Fiducial 4l → ZZ → pp

1

= 13 TeV, 3.2 fb s

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Total cross section vs. √s

[TeV] s 2 4 6 8 10 12 14 [pb]

tot ZZ

σ 2 4 6 8 10 12 14 16 18 20 22 24

) p ZZ (p ZZ (pp) =13 TeV) s LHC Data 2015 ( =8 TeV) s LHC Data 2012 ( =7 TeV) s LHC Data 2011 ( =1.96 TeV) s Tevatron Data (

1

66-116 GeV) 3.2 fb

ll

llll (m → ATLAS ZZ

1

66-116 GeV) 19.6 fb

ll

llll (m → CMS ZZ

1

66-116 GeV) 4.6 fb

ll

) (m ν ν ll(ll/ → ATLAS ZZ

1

60-120 GeV) 5.0 fb

ll

llll (m → CMS ZZ

1

) (on-shell) 9.7 fb ν ν ll(ll/ → CDF ZZ

1

60-120 GeV) 8.6 fb

ll

) (m ν ν ll(ll/ → D0 ZZ

ATLAS

MCFM, CT14 NLO

Prediction is NLO NNLO not yet ready

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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?

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Backup

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Candidate event (dilepton masses 95 and 88 GeV)

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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]

Stefan Richter for ATLAS: ZZ @ 13 TeV

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Monte Carlo signal samples

Powheg + Pythia 8: ZZ + 0 jets @ NLO
Sherpa: ZZ + 0-1 jets @ NLO + 2-3 jets @ LO
Sherpa: gg → ZZ + 0-1 jets @ LO

Stefan Richter for ATLAS: ZZ @ 13 TeV

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