Gideon BellaTel Aviv University

On behalf of the ATLAS collaboration

ATL-PHYS-PUB-2008-002ATL-PHYS-PUB-2009-078

Prospects of measuring ZZ and WZ polarization with ATLAS

Motivation

Gauge bosons are mostly produced singly, in electron-positron or quark-antiquark annihilation with transversal helicities.

Longitudinal helicity of W and Z is possible only in di-boson production or in top-decay.

The longitudinal helicity states are related to the spontaneous symmetry breaking of SU(2) x U(1).

Longitudinal W helicity has been observed so far in e⁺e⁻→W⁺W⁻ and in top decays. Longitudinal Z helicity has

not been observed.Polarization in WW and ZZ events can help in separating a

Higgs signal from di-boson continuum. Deviation from the SM expectation is an evidence for new

physics, e.g. anomalous triple gauge couplings.

4 September 20092 G. Bella DSPIN-09

Di-Boson production in lowest order

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W,Z

W,Z

(‘) (‘)

W, Z, γ

W,Z

W,Z

No ZZ in SM!

We consider only purely leptonic decays of W and Z

Angular variables (e.g. for W Z)⁻

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¯

* *,

qq

θ*q q

W⁻

Z

rest frame

ℓ¯

ν

* *, W⁻ rest frame

ℓ⁻

ℓ

Z rest frame

+

¯

Decay angle dependence

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For W:

For Z:

with:

- transversal (left-handed) helicity

- transversal (right-handed) helicity

- longitudinal helicity

2 2* * 2 ** 00

1 3 3 31 cos 1 cos sin

cos 8 8 4

d

d

*00

2 * * 2 * 2 **

1 3 3 31 2 cos cos 1 2 cos cos sin

cos 8 8 4

dA A

d

2 22 / l lA v a v a

00}spin-density matrix

elements

00 1

Decay angle distributions in MC

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Use high statistics generator level MC of pp at 14 TeV(Pythia, without detector effects)

ZZ WZ

Atlas Detector at LHC

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Weight: ~7000 tons~10 ⁸ electronic channels~3000 km of cables

2800 physicists

Lepton measurementElectrons: Muons:

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Triggered at first level by EM calorimeter Measured by inner detector

(η,ϕ) and EM calorimeter (E) at |η|<2.5 Energy resolution at 100 GeV:

Triggered at first level by MS (Muon Spectrometer)

Measured by MS and inner detector at |η|<2.5

Energy resolution vs. pT:

Event Selection (100 fb ¹ @ 14 TeV)⁻

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ZZWZσ·BR [pb] (from MC@NLO)0.0670.69

selection cuts:• 4 ℓ’s with PT > 7 GeV, and |η| < 2.5• ≥2 ℓ’s with PT > 20 GeV• |M(ℓℓ)-91.2| < 12 GeV

• 3 ℓ’s with PT > 7 GeV, and |η| < 2.5• Missing ET > 25 GeV• |M(ℓℓ)-91.2| < 12 GeV• ≤1 jet, PT < 30 GeV• 50 < MT(ℓν) < 90 GeV• ∢(ℓν)T > 40⁰

events selected:21942873

efficiency3.3%0.40%

backgr. sourcestt, Zbbtt, WW, ZZ, Z+jet, Z𝛾backgr. level<1%1%

Event reconstruction

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All 4 leptons measured ZZ

WZ 3 charged leptons measured. PT(ν) is the missing ET of the event. PL(ν) can be calculated from W-mass constraint – a quadratic equation with 2 solutions. We used the weighted mean of both solutions, with the theoretical SM cross-section used as the weight.

Resolution in WZ reconstruction

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WZ invariant mass - ŝ W⁻ decay angle

Extraction of ρ₀₀ from cosθ*ℓ distribution

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ZZ Use projection operators: ΛΤ = Λ₋₋+Λ₊₊ = 5cos²θ*ℓ - 1 ΛL = Λ₀₀ = 2 - 5cos²θ*ℓ

00 det 00

1

1 Nj i

i

CN

The sum is over all data events (N) in a given ŝ bin.

is the detector correction factor, calculated from MC for bin j in ŝ and |cosθ*ℓ |,

populated by event i (4x10 bins).

detjC

Extraction of ρ₀₀ from cosθ*ℓ distribution

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Decay angle distributions

Z in W⁻Z WZW⁻ W⁺

Use event-by-event ML fit to the theoretical distribution, corrected for detector effects, to obtain

ρ₀₀ in 3 different ŝ bins for Z, W⁻, W⁺.

Systematic effects

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• Parton Distribution Functions. for ZZ – use different sets (CTEQ, EHLQ2, MRST) for WZ – use 40 CTEQ6M error sets. • (for WZ only) Theoretical cross-section used for the weighting of the two solutions from the kinematic fit. Use cross-section with anomalous couplings within the Tevatron limits instead of the SM one (dominating).

• MC statistics

Total systematic errors are typically (20 – 50)% of the statistical ones.

Results

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ZZ WZ

ATLASPreliminary

Conclusions

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Z and W polarization in ZZ and WZ events can be measured, but needs at least 100 fb ¹ ⁻(not for next year…)

Dependence on ŝ can be observed for ZZ.Fraction of longitudinal polarization is

expected to drop with increasing energy

(W and Z become more “photon like”). Error is dominated by statistics. Better accuracy is expected at the super-LHC

Backup

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ZZ – detector correction

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ZZ – PDF systematics

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ZZ - results

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WZ – detector correction factors

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Z

W⁻

W⁺

WZ – systematic errors

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WZ – final results

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