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Correlations Between Neutral Bosons and Jets in Pb+Pb Collisions at 2.76 TeV

with the ATLAS DetectorZvi Citron

for the ATLAS Collaboration

ד" בס

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Introduction• Jet + bosons – the ‘golden channel’ for HI collisions

• Jets undergo energy loss in the medium• Jet + bosons – the ‘golden channel’ for HI collisions

• Jets undergo energy loss in the medium• Electroweak bosons do not

See more on photons at Iwona’s talk 11:00 on 15 Aug, in Parallel 4C!

See more on Z bosons at Jiri’s talk 11:40 on 15 Aug, in Parallel 4C!

• Jet + bosons – the ‘golden channel’ for HI collisions• Jets undergo energy loss in the medium• Electroweak bosons do not• A calibrated probe for jet energy loss!

ATLAS event display showing a Z → μμ + jet event candidate. •Fcal ΣET= 2.14 TeV(10-20%Centrality)•mμμ = 92.5 GeV•pT

Z = 102 GeV •pT

jet (R=0.2) = 46.3 GeV

• Jet + bosons – the ‘golden channel’ for HI collisions

See more on jets:•Martin S Plenary IIA•Aaron Parallel 2B•Martin R Parallel 3B

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• ATLAS has excellent jet, photon, electron, and muon reconstruction using charged tracking + calorimetry/muon spectrometry

•Tracking•Precise tracking and vertexing• coverage: |η|<2.5•B (solenoid) =2T•Pixels (Si): σ = 10 μm [rφ]•80M channels ; 3 layers and 3 disks ;•SCT (106 Si strips ): σ = 17 μm [rφ] •Transition Radiation Tracker

The ATLAS Detector

•Lar-Pb EM calorimeter (|η|<3.2)•e/γ trigger, identification; measurement•Granularity: 0.025x0.025 in Φxη•3 long. layers + presampler(0 <|η|<1.8) \ 180x103 channels

•Hadronic Calorimeter•|η|<1.7: Fe/scint. Tiles (Tilecal) •3.2 <|η|<1.5: Cu-Lar (HEC)•3.1<|η|<4.9: FCAL Cu/W-Lar

•Muon spectrometer (MS)•Air-core toroid magnetic field•Covers up to |η|=2.7 •Triggers•Filtering provided by the calorimeters•Tracking in B field for momentum•Measurement matching with Inner Detector (ID) to improve resolution and vertex capabilities

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Jet Reconstruction• Reconstruction algorithm anti-kt (0.2, 0.3, 0.4)• Input: calorimeter towers 0.1 x 0.1 (Δƞ x Δφ)• Event-by-event background subtraction:

• Anti-kt reconstruction prior to a background subtraction• Underlying event estimated for each longitudinal layer

and ƞ slice separately• Additional iteration step to avoid biasing subtraction from jets• Jets corrected for flow contribution to background• Fake rejection by matching jets to track jets or electron/photon

€

ETsubcell = ET

cell − ρ layer (η) × Acell

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Direct Photon Reconstruction•Subtract underlying event

• Iterative subtraction in Δη=0.1 slices, excluding jets

• Elliptic flow sensitive•Isolated photons

• Cut on a maximum energy in cone around photon

• Fragmentation photons reduced•Shower shape cuts

• Multiple layers of EM calorimeter, and hadronic calorimeter

• Rejection of jet fakes•Signal Extraction

• “Double sideband” method

Isolation E

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Photon – Jet Correlations

• To get at the jet quenching physics, consider:• Opening angle between leading jet and photon, Δφ• Transverse momentum ratio, xjγ=pT

jet/pTγ

• Rjγ = (1/Nγ)dNjγ/dxjγ, fraction of photon events that have a jet

• Form correlation between photon and leading jet with:• pT

jet > 25 GeV, |ηjet|<2.1• 60 < pT

γ < 90 GeV, |ηγ|<1.3• (For xjγ and Rjγ ) Δφ>7/8π, and xjγ>25/60

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Photon – Jet Corrections• Background Subtraction

• Use “double sideband” method to find the background• Subtract appropriately

• Unfold Jet Spectrum• Unfolding matrix for

inclusive jets (SVD) from PYTHIA embeddedinto data

• Apply to single events• pT

jet mapped to different values with differentweights

• Fill xjγ distribution• Photon efficiency

A

C D

BIsolated+tight

Raw xjγ distributions

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Photon – Jet Δφ Distributions

•Δφ between photon and jet (normalized by integral)•Shapes are consistent between data and simulation in all centrality, jet cone size•(R=0.2 jets on top, R=0.3 jets bottom; more central left to right)

40-80% 20-40% 10-20% 0-10%

R=0

.2R

=0.3

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Photon – Jet xjγ Distributions

•Ratio of jet and photon transverse momenta•Normalized per photon•Compare to generated level PYTHIA•Clear difference between data and PYTHIA in more central events•(R=0.2 jets on top, R=0.3 jets bottom; more central from left to right)

40-80% 20-40% 10-20% 0-10%

R=0

.2R

=0.3

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Photon – Jet Summary

Centrality dependent downward shift of <xjγ > (jets more quenched)

Centrality dependent downward shift of Rjγ (lower jet yield)

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Z Boson Reconstruction

•Z → ee• ET >20 GeV, |η|<2.5• Subtract underlying event energy from each electron• Background ~5%

•Z → μμ• pT > 10 GeV, |η|<2.7• Background ~1%

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Z Boson – Jet Correlations

• Similar to photon – jet analysis• Lower statistics • Higher purity

• Form correlation between Z boson and leading jet with:• pT

jet > 25 GeV, |ηjet|<2.1• pT

Z > 60 GeV• Δφ>1/2π, and xjZ>25/60

• Bin-by-bin unfolding of jet pT spectrum• Background contamination negligible

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Z Boson – Jet Results

•Ratio of jet and Z boson transverse momenta•Normalized per Z boson•Inset Δφ distribution, normalized to unity•Low statistics but data distributions in the momentum ratio are different from PYTHIA null hypothesis

R=0.2 R=0.4

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Z Boson – Jet CentralityR=0.2 R=0.4

0-20

%20

-80%

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Z Boson – Jet Summary

Clear evidence of quenching

Suggestive of increasing suppression with centrality (blue points not independent of black)

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Summary

•ATLAS has measured photon – jet and Z boson – jet correlations in L = 0.15 nb−1 of Pb+Pb @ √SNN=2.76 TeV•A calibrated probe of jet quenching in the medium•Full unfolding of jets in the data, comparison to generated level PYTHIA •Observation of centrality dependent jet quenching•Higher statistics will allow fuller look at the phase space

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Backup Information

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Triggers in Run 2011

Photon (e) triggers are based on LAr For ET>20 GeV, efficiency = 98.1 ± 0.1%

Pair efficiency:99.9 ± 0.1%

>90%

Muon triggers is a combination: L1 trigger with pT>4 GeVHLT trigger with pT>10GeV

95-99% weak centrality dependence

MB triggers: (LAr ET>50GeV) OR (ZDC & track)

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Photon-Jet Effect of Unfolding

No big changes from unfolding

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Z Boson-Jet Effect of Unfolding

Basic physics observable even without unfolding

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Photon – Jet Δφ Summary

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Major Systematic Uncertainties

•Boson purity/background subtraction• 10-20% in photons (ID cuts, isolation cuts, energy scale)• Z boson efficiency energy scale <2%

•Unfolding jet spectrum• <5% for both photons and Z bosons • (Unfolding does NOT introduce ‘new’ physics)

•Jet Energy Scale/Resolution• 3-5% for photons• ~5% for Z bosons