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Zvi Citron for the ATLAS Collaboration

בס"ד. Correlations Between Neutral Bosons and Jets in Pb+Pb Collisions at 2.76 TeV with the ATLAS Detector. Zvi Citron for the ATLAS Collaboration. Introduction. Jet + bosons – the ‘golden channel’ for HI collisions Jets undergo energy loss in the medium.

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Zvi Citron for the ATLAS Collaboration

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  1. בס"ד Correlations Between Neutral Bosons and Jets in Pb+Pb Collisions at 2.76 TeV with the ATLAS Detector Zvi Citron for the ATLAS Collaboration

  2. 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 • 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! • Jet + bosons – the ‘golden channel’ for HI collisions • See more on jets: • Martin S Plenary IIA • Aaron Parallel 2B • Martin R Parallel 3B • ATLAS event display showing a Z → μμ + jet event candidate. • Fcal ΣET= 2.14 TeV(10-20%Centrality) • mμμ = 92.5 GeV • pTZ = 102 GeV • pTjet (R=0.2) = 46.3 GeV 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!

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

  4. Jet Reconstruction • Reconstruction algorithm anti-kt (0.2, 0.3, 0.4) • Input: calorimeter towers 0.1 x 0.1 (ΔƞxΔφ) • 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

  5. 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

  6. Photon – Jet Correlations • To get at the jet quenching physics, consider: • Opening angle between leading jet and photon, Δφ • Transverse momentum ratio, xjγ=pTjet/pTγ • Rjγ = (1/Nγ)dNjγ/dxjγ, fraction of photon events that have a jet • Form correlation between photon and leading jet with: • pTjet> 25 GeV, |ηjet|<2.1 • 60 < pTγ< 90 GeV, |ηγ|<1.3 • (For xjγ and Rjγ ) Δφ>7/8π, and xjγ>25/60

  7. 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 • pTjetmapped to different values with differentweights • Fill xjγ distribution • Photon efficiency C D Isolated+tight A B Raw xjγ distributions

  8. Photon – Jet Δφ Distributions 40-80% 20-40% 10-20% 0-10% R=0.2 R=0.3 • 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)

  9. Photon – Jet xjγ Distributions 40-80% 20-40% 10-20% 0-10% R=0.2 R=0.3 • 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)

  10. Photon – Jet Summary Centrality dependent downward shift of <xjγ > (jets more quenched) Centrality dependent downward shift of Rjγ (lower jet yield)

  11. 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%

  12. Z Boson – Jet Correlations • Similar to photon – jet analysis • Lower statistics • Higher purity • Form correlation between Z boson and leading jet with: • pTjet> 25 GeV, |ηjet|<2.1 • pTZ> 60 GeV • Δφ>1/2π, and xjZ>25/60 • Bin-by-bin unfolding of jet pT spectrum • Background contamination negligible

  13. Z Boson – Jet Results R=0.2 R=0.4 • 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

  14. Z Boson – Jet Centrality R=0.2 R=0.4 0-20% 20-80%

  15. Z Boson – Jet Summary Clear evidence of quenching Suggestive of increasing suppression with centrality (blue points not independent of black)

  16. 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

  17. Backup Information

  18. 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% Muon triggers is a combination: L1 trigger with pT>4 GeV HLT trigger with pT>10GeV 95-99% weak centrality dependence MB triggers: (LAr ET>50GeV) OR (ZDC & track) >90%

  19. Photon-Jet Effect of Unfolding No big changes from unfolding

  20. Z Boson-Jet Effect of Unfolding Basic physics observable even without unfolding

  21. Photon – Jet Δφ Summary

  22. 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

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