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

2. Zvi Citron 2Introduction 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 ΣE T = 2.14 TeV(10- 20%Centrality) m μμ = 92.5 GeV p T Z = 102 GeV p T 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 See more on jets: Martin S Plenary IIA Aaron Parallel 2B Martin R Parallel 3B

3. Zvi Citron 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 3 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

4. Zvi Citron 4 Jet Reconstruction Reconstruction algorithm anti-k t (0.2, 0.3, 0.4) Input: calorimeter towers 0.1 x 0.1 (Δ ƞ x Δφ) Event-by-event background subtraction: Anti-k t 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. Zvi Citron 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. Zvi Citron 6 Photon – Jet Correlations To get at the jet quenching physics, consider: Opening angle between leading jet and photon, Δφ Transverse momentum ratio, x jγ =p T jet /p T γ R jγ = (1/N γ )dN jγ /dx jγ, fraction of photon events that have a jet Form correlation between photon and leading jet with: p T jet > 25 GeV, |η jet |<2.1 60 < p T γ < 90 GeV, |η γ |<1.3 (For x jγ and R jγ ) Δφ>7/8π, and x jγ >25/60

7. Zvi Citron 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 embedded into data Apply to single events p T jet mapped to different values with different weights Fill x jγ distribution Photon efficiency A CD B Isolated+tight Raw x jγ distributions

8. Zvi Citron 8 Photon – Jet Distributions 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=0.2 R=0.3

9. Zvi Citron 9 Photon – Jet Distributions Photon – Jet x jγ 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=0.2 R=0.3

10. Zvi Citron 10 Photon – Jet Photon – Jet Summary Centrality dependent downward shift of (jets more quenched) Centrality dependent downward shift of R jγ (lower jet yield)

11. Zvi Citron 11 Z Boson Reconstruction Z → ee E T >20 GeV, |η|<2.5 Subtract underlying event energy from each electron Background ~5% Z → μμ p T > 10 GeV, |η|<2.7 Background ~1%

12. Zvi Citron 12 Z Boson – Jet Correlations Similar to photon – jet analysis Lower statistics Higher purity Form correlation between Z boson and leading jet with: p T jet > 25 GeV, |η jet |<2.1 p T Z > 60 GeV Δφ>1/2π, and x jZ >25/60 Bin-by-bin unfolding of jet p T spectrum Background contamination negligible

13. Zvi Citron 13 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=0.2R=0.4

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

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

16. Zvi Citron 16Summary ATLAS has measured photon – jet and Z boson – jet correlations in L = 0.15 nb −1 of Pb+Pb @ √S NN =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. Zvi Citron 17 Backup Information

18. Zvi Citron 18 Triggers in Run 2011 Photon (e) triggers are based on LAr For E T >20 GeV, efficiency = 98.1 ± 0.1% Pair efficiency: 99.9 ± 0.1% >90% Muon triggers is a combination: L1 trigger with p T >4 GeV HLT trigger with p T >10GeV 95-99% weak centrality dependence MB triggers: (LAr E T >50GeV) OR (ZDC & track)

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

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

21. Zvi Citron 21 Photon – Jet Summary Photon – Jet Δφ Summary

22. Zvi Citron 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