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Heavy Ion Jet Physics with ATLAS

Heavy Ion Jet Physics with ATLAS. Wolf G. Holzmann. 23 rd Winter Workshop In Nuclear Dynamics Big Sky, Montana, February 11-17, 2007. M. Baker, R. Debbe, A. Moraes, R. Nouicer, P. Steinberg, H. Takai, F. Videbaek, S. White Brookhaven National Laboratory, USA.

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Heavy Ion Jet Physics with ATLAS

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  1. Heavy Ion Jet Physics with ATLAS Wolf G. Holzmann 23rd Winter Workshop In Nuclear Dynamics Big Sky, Montana, February 11-17, 2007

  2. M. Baker, R. Debbe, A. Moraes, R. Nouicer, P. Steinberg, H. Takai, F. Videbaek, S. WhiteBrookhaven National Laboratory, USA J. Dolejsi, M. SpoustaCharles University, Prague ★ ★ ★ ★ A. Angerami, B. Cole, N. Grau, W. Holzmann, M. LelchoukColumbia Unversity, Nevis Laboratories, USA ★ ★ ★ ★ ★ L. RosseletUniversity of Geneva, Switzerland A. DenisovIHEP, Russia A. Olszewski, B. Toczek, A. Trzupek, B. Wosiek, K. WozniakIFJ PAN, Krakow, Poland J. Hill, A. Lebedev, M. RosatiIowa State University, USA V. PozdnyakovJINR, Dubna, Russia S. TimoshenkoMePHI, Moscow, Russia P. Chung, J. Jia, R. Lacey, N N.. AjitanandChemistry Department, Stony Brook University, USA G. Atoian, V. Issakov, H. Kasper, A. Poblaguev, M. ZellerYale University, USA ATLAS HI Working Group

  3. Heavy Ion Physics at the LHC Phase Diagram for Nuclear Matter Pb+Pb collisions at the LHC will produce partonic matter at unprecedented T and  Will allow for detailed study and characterization of this high energy density partonic matter. Study evolution from RHIC -> LHC energies. ATLAS will target a comprehensive set of key observables (see Nathan Grau’s ATLAS overview talk) Here, I will exclusively focus on jet tomography.

  4. Jets as a tomographic probe of the medium Gyulassy et al., nucl-th/0302077 Jets in h+h collisions Jets in HI collisions Fragmentation: Fragmentation: Jet modification sensitive to gluon densities, path length, …. Jets as Tomographic Probes of the Medium!

  5. Jet tomography at RHIC STAR, PRL 93 (2004) 252301 Jets studied statistically via singles yields and correlations… RAA interm. pT correlations -h correlations high pT correlations Qualitatively successful, but quantitative interpretation difficult…

  6. Jet tomography at RHIC RAA not really constraining E-loss models? T. Renk, hep-ph/0607166 Correlation studies complicated by trigger bias effects? -h correlations suffer from statistics Plus no real fragment. function measurements, etc…

  7. Jet tomography at LHC Truly high pT jets will be produced copiously in Pb+Pb collisions at the LHC How can jet studies at the LHC improve on the situation? Can (and will) do RHIC type studies with better statistics Can (and will) do high pT jet reconstruction (event-by-event jet tomography, frag. functions, jet structure…) Why would you want to do this with ATLAS?

  8. The ATLAS Calorimeter ATLAS Calorimetery Hadronic Barrel EM Barrel Forward Finely segmented calorimeter coverage over full range and large  range EM EndCap Hadronic EndCap

  9. Measuring Jets in The ATLAS Calorimeter (Di)jets from PYTHIA in Calorimter Towers embedded in HIJING event Energetic jets clearly visible over the heavy ion background Large  coverage is important

  10. Segmentation of first EM sampling layer so fine that heavy ion background is ~ negligible (unique at LHC) Fine  -> rejection of neutral hadron decays Clean 1st sampling-> prompt  isolation Taking a closer look Jet Jet All too wide for single photons Background Back ground  x  = 0.0028 x 0.1

  11. Two Approaches to Jet Reconstruction in ATLAS First approach: use standard p+p cone algorithm with background subtraction A) Seeded Cone Algorithm Original cells Cloned cells Original towers Layer-by-layer subtraction (exclude seeds) Subtracted cells Currently also looking at methods to improve algorithm: seed selection, background subtraction, … New towers Reconstructed jets

  12. Jet Energy Resolution with Seeded Cone Algorithm Study of different event samples embeddedinto central Pb+Pb HIJING (b=0-2 fm) Results obtained from standard p+p cone algorithm w/ backgr.- subtraction Some recalibration still needed.

  13. Can we control the flowing background? Yes! Can measure dN/dϕ in different layers (and sections) of calorimeters e.g. EM Barrel η η η η Layer 1 Layer 2 Layer 3 Presampler ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ

  14. Two Approaches to Jet Reconstruction in ATLAS B) KT Algorithm clusters particles close in phase-space: dij = min(k2ti,k2tj)R2 , where R2=(i-j)2+(i-j)2 diB = k2ti Kt algorithm purposefully mimics a walk backwards along the fragmentation chain for all possible combinations: O(N3) Cacciari et al: “Fast” Kt optimization to O(NlogN)

  15. How fast is fast? M. Cacciari et al, hep-ph/0512210 “Fast” Kt algorithm outperforms cone algorithm, Becomes feasible in heavy ion environment!

  16. “Fast” Kt Finder: Discriminating Jets and Background Real Jets appear as narrow towers “Fake” Jets appear flat and broad Usejet topology to discriminate between jets and background!

  17. Discriminating Jets and Background: A First Look E T,max = maximum ET in calo cell <E T > = average ET in calo cell 1 2 3 4 1 3 4 2 Initial look seems promising. Other variables can also be constructed.

  18.  +Jet in ATLAS PYTHIA  + jet (75 GeV) superimposed on b=4 fm HIJING Pb+Pb event, full GEANT  Jet

  19. Background subtracted   +Jet in ATLAS PYTHIA  + jet (75 GeV) superimposed on b=4 fm HIJING Pb+Pb event, full GEANT  Jet

  20. +Jet in ATLAS  Δη×Δϕ = 0.003x0.1 One (of 64) rows in barrel EM calorimeter 1st sampling layer EM Layer 1 ET (GeV)  Isolated photon gives clean signal in EM first sampling layer Even in central Pb+Pb !

  21. Photon bremsstrahlung in jet cone? Direct  triggered angular correlations energy calibrated: - jet studies - mach cone studies +Jet in ATLAS Many interesting possibilities: let your imagination run wild :-)

  22. Summary and Outlook Jet modification studies at the LHC hold much potential for quantitative tomography of the partonic medium ATLAS is uniquely positioned to perform key jet measurements well Lots of ground work on jet reconstruction in heavy ion environment (seeded cone algorithm, fast Kt algorithm, different background subtraction schemes, etc…) being done in ATLAS Studies shown only an “amuse gueule” expect much more, soon New collaborators are welcome!

  23. Backup Slides

  24. Jet Position Resolution with Seeded Cone Algorithm Resolutions in f and h for <ET>~50 GeV Results obtained from standard p+p cone algorithm w/ backgr.- subtraction Some recalibration still needed.

  25. Segmentation of first EM sampling layer so fine that heavy ion background is ~ negligible Fine  -> rejection of neutral hadron decays Clean 1st sampling-> prompt  isolation The ATLAS Calorimeter Jet Jet All too wide for single photons Background Back ground  x  = 0.0028 x 0.1

  26. The ATLAS Calorimeter Δη×Δϕ in LAr Barrel: Layer 1: 0.003x0.1Layer 2: 0.025x0.025 Layer 3: 0.05x0.025 Finely segmented calorimeter coverage over full range and large  range

  27. Advantages of “Fast” Kt Algorithm Infrared and collinear safe Exceptionally suited to study jet sub-structure: - modification of jet topology in Pb+Pb - hard radiation within the jet New ways to distinguish jets and background Systematic cross-check to cone algorithm

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