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Recent Advances in Jet Algorithms and Their Applications in High-energy Proton-Antiproton Collisions

This paper discusses the recent advancements in jet algorithms and their applications in high-energy proton-antiproton collisions. Topics include the Tevatron and CDF performance, JetClu and MidPoint algorithms, W/Z/g(+jets) production, W+jet(s) production, and diphoton production. The results validate the quark flavor separation using secondary vertex mass and demonstrate the robustness of the QCD program at the Tevatron.

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Recent Advances in Jet Algorithms and Their Applications in High-energy Proton-Antiproton Collisions

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  1. Alberto Cruz On behalf of the CDF collaboration XXXIV International Symposium on Multiparticle Dynamics

  2. Chicago  Florida Booster CDF DØ Tevatron p source Main Injector Fermilab

  3. Long Term Luminosity Projection (by end FY2009) Base Goal -> 4.4 fb-1 Design -> 8.5 fb-1 Tevatron • proton-antiproton collisions • Main injector • (150 GeV proton storage ring) • antiproton recycler (commissioning) • Electron cooling this year • Operational on June’05 • 40% increase in Luminosity • 36 bunches (396 ns crossing time) Increasing Luminosity: RUN IIa (2001~2005) ~1fb-1 RUN I (1992-95) ~0.1fb-1

  4. Tevatron Performance Recent Luminosity Record of 10.3x1031 sec-1cm-2 (July 16, 2004)

  5. CDF Run II Data • CDF Efficiency > 80% • DAQ runs with 5% to 10% dead time • Rest coming from very careful operation • of detector’s HV due to machine losses • (…to preserve silicon & trackers…) CDF -> ~450 pb-1 on tape

  6. The Jet Algorithm Allows us to “see” the partons (or at least their fingerprints) in the final hadronic state. In proton-antiproton collisions we can occasionally have a “hard” parton-parton scattering resulting in large transverse momentum outgoing partons.

  7. Jet algorithms & physics • Final state partons are revealed through collimated flows of hadrons called jets • Measurements are performed at hadron level & theory is parton level (hadron  parton transition will depend on parton shower modeling) • Precise jet search algorithms necessary to compare with theory and to define hard physics • Natural choice is to use a cone-based algorithm in - space (invariant under longitudinal boost)

  8. Run II -> MidPoint algorithm • Define a list of seeds using CAL towers with E > 1 GeV • Draw a cone of radius R around each seed and form “proto-jet” • Draw new cones around “proto-jets” and iterate until stable cones • Put seed in Midpoint (-) for each pair of proto-jets separated by less than 2R and iterate for stable jets • Merging/Splitting T Cross section calculable in pQCD Arbitrary Rsep parameter still present in pQCD calculation …

  9. Comparison of JetClu and MidPoint for HERWIG MC Comparison of the JetClu to MidPoint cone algorithms Differences between MidPoint and JetClu found to be due to “ratcheting”. JetClu  0.5-2% higher ET jets

  10. W/Z/g(+jets) production: introduction • QCD-wise, are W/Z/g cross sections of interest? • Smaller subset of diagrams, different mix of initial partons • Below is a set of LO diagrams for W/Z and W/Z/g + 1 jet • Inclusive distributions are not affected by jet finding uncertainties • More theoretical work is needed, e.g.: • W inclusive: known at the level of NNLO • W + 1 jet: known at the level of NLO • W + 2, 3, 4 jets: known at the level of LO • (MCFM does proved W + 2 jets at NLO, it just isn’t an event generator)

  11. W+jet(s) Production (JetClu R=0.4) • Background to top and Higgs Physics • Stringent test of pQCD predictions • Test Ground for ME+PS techniques • (Special matching  MLM, CKKW to avoid • double counting on ME+PS interface) Inclusive s (nb) Run I (1.8 TeV): LO: 1.76 NLO: 2.41 NNLO: 2.50 CDF I: 2.380.24 Run II (1.96 TeV): LO: 1.94 NLO: 2.64 NNLO: 2.73 CDF II: 2.640.18 W + 1 parton +PS W+ 2 partons QCD corrections cover this difference. 40% higher than the RUNI result Alpgen + Herwig LO  large uncertainty

  12. W+ jet(s) Production (JetClu R=0.4) ME+PS implementation tested using the Nth jet spectrum in W+Njet events. Dijet Mass in W+2jets 1st jet in W + 1p Energy-scale 2nd jet in W + 2p 4th 3rd

  13. Diphoton Production • General agreement with NLO predictions Data: 2 isolated γs in central region, ET1,2 > 14, 13 GeV • Testing NLO pQCD and resummation methods • Signature of interesting physics • One of main Higgs discovery channels at LHC

  14. γ+heavy flavour production • Probes heavy-quark PDFs • b/c-quark tag based on displaced vertices • Secondary vertex mass discriminates flavour MC templates for b/c & (uds) used to extract b/c fraction in data

  15. γ+heavy flavour production γ+b-quark γ+c-quark Good agreement with LO pQCD within still very large stat. errors Validates quark flavour separation using secondary vertex mass

  16. Summary • Tevatron and CDF are performing well • Data samples already significantly exceed those of Run I • On track for accumulating 4-8 fb-1 by 2009 • Robust QCD program is underway • Jets, photons, W+jets, heavy flavors • Jet energy scale is the dominant systematics – improvements on the way • Heavy flavor identification is working well • Verifying and tuning tools: NLO calculations, Monte Carlo generators, resummation techniques, combining ME with PS • NLO does well for hard aspects • LO + Pythia give reasonable description of W+n jets • We don’t see any discrepancies.

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