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Jet Physics at the Tevatron
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  1. Jet Physics at the Tevatron Lee Sawyer Louisiana Tech University On Behalf of the CDF & D0 Collaborations

  2. Jet Physics at the Tevatron • Why do QCD at the Tevatron? • The Experiments • From Detector Signals to Partons • Some Results • From CDF: • Jet cross-sections: Cone jets and kT jets • Jet Shapes • From DØ: • Inclusive jet cross-section • Dijet Cross-Section • Dijet Azimuthal Decorrelations "Jet Physics at the Tevatron"

  3. The Fermilab Tevatron • pp Collider at √s = 1.96 TeV (increased from 1.8 TeV in Run I) • Main Injector replaces the old “Main Ring” • Other improvements • p source • Recycler • electron cooling aimed at improving p beam lifetime, increase luminosity. • Increasing luminosity: • Run I (1992-95) ~0.1 fb-1 • Run IIa (2001~2005) ~1 fb-1 • Run IIb (2006-2009) ~4-8 fb-1 "Jet Physics at the Tevatron"

  4. Tevatron Performance Instantaneous luminosities approaching 1032 cm-2 s-1 On 16 July, store# 3657 B0Lum 110.16 x 1030 cm-2 s-1D0Lum 91.32 x 1030 cm-2 s-1 Integrated luminosity around 400 pb-1 per experiment. Should reach 1fb-1 by 2005 shutdown. "Jet Physics at the Tevatron"

  5. The CDF Detector • From Run I • Central Calorimeter • Solenoid • Muon System (with Upgrades) • For Run II • Plug and MiniPlug Calorimeters • TOF and central Drift Chamber • Silicon Microstrip Tracker • Forward Muon Detectors => Calorimeter coverage extended (|h|<3.6) while maintaining excellent tracking and vertex resolution. "Jet Physics at the Tevatron"

  6. CDF Data Taking Performance Around 450 pb-1 on tape Efficiency > 80% "Jet Physics at the Tevatron"

  7. The DØ Detector "Jet Physics at the Tevatron"

  8. DØ Data Taking Performance Around 400 pb-1 recorded Efficiency regularly above 85%. "Jet Physics at the Tevatron"

  9. Inclusive jet spectrum x2 QCD at the Tevatron • Higher √s means higher cross-sections • Probe proton structure at large x • Test pQCD with increased statistics • Search for high mass states (Z’, W’, compsiteness, etc.) • QCD signals form the primary background to most of the other measurements at the Tevatron "Jet Physics at the Tevatron"

  10. kT jet Cone jet What Is a Jet? • (For details on jet algorithms, see talk by Bernard Andrieu.) • DØ Run II cone algorthim w/ Rcone=0.7, pTmin=8 GeV/c, f=50% • Any “particle” (MC, cal tower, etc) used as a seed. • Make a cone in DR=√(Dh)2+(Df)2 < Rconearound seed • Add particle 4-vectors in cone => “proto-jet” • Draw new cone around proto-jet, iterate until stable solution found => cone axis = jet axis • Remove proto-jets w/ pT,<pTmin. • Merge jets if more than overlap fraction f of pTjet is contained in the overlap region; otherwise split jets. • Use midpoints between pairs of jets as seeds. • CDF JETCLU algorthim w/ Rcone=0.7 • Adds ET’s of cluster’s in cone (“Snowmass”) • Does not use midpoints between pairs of jets as seeds. • kT algorthim • Not a “fixed cone” algorithm • Use relative momenta of particles, merge by pairs. • Dmin = min(pT2(i),pT2(j)) DRi,j/D • D = Jet Size parameter. "Jet Physics at the Tevatron"

  11. “calorimeter jet” CH hadrons FH  EM “particle jet” Time “parton jet” From Detector Signals to Partons • We do not see quarks and gluons • We do not really see p,K,p,g,etc • How do we go from calorimeter ADC counts to p of the outgoing partons? => Jet Energy Scale • Factors impacting the JES include • Energy Offset (i.e. energy not from the hard scattering process) • Detector Response • For DØ, EM energy scale determined from Z→ee. Use pT balance in g+jets, measured linearity corrections from calorimeter calibration. Extrapolate for very high pT • Out-of-Cone showering • Resolution => Unsmearing • Energy Scale uncertainties typically are the largest systematic errors in jet measurements. "Jet Physics at the Tevatron"

  12. Inclusive Jet Cross-Section Extends the Run I CDF measurement by approx. 150 GeV Run I/Run II comparison plot includes 3% energy scale uncertainty band. Luminosity uncertainites not included. "Jet Physics at the Tevatron"

  13. Inclusive Jet Cross-Section as a Function of Rapidity "Jet Physics at the Tevatron"

  14. Inclusive Jet Cross-Section as a Function of Rapidity Increased uncertainty in PDFs in forward region Good agreement between theory and data at all rapidities "Jet Physics at the Tevatron"

  15. KT Jet Cross-Section • CDF measures inclusive jet production using the KT algorithm • Jets in the region 0.1 < |Y| < 0.7 and • PT > 72 GeV/c. • Results based on 145 pb-1 "Jet Physics at the Tevatron"

  16. KT Cross-Section vs D Data diverges from NLO prediction as D gets large, due to soft contributions "Jet Physics at the Tevatron"

  17. Dijet Cross-Section • DØ measures the cross-section for dijet production in three rapidity bins • 0<Y<0.5 • 1.5<Y<2.0 • 2.0<Y<2.4 • ds/dMjj measured for central rapidities • Good agreement between data and NLO pQCD "Jet Physics at the Tevatron"

  18. Dijet Cross-Section Data/Theory comparison for central rapidities "Jet Physics at the Tevatron"

  19. Dijet Mass Spectrum "Jet Physics at the Tevatron"

  20. Nifty Pictures I: Highest Mass Dijet Event From DØ "Jet Physics at the Tevatron"

  21. Dijet Mass = 1364 GeV (probing distance ~10-19 m) ET = 666 GeV h= 0.43 CDF (f-r view) ET = 633 GeV h = -0.19 Nifty Pictures II: Highest Mass Dijet Event From CDF "Jet Physics at the Tevatron"

  22. Dijet Azimuthal Decorrelations • Jet separation in f is sensitive to final state radiation. • At LO, Df=p. • At higher order, a hard third jet (k┴>0) leads to Df<p. • Measuring the dijet Df spectrum tests O(as3) predictions • No need to explicitly measure third or greater jet "Jet Physics at the Tevatron"

  23. Dijet Azimuthal Decorrelations • Use central inclusive dijet sample • Data binned in pT of the leading jet • Normalize cross-section for each pT-bin to inclusive cross-section • Only look at Df>p/2 to avoid overlap region between jets • Hard leading jets have pT spectra more sharply peaked near p. "Jet Physics at the Tevatron"

  24. Dijet Df Comparisons • Comparison to fixed-order pQCD predictions • Leading order (dashed blue curve) • Divergence at ΔΦ =  (need soft processes) • No phase-space at ΔΦ<2/3 (only three partons) • Next-to-leading order (red curve) • Good description over the whole range, except in extreme ΔΦ regions "Jet Physics at the Tevatron"

  25. Dijet Df Comparisons • Comparison to Monte Carlo predictions • Herwig 6.505 (default) • Good overall description! • Slightly too high in mid-range • Pythia 6.223 (dash line=default) • Very different shape • Too steep dependence • Underestimates low ΔΦ • Vary PARP(67) = 1 → 4 • Varies ISR • Radiation starts for Q*PARP(67) • With more ISR, closer agreement to data. "Jet Physics at the Tevatron"

  26. Jet Shapes • CDF looks at the fraction of a jet ET within a subcone • Define Y = ET(r)/ET(R) • Energy flow variable • Sensitive to multiple gluon emission from the primary parton • Also sensitive to underlying event. "Jet Physics at the Tevatron"

  27. Jet Shapes Study uses Midpoint Algorithm w/ R=0.7. Y is corrected to the hadron level. Central jets: Low pT High pT "Jet Physics at the Tevatron"

  28. Jet Shape vs pT "Jet Physics at the Tevatron"

  29. Lagniappe • In addition to the study of high pT QCD, there is a rich program of diffractive studies and elastic scattering measurements at both experiments • About 40% of pp total cross-section is elastic or diffractive. • Portions of upgrades designed to enhance this capability • See talk by Mary Convery on Saturday for “Diffractive Results from CDF” • Want to mention a few low pT results from DØ "Jet Physics at the Tevatron"

  30. DØ Detector Details • In addition to Calorimeter, can tag interaction with luminosity monitors near beampipe. • Forward Proton Detector added to elastic scattering measurements • Series of 18 Roman Pots arranged in 9 spectrometers • Now fully commissioned and part of the DØ readout. "Jet Physics at the Tevatron"

  31. Diffractive Z Production • Define a “rapidity gap” between calorimeter and luminosity monitors • In Run I, identified a handful of events consistent with W→en and Z→ee with associated rapidity gap • In Run II, have looked for Z→mm plus forward rapidity gap. "Jet Physics at the Tevatron"

  32. Diffractive Z Production “North” = Negative Rapidities “South” = Positive Rapidities Evidence for diffractive Z production, mass consistent. More work to be done (backgrounds, efficiencies,..) "Jet Physics at the Tevatron"

  33. Elastic Scattering • First results from Forward Proton Detector • Measurement of ξ = Fraction of proton longitudinal momentum lost in the scattering "Jet Physics at the Tevatron"

  34. Conclusions • Rich range of QCD topics to be pursued in Run II • First results from both experiments show generally good agreement with theory for cross-sections • More detailed comparisons to theory needed for details of event and jet shapes. • First DØ being produced for diffractive and elastic physics • Both experiments will be able to explore high pT and Mjj regions over a wide range of rapidities • Test high-x gluon contributions • Look for evidence of new physics "Jet Physics at the Tevatron"