1. April 5, 2003Gregory A. Davis1 Jet Cross Sections From DØ Run II American Physical Society Division of Particles and Fields Philadelphia, PA April 5, 2003

2. Gregory A. Davis2 Motivation We present here two measurements in QCD: the inclusive jet cross section and the dijet cross section These measurements will improve our understanding of proton structure by constraining future PDFs As the highest Q 2 processes we observe, they are obvious places to look for new physics

3. April 5, 2003Gregory A. Davis3 Run II at Fermilab New for Run II Main Injector and Recycler are new Luminosity has increased Bunch crossings are every 396 ns instead of 3500 ns Center of mass energy has increased from 1.8 to 1.96 TeV

4. April 5, 2003Gregory A. Davis4 Jets in Run II

5. April 5, 2003Gregory A. Davis5 The DØ Upgrade The upgrade centers on tracking and triggering New tracking helps the EM scale which helps the jet energy scale However, to the calorimeter, the solenoid is extra material A preshower detector outside the solenoid will compensate for this in the future

6. April 5, 2003Gregory A. Davis6 The Calorimeter Calorimeter retained from Run I Uranium-Liquid Argon Calorimeter stable, uniform response, radiation hard Compensating: e/   1 Uniform hermetic coverage |  |  4.2 recall    ln[tan(  /2)] Longitudinal Segmentation 4 EM Layers (2,2,7,10) X o 4  5 Hadronic Layers (6 ) Transverse Segmentation   0.05  0.05 in EM 3   0.10  0.10 otherwise

7. April 5, 2003Gregory A. Davis7 Our Jet Algorithm We use a four-vector cone algorithm with a radius of 0.7 in  -  space Identify seed tower in the calorimeter Using the event’s vertex, assign a four- vector to that seed Add all other other four-vectors inside R to generate the jet’s four-vector If the jet’s four-vector does not line up with the seed’s repeat using the new jet four-vector as the seed Changes from Tevatron Run I We use the midpoints between jets as seeds for new jets We use four-vectors instead of scalar quantities R=0.7  0    The Jet Definition The Jet’s Properties

8. April 5, 2003Gregory A. Davis8 Jet Energy Scale Measured jet energy is corrected to particle level O: offset energy includes calorimeter noise and extra interactions R: calorimeter response includes non-linearities and dead material measured from momentum imbalance in photon + jet events S: showering correction The percentage of energy that is in the cone Uncertainties increases with energy due to extrapolation. Current  + jet statistics extend only to about 200 GeV

9. April 5, 2003Gregory A. Davis9 Jet p T Resolution Jet Momentum Resolution We use this resolution to unsmear our data The Jet Resolution is measured by studying dijet asymmetry

10. April 5, 2003Gregory A. Davis10 A Typical Dijet Event p T = 432 GeV/c p T = 394 GeV/c  = 0  = 4  = -4  = 2   =   = 0 Dijet Mass = 838 GeV Missing E T = 36 GeV Run 163526 Event 12649416

11. April 5, 2003Gregory A. Davis11 Uncorrected Inclusive Jet Cross Section The inclusive cross section has been measured at |  | < 0.5 New cone algorithm for Run II adds four-vectors instead of the scalar E T uses midpoints for IR safety A similar plot exists for the dijet cross section

12. April 5, 2003Gregory A. Davis12 Inclusive Jet Cross Section Event and jet efficiencies are estimated from data 10% normalization uncertainty is not shown The theory is NLO pQCD calculated with JETRAD

13. April 5, 2003Gregory A. Davis13 Inclusive Jet Cross Section

14. April 5, 2003Gregory A. Davis14 Dijet Cross Section Event and jet efficiencies are estimated from data 10% normalization uncertainty is not shown The theory is NLO pQCD calculated with JETRAD Both jets must be central: |  | < 0.5 Dijet Mass Spectrum

15. April 5, 2003Gregory A. Davis15 Dijet Cross Section

16. April 5, 2003Gregory A. Davis16 In Conclusion We have early measurements of two of the standard QCD observables The MRST2001 and CTEQ6M PDFs differ significantly; however, at present both are consistent with our measurements With more work and more statistics, our large energy scale uncertainties will come down We will soon have excellent new measurements

17. April 5, 2003Gregory A. Davis17 Extra Slides

18. April 5, 2003Gregory A. Davis18 Different Jets Our model says the hard interaction is between partons: quarks or gluons. The directly resulting partons constitute the “parton jet” Partons hadronize and turn into observable particles, like  and , which constitute the “particle jet” Our data is the “calorimeter jet” made of energy deposition in the detector We correct to the energy of the particle jets

19. April 5, 2003Gregory A. Davis19 Unsmearing The steeply falling cross section means that we get more jets migrating into each bin than out To unsmear this, we guess an ansatz function, f, for the true cross section and smear it with our jet resolution We fit the smeared ansatz, F, to the data Lastly, we correct our data by the ratio of the ansatz to the smeared ansatz Steeply Falling Spectrum

20. April 5, 2003Gregory A. Davis20 Unsmearing Because the cross section is steeply falling, imperfect jet resolution causes the cross section to shift to the right.

21. April 5, 2003Gregory A. Davis21 DØ Luminosity