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

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

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

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

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

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

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

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

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

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 Event

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

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

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

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

April 5, 2003Gregory A. Davis15 Dijet Cross Section

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

April 5, 2003Gregory A. Davis17 Extra Slides

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

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

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.

April 5, 2003Gregory A. Davis21 DØ Luminosity