# Presentation: 091229

## QCD Results from D0

- Run II D0 Detector
- Run II Jet Physics
- Run II Jet Algorithms
- Jet Energy Scale and Resolution
- Inclusive Jet Cross Section
- Dijet Mass Cross Section
- Summary and Outlook

## Run II D0 Detector

## Run II D0 Detector

- New magnetic inner tracking system
- 2T superconducting solenoid
- 800k channel silicon strip detector
- scintillating fiber tracker
- preshower detectors
- Faster calorimeter readout electronics for shorter bunch spacings
- Upgraded muon system
- new forward muon detectors
- additional scintillation counters in central

- Increased shielding
- Forward proton detector
- New trigger & DAQ system
- 10 kHz accept rate at L1
- 1 kHz accept rate at L2
- 50 Hz accept rate at L3

## Run II Jet Physics

- Tevatron energy increased from 1.8 TeV to 1.96 TeV
- Inclusive jet cross section increases by 20% at pT=100GeV and by 100% at pT=500GeV
- Tevatron will collect 4-8 fb-1 of data by 2009
- Extend jet measurements to higher jet energies
- Proton structure at large x (~0.5)
- Also there will be improved PDF measurements with W asymmetry and Drell-Yan.
- Searches for new physics
- Tevatron experiments will use Run II jet algorithms
- Better comparison with higher order theoretical calculations

## Run II Jet Algorithms

- Agreed at the Workshop on “QCD and Weak Boson Physics in Run II, 1999”
- Cone Jet Algorithms (Improved Legacy Cone Algorithm)
- Rcone = 0.7 in x
- Recombination in E-scheme
- 4-vector sum with true pT and massive jets
- Seeds + midpoints
- Approximate seedless algorithm
- Infrared and collinear safe
- Split/merge jets with >50% overlap
- kT Jet Algorithms

## Jet Energy Scale

- From the back-to-back +jet events

- Current analyses are for ||<0.5
- Results for ||>0.5 later

- Total error

- Statistical
- error

- |jet|

- |jet|

- Ejet(GeV)

- Ejet(GeV)

- Corr. Factor

- Corr. Factor

- Uncertainty

- Uncertainty

## Jet pT Resolution

- Jet pT Resolution
- From the pT imbalance in the back-to-back dijets

## Inclusive Jet Cross Section

- Test of QCD at the highest energy
- Probe the gluon distribution at large x

## Triggers

- Jet Triggers
- Level 1
- Hardware trigger with x=0.2x0.2 towers
- Fast calorimeter readout
- Coverage ||<2.4
- Ability to prescale the trigger
- Level 2
- Software trigger running on special processor
- 3x3 or 5x5 trigger towers
- Level 3
- Software trigger

- Reconstruction code optimized for speed using precision readout
- No split and merge

## Data Sample

- Run Selection
- Reject runs with
- Not enough events
- Problems in tracking detectors
- Noisy calorimeter
- Hot cells in calorimeter
- L3 Trigger requirement
- Simple cone jet at L3
- pT calculated assuming zvtx=0.
- Offline pT cut to ensure trigger is 100% efficient

- Vertex requirement
- Ntracks for vertex  5
- |zvtx|<50 cm
- Missing Energy requirement
- MET < 0.7*pTmax
- Jet Selection
-  /e rejection
- Fake jets due to noises and hot cells removed

| trigger | pT(GeV/c) |
| --- | --- |
| 25 GeV trig | 60 – 100 |
| 45 GeV trig | 100 – 140 |
| 65 GeV trig | 140 – 180 |
| 95 GeV trig | > 180 |

## Unsmearing

- In order to compare the experimental data and theoretical predictions, the detector resolution has to be taken into account.
- Method
- Assume that the correct inclusive jet cross section can be described by an analytic function (ansatz funciton) fn.
- The ansatz functions are smeared according to the jet pT resolution.

## Unsmearing

- The smeared functions are fitted to the data distribution.
- The ratio of the   unsmeared and the smeared ansatz funtions gives the correction factors.

## Inclusive Jet Cross Section

- Error dominated
- by current energy
- scale error

- Agreement in 7 order of magnitudes!

- Current result
- Is consistent
- with both PDFs.

## Dijet Mass Cross Section

- Probe the proton structure at large x
- Searches for new physics (quark compositeness, excited quarks, Z’, W’, and other exotic particles)

## Dijet Mass Resolution

- PYTHIA dijet events smeared using the measured jet pT resolution.
- The original mass compared to the smeared mass.

## Data Sample

- The same data sample as the inclusive jet cross section.
- Dijet mass requirements for triggers to be 100% efficient.

| trigger | Mjj(GeV) |
| --- | --- |
| 25 GeV Trig | 150 - 180 |
| 45 GeV Trig | 180 – 300 |
| 65 GeV Trig | 300 – 390 |
| 95 GeV Trig | 390 - 1400 |

## Event Display

- Highest two-jet mass event
- with Mjj = 926 GeV

## Unsmearing

- The same method as in the inclusive jet cross section
- The ansatz function:

## Dijet Mass Cross Section

- Good agreement
- Between data and
- QCD predictions

## Summary and Outlook

- Preliminary results on the inclusive jet pT cross section and the dijet mass cross section are presented for ||<0.5.
- Analyses with the increased data sample (>120pb-1) will be presented in the winter conferences.
-   coverage will extend to 2.4.
- The highest Mjj will reach ~1.3 TeV.
- Some of other planned analyses
- Inclusive cross section ratio in 
- tests of QCD with less systematic uncertainties.
- Cross section measurements with kT algorithm.
- Three jet cross section