# Presentation: 752530

## Jets at the Tevatron

- B. Paul Padley
- Dept of Physics and Astronomy
- Rice University

## Exciting Time

- Obviously we have entered an era of new energy and luminosity.
- Clearly the is a great potential to do new and exciting things
- And the old stuff even looks new and exciting.
- Probing ~10-19 m scale

## Highest mass di-jet event  so far

- ET = 666 GeV
-  =  0.43

- (corrected mass = 1364 GeV/c2):

- ET = 633 GeV
- = -0.19

- r-view

## D0 Highest Dijet Mass Event

- 1206 GeV/c2 dijet mass event

## Clusters

- Use Run II cone algorithm
- Combine particles in a R=0.7 cone
- Use the four vector of every tower as a seed
- Rerun using the midpoints between pairs of jets as seeds
- Overlapping jets merged if the overlap area contains more than 50% of lower Pt jet, otherwise particles assigned to nearest jet.

- Cf. Blazey et al. Hep-ex/0005012v2

##  Jets

**Both groups are using E-scheme recombination**

- Cf. Blazey et al. Hep-ex/0005012v2

## Inclusive Jet Cross Section

** ****Extending Run 1 reach by ~150 GeV    **
**Run II Data shown taken between Feb 2002 and summer 2003**
**(They are planning on new results using Kt algorithm in May)**
**      **
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## Inclusive Jet Cross Section

**Excitement about Run 1 high E**T** excess**
**SM explanation: gluon PDF not well constrained at high x**

- Use EKS pQCD
- _[http://zebu.uoregon.edu/~soper/EKSJets/jet.html](http://zebu.uoregon.edu/~soper/EKSJets/jet.html)_

## Inclusive Jet Cross Section Ratio

- Both plots the same with extra point on the lower

## Inclusive Jet Cross Section

**Notice forward jets**

## Inclusive Jet Cross Section

## Inclusive Jet Cross Section

- JES uncertainty <7% dominant contributor to systematic uncertainties

## Dijet Cross Section

## Dijet Cross Section

## Dijet Decorrelations

- In leading order pQCD dijets are back to back

- Jet 1

- Jet 2

****

## Dijet Decorrelations

- Three jet events in leading order pQCD
- As Kt goes to 0,  goes to 
- If Kt is large

- Jet 1

- Jet 2

****

- Jet 3

- Kt

******distribution is directly sensitive to QCD higher order radiation without explicitly measuring the third jet**

## Dijet Decorrelations

## Decorrelations, Compare to LO

## Decorrelations, Compare to LO

- Same plots as previous page, just spread out for print visibility

**Poor fit to data at low Pt. **
**At higher Pt’s it gets better, especially in the 2.3 to 2.8 regime**

## Dijet Decorrelations

**Dijet ********Decorrelations**

## Dijet Decorrelations

- Pythia Tune A reproduces the data best.

## Underlying Event Studies 

**“****Back-to-Back”**
**(********12****>150****o****,ETj****2****/ETj****1****>0.8)**

**Transverse regions are sensitive to underlying event**

## Underlying Event Studies 

-  Direction of leading jet to define 3 ****** **regions

**Min-Bias**
**0.25 per unit ********-******

-  Run 2

##  Underlying Event Studies (2)

**Shows the **_average charged particle density_**, dN/d****d****, in the ****“transverse” region**** (p****T**** > 0.5 GeV/c, |****| < 1) versus E****T****(jet#1) for ****“Leading Jet”**** and ****“Back-to-Back”**** events compared with ****PYTHIA Tune A**** and ****HERWIG**** after CDFSIM.**

- Current MC with default parameters fail to reproduce data.
- In Run 1 PYTHIA 6.206 tuned adding multi-parton interactions.

-  Run 2 data:

##  Underlying Event Studies (3)

** ****Underlying Event Studies (3)**

- PYTHIA tuned (on Run 1 data) reproduces Run 2 data well
- HERWIG (with no multi-parton interaction) works only at high E**T****j1**.

- _Average charged PTsum density_**, dPT/d****d****, in the ****“transverse” region**** **
**     ****(p****T**** > 0.5 GeV/c, |****| < 1) versus E****T****(jet#1) for ****“Leading Jet”**** and ****“****Back-to-Back”**** events compared with ****PYTHIA**** Tune A and ****HERWIG**** after CDFSIM.**

## Conclusions

- First results have been shown – no surprises
- Highest mass DY events have been observed
- Jet Energy Scale is dominant error in inclusive cross sections
- Pythia Tune A (by Rick Field) on the Run 1 data is reproducing soft data well.