# Presentation: 586392

## First DØ Jet Results From Run II

**Fermilab Tevatron**
- Run I vs. Run II
- Data collection
**DØ Detector and jets**
- DØ Calorimeter
- jet algorithms
- jet energy scale
**First DØ Run II jet results**
- Inclusive jet cross section
- Dijet cross section
**Conclusions/Outlook**

## The Fermilab Tevatron

**Run I (1992-95) ****~100 pb****-1**** recorded**
**Run IIa (2001-05)  ****2 fb****-1**
**6 month shutdown to install new silicon**
**Run IIb (2006-08?) ****~ 6-11 fb****-1**
**Depending on LHC schedule **
**Note the energy is also slightly boosted**
**Run I CM energy was 1.8 TeV **
**Run II CM energy is 1.96 TeV**

## Hadron-hadron collisions

- Complicated by
**parton distributions — **
- a hadron collider is really a broad-band quark and gluon collider
- both the initial and final states can be colored and can radiate gluons
**underlying event from proton remnants**

## Hadron-hadron collisions

***Q******2***** = Momentum Transfer**
**     ****= *****E******T******2****** ***

## (Run I) x-Q2 reach of DØ

**(Run I) x-Q****2**** reach of DØ**

**DØ’s most complete cross section measurement extends over |********| < 3.0**
**complements  HERA x-Q****2**** range**
**Used in CTEQ6 and MRST2001 fits to determine gluon at large x**
**Enhanced gluon at large x**

- CTEQ6M comparison

**Notes:**
To fully illustrate the weight of this result
Shown here is the region of Q2 versus x phase space which is explored in this Run I measurement.
The dark purple region shows the region 
explored by both D0 and CDF using the central rapidity cross sections, 
and the lighter purple region shows the extended reach of the D0 forward cross section measurements.

The 90 data bins with their full correlations of uncertainties were
Used in CTEQ6 and MRST2001 fits to determine gluon at large x “better than ever before”
The most significant change to the pdfs was an Enhanced gluon distribution at large x

The data points shown to the right show the measurement with the CTEQ6M prediction overlayed.
And the data-theory/theory result is shown in the strip charts to the bottom 
Showing the total uncertainty for each rapidity interval as a function of jet transverse energy.

## 
Cross section increases with CM energy especially at high jet pT

Note that at 400 GeV/c the cross section is doubled

**Cross section increases with CM energy especially at high jet p****T**
**Note that at 400 GeV/c the cross section is doubled**

**Run I vs. Run II**

## Data collection

## Run 2a DØ Upgrade

**Central tracking **
**2 Tesla Solenoid magnetic field**
**Silicon Microstrip Tracker**
**Scintillating Fiber tracker**
**Calorimetry:**
**LAR: electronics readout and trigger**
**Replace intercryostat detectors **
**Central/forward preshower detectors**
**Muon **
**scintillator layers for fast triggering**
**extended drift chamber coverage**
**Beamline shielding**

**New trigger system and DAQ to handle higher event rate **
**Forward Proton Spectrometer**

**Notes:**
Shown here is a summary of d0’s upgrades for Run II.

The Calorimeter geometry remains essentially the same
But the electronics in the readout and trigger have been overhauled 
In order to keep up with the increased data rate.

Other major changes were made to other subsystems.

## DØ Calorimeter

-  Using Run I calorimeter
-  Uranium-Liquid Argon
-  stable, uniform response, radiation hard, fine segmentation
-  Compensating (e/ ~1)
-  Uniform, hermetic, full coverage || < 4.2
-  Good energy resolution
-  New readout electronics to operate in Run II environment

## Jets at DØ: Calorimetry

**Uniform, hermetic with full coverage**** **
**|****| < 4.2 (******** ******** 2****o****), ********int ****~ 7.2  (total)**** **
**Single particle energy resolution**
**e: ****/E = 15% / ********E ******** 0.3%   **
******: ****/E = 45% / ********E ******** 4%****   **

**Fine segmentation**** **
**5000 pseudoprojective towers **
**( 0.1 ******** 0.1 ) in ********, ******
**4 EM layers**
**shower-max (EM3): 0.05 ******** 0.05**
**4/5 Hadronic ( FH + CH )**
**L1/L2 fast Trigger readout towers**

**|********| = 0.5**

**Notes:**
The primary instrument for the id and measurement of jets in both Run I and Run II is in D0’s Calorimetry.

It is primarily a liq ar sampling calorimeter with Uranium absorber
contained in 3 cryostats surrounding the central tracking system which encloses the interaction region.
It is uniform and hermetic with full coverage to an angle of 2 degrees with the beamline.

Single particle energy resolutions was measured in a calibration beam to be
15 and 45% over root E for electrons and hadrons, respectively.
It has around 50k readout channels available in the precision readout
and a separate readout chain summing channels longitudinally and by .2 in pseudo-rapidity and phi
in order to enable trigger decisions in the high rate environment at the first and second levels of the trigger.

## Highest Energy Dijet event in this sample

**Dijet Event**

**M****jj ****= 838 GeV/c****2**
**p****T****(1) = 432 GeV/c**
**p****T****(2) = 396 GeV/c**

****

****

## Run I/Run II Jet Algorithms

**Run I/Run II Jet Algorithms**

**Run I Legacy Cone:**
**Draw a cone of fixed size in ******** space around a seed**
**Compute jet axis from E****T****-weighted mean and jet ET from  ********E****T****’s**
**Draw a new cone around the new jet axis and recalculate axis and new E****T**
**Iterate until stable**
**Algorithm is sensitive to soft radiation**

**Improved Run II Cone :**
**“****Joint CDF/DØ/Theory Jet Working Group”**
- Use 4-vectors instead of ET
- Add additional midpoint seeds between pairs of close jets
- Split/merge after stable protojets found
- Improved infrared safety at NLO
**Run II: Algorithms under study:**
**k****T : ****(used for some results in Run I)**
- Uses relative momentum of particles
**Preprocessors for k****T**** or cone :**
- Cell Nearest Neighbor
- Energy Flow algorithm (tracking)

**Jet Energy Scale**

**Corrected jet energy**
**(to particle level)**

**Measured jet energy**
**(as measured in calorimeter)**

**Jet response**

**Showering Correction**
**(for energy leaking**
** ****in/out of cone)**

**Offset Correction**
**(for energy not **
**associated **
**with hard scatter)**

## Main corrections:

**Jet Energy Scale**

**Jet Response:**
** ****measure response of particles making up the jet **
** ****use photon + jet data - calibrate jets against the better calibrated EM scale**
** ****extrapolate using log-linear fit**

## Main corrections:
Subtraction of offset energy not originating from hard scatter
use minimum bias data (only requirement is an inelastic interaction) to extract this
Correction for jet energy in/out of cone
detector effect of jet calorimeter energies showering out of cone or different jet showering energy into cone
corrected with jet data + MC 
Current level of uncertainty
9% for central jets between 50-200 GeV
Dominated by response measurement
statistics
electromagnetic scale

- Subtraction of offset energy not originating from hard scatter
**use minimum bias data (only requirement is an inelastic interaction) to extract this**
- Correction for jet energy in/out of cone
**detector effect of jet calorimeter energies showering out of cone or different jet showering energy into cone**
**corrected with jet data + MC **
**Current level of uncertainty**
- 9% for central jets between 50-200 GeV
- Dominated by response measurement
**statistics**
**electromagnetic scale**

**Jet Energy Scale**

## First DØ Run II Jet Results

**Inclusive Jet Cross Section**
**Dijet cross section as a function of dijet mass**

## 
Data clean up cuts and jet energy scale corrections applied
No resolution or efficiency corrections

Clearly see trigger turn-ons
Triggers properly normalized

**Data clean up cuts and jet energy scale corrections applied**
**No resolution or efficiency corrections**
**Clearly see trigger turn-ons**
**Triggers properly normalized**

**Raw Inclusive Jet Cross Section**

## Jet Resolution and Unsmearing

**The jet cross section falls steeply with p****T****. **
- Mismeasured lower energy jets migrate to higher pT bins more often than higher jets migrating to lower bins
**This means the inclusive jet cross section is “smeared” preferentially to higher p****T**
- We extract the smearing correction by  by “guessing” the form of the true cross section, smearing it with the measured jet resolutions and fitting it to the data
- We then correct the measured cross section with the smearing corrections.

**DØ L3 **
**simulation**

**Jet p****T**

## Jet resolution measured from the data

**Jet Resolution**

**Use dijet momentum imbalance to measure jet resolutions**
**Define dijet asymetry as **
**Then the jet resolution is:**

## Unsmearing Correction

**Ansatz functions compared to the data after smearing with jet resolution and fitting**

**Unsmearing corrections**

## 

Finally...
After all corrections and unsmearing

**Finally...**
- After all corrections and unsmearing

**Inclusive Jet Cross Section**

## Fully corrected inclusive jet cross section

Red line - total systematic uncertainty (mainly JES)
Additional 10% from luminosity uncertainty not plotted

**Red line - total systematic uncertainty (mainly JES)**
**Additional 10% from luminosity uncertainty not plotted**

**Comparison with Theory**

## 

Partially corrected jet cross section as a function of dijet mass
same inclusive jet trigger set used as for total inclusive jet cross section analysis
All corrections except unsmearing correction have been applied

**Partially corrected jet cross section as a function of dijet mass**
**same inclusive jet trigger set used as for total inclusive jet cross section analysis**
**All corrections except unsmearing correction have been applied**

**“****Raw” Cross Section as a Function of Dijet Mass**

## Same prescription with resolution and smearing corrections

**Same prescription with resolution and smearing corrections**

**Dijet Mass Resolution**

**Unsmearing Correction for M****jj**

## Dijet Mass - comparison with theory

Red line - total systematic uncertainty (mainly JES)
Additional 10% from luminosity uncertainty not plotted

**Red line - total systematic uncertainty (mainly JES)**
**Additional 10% from luminosity uncertainty not plotted**

**Dijet Mass Distribution**

## Conclusions

- First measurements of Inclusive Jet and Dijet Cross Sections at CM energy = 1.96 TeV
- Based on 34 pb-1 of data
- more data already collected and we continue to collect data
- working hard to reduce systematic uncertainties
**especially in jet energy scale**
- Other ongoing analyses
- Jet algorithms
- Jet energy flow
- dijet angular distributions
- Data collection prospects
- Higher energy and luminosities mean much larger high ET dataset for QCD studies
- Improved detector should lead to an improved Jet Energy Scale, the largest systematic error for QCD studies